fm global 1 54 roof loads for new construction

July 2011 of 81

ROOF LOADS FOR NEW CONSTRUCTION

Table of ContentsPage

1.0 SCOPE ................................................................................................................................................... 41.1 Changes .......................................................................................................................................... 4

2.0 LOSS PREVENTION RECOMMENDATIONS ....................................................................................... 42.1 Use of Other Codes and Standards .................................................................................................. 42.2 Roof Loads and Load Combinations ............................................................................................... 4

2.2.1 Roof Live Load Reduction ................................................................................................... 42.3 Snow Loads ..................................................................................................................................... 5

2.3.1 General .................................................................................................................................. 52.3.2 Snow Load Notation .............................................................................................................. 52.3.3 Ground Snow Loads ............................................................................................................. 62.3.4 Snow Density ........................................................................................................................ 62.3.5 Flat-Roof Snow Loads ........................................................................................................... 72.3.6 Minimum Snow Loads for Low-Sloped Roofs ....................................................................... 72.3.7 Sloped-Roof Snow Loads ..................................................................................................... 72.3.8 Unbalanced Roof Snow Loads ............................................................................................. 82.3.9 Hip and Gable Roofs .............................................................................................................. 82.3.10 Curved and Domed Roofs ................................................................................................... 92.3.11 Valley Roofs ....................................................................................................................... 102.3.12 Drifts on Lower Roofs — Snow Loads .............................................................................. 11

2.4 Rain-on-Snow Surcharge ............................................................................................................... 182.5 Rain Loads .................................................................................................................................... 18

2.5.1 General ................................................................................................................................ 182.5.2 Bases for Design Rain Loads ............................................................................................. 182.5.3 Designing for Stability Against Ponding ............................................................................... 202.5.4 Roof Drainage ...................................................................................................................... 21

2.6 Other Roof Loads and Roof Overloading ..................................................................................... 412.7 Use of Eurocode ............................................................................................................................ 41

2.7.1 Eurocode for Snow Loads ................................................................................................... 412.7.2 Eurocode for Roof Live Load (Imposed Load) .................................................................... 422.7.3 Eurocode for Rain Loads ..................................................................................................... 42

2.8 Use of ASCE 7 for Snow Loads ..................................................................................................... 432.8.1 Factors .................................................................................................................................. 432.8.2 Hip and Gable Roofs ............................................................................................................ 43

2.9 Plan Review and Submissions ....................................................................................................... 442.9.1 General ................................................................................................................................. 442.9.2 Other Codes and Standards ................................................................................................ 45

3.0 SUPPORT FOR RECOMMENDATIONS ............................................................................................. 453.1 General ........................................................................................................................................... 45

3.1.1 Use of Other Codes and Standards .................................................................................... 453.1.3 Siphonic Drainage ................................................................................................................ 46

4.0 REFERENCES ..................................................................................................................................... 474.1 FM Global ...................................................................................................................................... 474.2 Others ............................................................................................................................................ 47

APPENDIX A GLOSSARY OF TERMS ..................................................................................................... 47A.1 Roof Loads and Drainage ............................................................................................................. 47

A.1.1 Controlled Roof Drains ........................................................................................................ 47A.1.2 Design Roof Line ................................................................................................................ 47

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A.1.3 Ponding and Ponding Cycle ............................................................................................... 47A.1.4 Dead Load .......................................................................................................................... 48A.1.5 Live Load ............................................................................................................................ 48A.1.6 Total Load ........................................................................................................................... 48A.1.7 Tributary Loaded Area (TA) ................................................................................................ 49A.1.8 Roof Strength ...................................................................................................................... 49A.1.9 Safety Factor ....................................................................................................................... 49

APPENDIX B DOCUMENT REVISION HISTORY ..................................................................................... 50APPENDIX C SUPPLEMENTARY INFORMATION .................................................................................... 50APPENDIX E ILLUSTRATIVE EXAMPLES AND JOB AIDS ................................................................... 65

E.1 Snow Loading Illustrative Examples ............................................................................................. 65E.2 Roof Drainage and Rain Loading Illustrative Examples ............................................................... 68E.3 Job Aids—Snow and Rain Loads and Drainage .......................................................................... 75

List of FiguresFig. 1. Snow loads for hip and gable roofs. ................................................................................................... 9Fig. 2a. Snow loads for curved and dome roofs. ........................................................................................ 10Fig. 2b. Unbalanced snow load distribution on dome roofs ........................................................................ 11Fig. 3. Snow loads for valley roofs .............................................................................................................. 11Fig. 4a. (To be used with Table 3) Snow loads for lower roofs. ................................................................. 12Fig. 4b. Snow drift intersection at lower roofs. ............................................................................................ 12Fig. 5. Snow loads for lower roof of adjacent structures ............................................................................ 16Fig. 6. Sliding snow load for lower roofs (upper roof snow load not shown) ............................................. 17Fig. 7. Snow load at roof projections ........................................................................................................... 17Fig. 8a. Typical primary and overflow systems for pitched roofs ................................................................ 19Fig. 8b. Typical primary and overflow drainage systems for flat roofs ....................................................... 20Fig. 9. Flat and sloped roofs with interior roof drains ................................................................................. 29Fig. 10. Sloped roof with roof edge drainage .............................................................................................. 29Fig. 11. Diagram of Siphonic Roof Drain System ......................................................................................... 37Fig. 12. Elevation View of Siphonic System and Disposable (Available) Head ......................................... 38Fig. 13. Siphonic Roof Drain [photo courtesy of Jay R. Smith Mfg. Co.] ................................................... 39Fig. 14. Siphonic Roof Drain for Gutters (without dome strainer or debris guard) ..................................... 39Fig. 15. Typical tributary loaded areas for primary and secondary members ............................................ 49Fig. 16a. Ground snow load (Pg) in psf for Western United States. ........................................................... 51Fig. 16b. Ground snow load (Pg) in psf for Eastern United States. (To obtain kN/m2, multiply by 0.048) . 52Fig. 17a. Ground Snow Load (Pg) in kN/fm2 for Western China ................................................................. 57Fig. 17b. Ground Snow Load (Pg) in kN/m2 for Eastern China ................................................................... 58Fig 18. Roof Live load reduction Flow Chart/Decision Tree ....................................................................... 59Fig. 19. Rainfall intensity (i) in inches per hour for the western United States (to convert to

millimeters per hour multiply by 25.4.) ........................................................................................... 60Fig. 20. Rainfall intensity (i) in inches per hour for the central and eastern United States

(to convert to millimeters per hour multiply by 25.4. ...................................................................... 61Fig. 21a. Rainfall intensity (i) in inches per hour for Puerto Rico (to convert to millimeters

per hour multiply by 25.4. ............................................................................................................. 62Fig. 21b. Rainfall intensity (i) in inches per hour for Hawaiian Islands (to convert to millimeters

per hour multiply by 25.4). ........................................................................................................... 63Fig. 22. Rainfall intensity (i) in inches per hour for Alaska (to convert to millimeters per hour

multiply by 25.4). ............................................................................................................................ 64Fig. E1.1. Design snow loads for Example 1 .............................................................................................. 65Fig. E1.2. Design snow loads for Example 2 .............................................................................................. 66Fig. E1.3. Design snow loads for Example 3 (Leeward Drifting) ................................................................ 67Fig. E1.4. Design snow loads for Example 4 (Windward Drifting) .............................................................. 68Fig. E1.5.1 Flat roof plan for Example 5 ..................................................................................................... 69Fig. E1.5.2 Sloped roof plan for Example 5 ................................................................................................ 70Fig. E1.5.3 Sloped Roof Section for Example 5 ......................................................................................... 71Fig. E1.6. Roof plan for Example 6 ............................................................................................................. 72

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Fig. E1.7. Roof plan for Example 7. ............................................................................................................ 73

List of TablesTable 1. Ground Snow Load (Pg) versus Balanced Flat-Roof Snow Load (Pf), Density (D), and

Height of Balanced Snow Load (hb) for Flat and Low-sloped Roofs ............................................. 7Table 2. Roof Slope Factor Cs ...................................................................................................................... 8Table 3. Ground Snow Load (Pg) versus Balanced Snow Load (Pf), Density (D), Balance Snow

Load Height (hb), Drift Height (hd), Max Drift Load (Pd) and Max Load (Pd+Pf) ......................... 14Table 3, Continued. Ground Snow Load (Pg) versus Balanced Snow Load (Pf), Density (D), Balance

Snow Load Height (hb), Drift Height (hd), Max Drift Load (Pd) and Max Load (Pd+Pf) ............... 15Table 4. Rain-on-Snow Surcharge Load ..................................................................................................... 18Table 5. Flow Capacity for Roof Drains and Pipinga .................................................................................. 25Table 6. Hydraulic Head Versus Flow Capacity for Roof Scuppers(Depth of water over

invert versus flow of water through scupper) ................................................................................. 26Table 7. Conversion of Rainfall Intensity to Flow Rate and Rain Load per Unit Area ............................... 27Table 8. Hydraulic Head versus Roof Drain Flow ...................................................................................... 27Table 9. Rainfall Intensity Conversion Rates ............................................................................................... 40Table 10. Schedule 40 Pipe Dimensions and Geometric Properties .......................................................... 40Table 11. Standard Atmospheric Pressure at Various Elevations ................................................................ 41Table 12. Ground Snow Load (Pg) for Alaskan Locations, psf (kN/m2) ....................................................... 53Table 13. Ground Snow Load (Pg) for Locations in Korea, psf and kPa .................................................... 53Table 14. Ground Snow Load (Pg) for Locations in Japan, psf and kPa ..................................................... 54Table 15. Ground Snow Load (Pg) for Locations in China* ........................................................................ 55Table 15. Continued ..................................................................................................................................... 56

Roof Loads for New Construction 1-54FM Global Property Loss Prevention Data Sheets


1.0 SCOPE

This loss prevention data sheet presents recommendations principally for snow and rain loadings anddrainage for the design of new roofs of buildings and other structures.

In general, it is the function of this data sheet to present background details and guidelines for buildingdesigners to use in carrying out the requirements or intent of typical building and plumbing codes regardingdesign roof loads and roof drainage.

It should be noted that the various recommendations presented here are not based on the worst conditionspossible, or even the worst conditions recorded. A probabilistic approach is used to establish design valuesthat reduce the risk of a snow-load-induced or rain-load-induced roof collapse to an acceptably low level.

1.1 Changes

July 2011. Corrections were made to Table 12, Ground Snow Load for Alaskan Locations.

2.0 LOSS PREVENTION RECOMMENDATIONS

2.1 Use of Other Codes and Standards

Refer to Sections 2.7 and 2.8 for the use of the Eurocode and ASCE 7, respectively.

2.2 Roof Loads and Load Combinations

Design the roof to resist the effects of dead loads in combination with the more demanding of the followingroof live or environmental (e.g., rain or snow) loads:

a) The balanced (uniform) or unbalanced snow loads, including snow drift surcharge and rain-on-snowsurcharge where applicable, in accordance with Section 2.3

b) The rain loads in accordance with Section 2.5 and precluding (i.e., ruling out in advance) instabilityfrom ponding

c) Superimposed roof live loads, as specified, to account for the use and maintenance of the roof andthe occupancy of the building/structure

d) A minimum roof live load of 20 psf (1.0 kN/m2) for flat roofs, sloped roofs less than 4 in./ft (18.4 degrees)and curved roofs with rise less than 1⁄8 of span, except when a reduction in the minimum roof live loadis appropriate, as described in Section 2.2.1.

2.2.1 Roof Live Load Reduction

2.2.1.1 Reductions in the minimum unfactored (characteristic) roof live load of 20 psf (1.0 kN/m2) forlightweight roof constructions (lightweight roof constructions include metal roofs, insulated steel deck,boards-on-joists, plywood diaphragm, and similar constructions), when permitted by applicable building codesand standards, are only allowed whenever both of the following conditions are met:

a) The roof slopes at least 1⁄4 in./ft (1.2 degrees or 2%), and

b) The roof snow load is zero, or the supported combined unfactored (characteristic) dead load plusresultant roof live load (reduced) is at least 28 psf (1.4 kN/m2).

See Figure 18 for a roof live load reduction flow chart.

Note that for purposes of foundation design only (e.g., footings, grade beams, piles, and caissons), the useof roof live loads and live reduction techniques as permitted by applicable building codes and standards areacceptable without revision or exception.

Do not use roof live load reduction for the following:



1) Roofs that can have an occupancy function such as roofs on which an assembly or congregation ofpeople is allowed or intended (e.g., some roof gardens (vegetated green roofs); or roofs that function asa balcony, elevated terrace, or viewing platform).

2) Roof used for storage, including car parking garage roofs.

3) Roofs where the code-required unfactored (characteristic) live load is greater than 75 psf (3.6 kPa).

2.2.1.2 Where roof live load reduction is permissible under Section 2.2.1.1, use the following roof live loadreduction procedure (where TA = Tributary Area):

1) TA ≤ 200 ft2 (19 m2): No roof live load reduction allowed; use 20 psf (1.0 kPa).

2) 200 ft2 (19 m2) < TA = < 600 ft2 (56 m2):Roof live load (psf) = (1.2 - 0.001(TA))(20 psf)orRoof live load (kPa) = (1.2 - 0.0108(TA)) (1.0 kPa)

3) TA > 600 ft2 (56 m2): Roof live load = 12 psf (0.6 kPa)

For example, if TA = 400 ft2 (37.5 m2), then the minimum reduced roof live load is 0.8 x 20 psf (1.0 kPa) =16 psf (0.8 kPa).

For a continuous structural roof system, such as a concrete slab, use a tributary length equal to the span(use the lesser span for a two-way slab system), and use a tributary width not greater than 1.5 x tributary span;in other words: TA = 1.5 (tributary span)2. The same technique can be used for one-way systems such asmetal roof deck, standing seam roofs, of lap seam roofs; however, based on typical spans, the TA will generallybe less than 200 ft2 (19 m2) and therefore will not be eligible for roof live load reduction.

2.3 Snow Loads

2.3.1 General

Determine roof snow loads in accordance with the guidelines of this section; however, ensure the roof loadsare not less than the minimum live loads or snow loads designated by the applicable building code, nor lessthan the rain loads covered in Section 2.5. For roofs of unusual shape or configuration, use wind-tunnel oranalytical modeling techniques to help establish design snow loads.

2.3.2 Snow Load Notation

Ce = exposure factorCs = slope factor

Ct = thermal factorD = snow weight density (pcf [kN/m3]) of drifted snow

hb = height of balanced uniform snow load (ft [m]) (i.e., balanced snow load Pf or Ps divided by D)

hc = clear height from top of balanced snow load (ft [m]) to the closest point(s) on adjacent upper roof; tothe top of parapet; or to the top of a roof projection

hd = maximum height of snow drift surcharge above balanced snow load (ft [m])

hr = difference in height between the upper roof (including parapets) and lower roof or height of roofprojection (ft [m])

Pd = maximum intensity of drift surcharge load (psf [kN/m2 or kPa])

Pf = flat-roof snow load (psf [kN/m2 or kPa])

Pg = ground snow load (psf [kN/m2 or kPa])

Ps = sloped-roof snow load (psf [kN/m2 or kPa])

S = separation distance between buildings (ft [m])



Wb = horizontal distance of roof upwind of drift (ft [m]), but not less than 25 ft (7.6 m). Wb equals the entireupwind distance of roofs with multiple elevation differences, provided the predicted drift height at eachelevation difference exceeds hc

Wd = width of snow drift surcharge (ft [m])

Wp = width of rooftop projection (ft [m])

Ws = width of sloped upper roof, from ridge to eave (ft [m])Θ = roof slope from horizontal (degrees, rise: run, in./ft)

2.3.3 Ground Snow Loads

Ground snow loads (Pg) used in determining design snow loads for roofs are given in the two-part map forthe contiguous United States in Figures 16a and 16b. The maps are provided in the publication MinimumDesign Loads for Buildings and Other Structures by the American Society of Civil Engineers/StructuralEngineering Institute (ASCE/SEI Standard 7-05). The maps present snow-load zones with estimated groundsnow loads based on a 50-year MRI and provide the upper elevation limit for the presented ground snowloads. At locations where the elevation exceeds that indicated on the ground snow load maps, and in areaszoned as CS (where the amount of local variation in snow loads is substantial enough to preclude meaningfulmapping; these include mountainous locations and areas close to large water bodies), site-specific CaseStudies (CS) are needed to determine accurate ground snow conditions. The local office of the NationalWeather Service, local building codes, or the building authority or official having jurisdiction (AHJ), should becontacted for locations where site-specific Case Studies (CS) are applicable.

Use ground snow loads based on a 50-year MRI. Approximate multiplication factors for converting from lesserMRI ground snow loads to 50-year MRI ground snow loads are:

50-year = 2.25 x 5-year50-year = 1.82 x 10-year50-year = 1.20 x 25-year50-year = 1.15 x 30-year

Ground snow loads are zero for Puerto Rico and most of Hawaii, although for mountainous regions in Hawaii,consult local building officials to verify ground snow load conditions.

Ground snow loads (Pg) for Alaska are presented in Table 12 for specific locations only and generally donot represent appropriate design values for other nearby locations. In Alaska, extreme local variationspreclude statewide mapping of ground snow loads.

2.3.3.1 Ground Snow Loads in China

Use a snow load Importance Factor (l) of 1.2 for ground snow loads in China. Apply the 1.2 Importance Factorto the ground snow load values (Pg) from Figures 17a and 17b China Ground Snow Load. Figures 17a and17b represent the 50-year ground snow loads. Note that the Pg values for the various cities in Table 15Ground Snow Load (Pg) for Locations in China include the 1.2 Importance Factor. For example, for Dalian:From Figure 17b, Pg = 0.53 kN/m2; and from Table 15, Pg = 0.64 kN/m2.

2.3.4 Snow Density

Determine bulk snow (weight) density (to evaluate the heights of roof snow loads) (D) as a function of theground snow load (Pg) according to Table 1 or the following formulas:

English Units:

D (pcf) = 0.13 Pg + 14 ≤ 30 where Pg in psf

Metric Units:

D (kN/m3) = 0.43 Pg + 2.2 ≤ 4.7 where Pg in kN/m



2.3.5 Flat-Roof Snow Loads

To determine the balanced (uniform) snow load (Pf) on an unobstructed flat roof, including any roof with aslope less than 5° (1 in./ft or 8%), use Table 1 or the following formulas:

Pf = Pg where Pg ≤ 20 psf(1.0 kN/m2)

Pf = 0.9 Pg ≥ 20 psf (1.0 kN/m2) where 20 psf < Pg ≤ 40 psf (1.0 < Pg ≤ 1.9 kN/m2)

Pf = 0.8 Pg ≥ 36 psf (1.7 kN/m2) where Pg > 40 psf (1.9 kN/m2)

2.3.6 Minimum Snow Loads for Low-Sloped Roofs

The minimum allowable snow loads are the balanced snow loads (Pf) of Section 2.3.5 or Table 1 and appliedto shed, hip and gable roofs with slopes less than 15°, and curved roofs where the vertical angle (see Fig.2a) from the eave to the crown is less than 10°. The formulas in Section 2.3.5 satisfy the following mini-mum snow load guidelines: for locations where the ground snow load (Pg) is 20 psf (1.0 kN/m2) or less, theflat roof snow load (Pf) for such roofs is not less than the ground snow load (Pg); in locations where the groundsnow load (Pg) exceeds 20 psf (1.0 kN/m2), the flat-roof snow load (Pf) for such roofs is not less than 20psf (1.0 kN/m2).

In building codes, minimum roof live loads and live load reductions do not apply to snow loads. Snow loadsgreater than such live loads govern the determination of design loads.

Table 1. Ground Snow Load (Pg) versus Balanced Flat-Roof Snow Load (Pf), Density (D), andHeight of Balanced Snow Load (hb) for Flat and Low-sloped Roofs

English UnitsGround Snow Load, Pg (psf) Balanced Flat-Roof Snow Load, Pf (psf)

Pg 5 10 20 25 30 35 40 50 60 70 80 90 100Pf 5 10 20 23 27 32 36 40 48 56 64 72 80

Density D, (pcf) Balanced Flat-Roof Snow Load Height, hb, (ft)D 14.7 15.3 16.6 17.3 17.9 18.6 19.2 20.5 21.8 23.1 24.4 25.7 27.0hb 0.3 0.7 1.2 1.3 1.5 1.7 1.9 2.0 2.2 2.4 2.7 2.8 3.0

Metric UnitsGround Snow Load, Pg (kN/m2) Balanced Flat-Roof Snow Load, Pf (kN/m2)

Pg 0.25 0.5 0.6 0.9 1.0 1.4 1.9 2.0 3.0 4.0 5.0Pf 0.25 0.5 0.6 0.9 1.0 1.3 1.7 1.7 2.4 3.2 4.0

Density D, (kN/m3) Balanced Flat-Roof Snow Load Height hb (m)D 2.3 2.4 2.5 2.6 2.6 2.8 3.0 3.1 3.5 3.9 4.4hb 0.1 0.2 0.2 0.3 0.4 0.4 0.6 0.6 0.7 0.8 0.9

Note: Linear interpolation is appropriate.

2.3.7 Sloped-Roof Snow Loads

Determine balanced (uniform) snow load (Ps) on sloped roofs, such as shed, hip, gable, and curved roofs,by multiplying the flat-roof load (Pf) by the roof slope factor (Cs):

Ps = Cs x Pf

Values of Cs are given in Table 2. Use cold roof values. The exception is warm roof values that apply forun-insulated glass or metal panel, plastic (e.g., acrylic or reinforced plastic panels), and fabric roofs withR-value less than 2.0 ft2•hr.•°F/Btu (0.4 m 2-°K/W) of buildings continuously heated above 50°F (10°C); notethat to take advantage of warm roof slope factor values, ensure the building has a maintenance techni-cian on duty at all times and a temperature alarm system battery back-up is in place to warn of heating fail-ures.

Use ‘‘slippery surface’’ values only where the sliding surface is metal (aluminum, copper, galvanized or enam-eled steel panels, such as on all-metal buildings) and is unobstructed with sufficient space below the eavesto accept all sliding snow; if it is reasonable to assume snow guards could be installed (e.g., where a slopedroof overhangs a sidewalk) consider the roof obstructed. Note that for curved and dome roofs the ”verticalangle” (see Fig. 2a) is measured from the eave to the crown.



Table 2. Roof Slope Factor Cs

Roof Slope, degrees(Rise:Run)

Cs Values 1,2

Unobstructed Slippery Surfaces All Other SurfacesCold Roof Warm Roof Cold Roof Warm Roof

≤ 5° (1:12) 1.0 1.0 1.0 1.014° (3:12) 1.0 0.8 1.0 1.0

18.4° (4:12) 0.94 0.74 1.0 1.026.6° (6:12) 0.79 0.62 1.0 1.030° (7:12) 0.73 0.57 1.0 1.0

33.7° (8:12) 0.66 0.52 1.0 0.9145° (12:12) 0.46 0.36 1.0 0.63

60° 0.19 0.14 0.4 0.2570° 0 0 0 0

1. Use “cold roof” and “all other surfaces” values unless conditions in Section 2.3.7 apply.2. Linear interpolation is appropriate within any column.

2.3.8 Unbalanced Roof Snow Loads

Consider balanced and unbalanced snow loads as separate load cases. Consider winds from all directionswhen establishing unbalanced snow loads. For design purposes, unbalanced and drifting snow due toorthogonal wind directions (90° to each other) are considered to occur simultaneously; however, winds fromopposite directions, 180°, are not considered to occur simultaneously.

2.3.9 Hip and Gable Roofs

2.3.9.1 Unbalanced Snow Load

Consider the balanced snow load case for all roof slopes. The unbalanced snow load case need only beconsidered for roof slopes between 5° and 70° (1 on 12, and 33 on 12 slopes) inclusive. Balanced andunbalanced snow loading diagrams appear in Figure 1. Apply no reduction in snow load for roof slopes upto and including 15° (i.e., Cs = 1.0 and therefore Ps = Pf) and the snow surface above the eave need not beat a higher elevation than the snow surface above the ridge. Determine snow depths by dividing the snowloads by the appropriate snow density (D) from Table 1.



2.3.9.2 Ice Dam Load

For typical heated building structures that drain water over their overhanging roof eaves, and where the roofassembly has an R-value of less than 25 ft2•hr.•°F/Btu (4.4 m 2•°K/W), apply a uniform snow load of 2Pf

to the overhanging roof eaves; if the R-value of the roof assembly cannot be verified, assume that the load2Pf is applicable. The load 2Pf is intended to account for the effects of ice dams along the overhanging roofeave, and need not be combined with any design load other than the dead load of the roof.

2.3.10 Curved and Domed Roofs

Consider unbalanced snow loads for slopes where the ‘‘vertical angle’’ from the eave to the crown is between10° and 60°. Consider portions of curved roofs having a roof slope exceeding 70° free of snow; considerthe point at which the roof slope exceeds 70° the ‘‘eave’’ for such roofs. Unbalanced loading diagrams, CasesI, II, and III, for curved roofs with roof slopes at the eave of less than 30°, 30° to 70°, and greater than 70°,appear in Figure 2a. If another roof or the ground surface abuts a Case II or III curved roof at or within3 ft (0.9 m) of the eave, ensure the snow load is not decreased between the 30° roof slope point and theeave, but remains constant at 2.0 Ps as shown by the dashed line.

Fig. 1. Snow loads for hip and gable roofs.



For domed roofs, see Fig. 2b for unbalanced snow load distribution with a single wind orientation. Note thatsince unbalanced snow loads due to orthogonal (90 degree) wind directions are assumed to act concur-rently, consider also the load distribution with the unbalanced snow load on one-half the roof (180 degrees),two linearly decreasing to zero snow zones of 22.5 degrees each, and the remaining area (135 degrees)free of snow. Determine the governing orientation of the unbalanced snow load based on the maximumdemand on the structure.

2.3.11 Valley Roofs

Valleys are formed by multiples of folded plate, gable, saw-tooth, and barrel vault roofs. No reduction inbalanced or unbalanced snow load is allowed for any roof slope (i.e., Cs = 1.0 and Ps = Pf). For valleys formedby roof slopes of 5° (1 on 12) and greater, consider unbalanced snow loads. The unbalanced snow loadshould increase from one-half the balanced load (0.5 Pf) at the ridge (or crown) to two times the balancedload at the valley (2.0 Pf) (see Fig. 3). The snow surface above the valley, however, need not be at a higherelevation than the snow surface at the ridge (or crown). Determine snow depths by dividing the snow loadsby the appropriate snow density (D) in Table 2. The above snow load methodology is also applicable to mul-tiple gable and barrel vault roofs.

Fig. 2a. Snow loads for curved and dome roofs.



2.3.12 Drifts on Lower Roofs — Snow Loads

In areas where the ground snow load (Pg) is less than 5 psf (0.25 kN/m2) or the ratio hc/hb is less than 0.2,drift loads on lower roofs need not be considered. Otherwise, design lower levels of multilevel roofs to sustainlocalized loads from snow drifts caused by wind over upper roofs of the same structure, adjacent structures,or terrain features within 20 ft (6 m) (leeward drifting); sliding snow; or snow drifts formed on lower roofs bywindblown snow across the lower roof (windward drift).

Examine the following three load cases when determining the maximum demand placed on the supportingstructure of the lower roof:

a) Balanced Snow + Leeward Drift

b) Balanced Snow + Windward Drift

c) Balanced Snow + Sliding

Note that drift load need not be combined with sliding snow load.

Fig. 2b. Unbalanced snow load distribution on dome roofs

Fig. 3. Snow loads for valley roofs



Note that more than one load case may govern the structural design. For example, for a low roof joist spanningperpendicular to the line of the roof step (i.e., parallel to the worst-case wind direction for drifting), load case(a) may produce maximum shear, but load case (c) may produce maximum bending.

2.3.12.1 Drift Load

Take the drift load on lower roofs as a triangular surcharge loading superimposed on the balanced roof snowload (Pf), as shown in Figure 4a. Note that the upper roof may be flat or sloped. For upper roof slopes lessthan 30-degrees, use an upwind distance (Wb) equal to the upper roof width parallel to the wind direction(e.g., eave to eave distance for a sloped roof). For upper roof slopes of 30-degrees or greater, use an upwinddistance (Wb) equal to 85% of the upper roof width.

Where intersecting snow drifts of lower roofs due to perpendicular wind directions are possible, at thetheoretical drift intersection the larger snow drift governs; the two drift loads need not be superimposed tocreate a combined (additive) drift load. See Figure 4b.

Note that parapet walls on high roofs will not substantially reduce leeward drifting on adjacent low roofs;therefore, do not credit high roof parapets as a method of reducing low roof leeward drifting.

Determine maximum drift height (hd) in ft (m) from Table 3 or the following formulas:

Fig. 4a. (To be used with Table 3) Snow loads for lower roofs.

Fig. 4b. Snow drift intersection at lower roofs.



English Units:

hd (ft) = 0.43 √3

Wb √4

Pg+10 − 1.5 ≤ hc

where Pg in psf; Wb and hc in ft

Metric Units:

hd (m) = 0.42 √3

Wb √4

Pg+0.48 −0.457 ≤ hc

where Pg in kN/m2; Wb and hc in meters

Drift surcharge load (maximum intensity), Pd = (hd × D) ≤ (hc × D)

Maximum snow load (at wall) = (Pd + Pf) ≤ (hr × D)

The drift surcharge load (Pd) and the maximum snow load at the wall (see Fig. 4a) may also be determinedby Table 3, provided the product of the density (D) and hc or hr does not govern.

Drift width (Wd) is equal to 4 hd except for rare cases when the calculated hd exceeds hc . For these cases,the minimum Wd is established so that the cross-sectional area of the drift (0.5 Wd × hc) is equal to thecross-sectional area of the hypothetical drift (0.5hd × 4hd = 2hd

2) that would be computed if hd were lessthan hc; however, Wd cannot be less than 6 hc and need not be greater than 8 hc. Thus,

Wd = 4 hd,except when hd > hc , then Wd = 4 h

d

2

hc

(but 8hc ≥ Wd ≥ 6hc)

If Wd exceeds the width of the lower roof (this occurs frequently with canopy roofs), truncate the drift at thefar edge of the roof and do not reduce it to zero.



Table 3. (To be used with Figure 4a) Ground Snow Load (Pg) versus Balanced Snow Load (Pf), Density (D), BalanceSnow Load Height (hb), Drift Height (hd), Max Drift Load (Pd) and Max Load (Pd+Pf)

English Units:Ground Snow Load, Pg (psf)

Balanced Snow Load, Pf (psf)Pg 5 10 15 20 25 30 35 40 50 60 70 80 90 100Pf 5 10 15 20 23 27 32 36 40 48 56 64 72 80

Density, D (pcf)Balanced Snow Load Height, hb (ft)

D 14.7 15.3 16.0 16.6 17.3 17.9 18.6 19.2 20.5 21.8 23.1 24.4 25.7 27.0hb 0.3 0.7 0.9 1.2 1.3 1.5 1.7 1.9 2.0 2.2 2.4 2.7 2.8 3.0

UpwindDistanceWb (ft)

Drift Height, hd (ft)a

Max. Drift Load, Pd (psf)a

Max. Load at Wall, Pd + Pf (psf)a

25 hd 0.97 1.16 1.31 1.44 1.56 1.66 1.76 1.84 2.00 2.14 2.26 2.37 2.47 2.57Pd 14 18 21 24 27 30 33 35 41 47 52 58 63 64

Pd+Pf 19 28 36 44 50 57 65 71 81 95 108 122 135 14450 hd 1.61 1.85 2.04 2.21 2.35 2.48 2.60 2.71 2.91 3.08 3.24 3.38 3.51 3.62

Pd 24 28 33 37 41 44 48 52 60 67 75 82 90 98Pd+Pf 29 38 48 57 64 71 80 88 100 115 131 146 162 178

100 hd 2.42 2.72 2.96 3.17 3.35 3.52 3.67 3.81 4.05 4.27 4.47 4.65 4.81 4.96Pd 36 42 47 53 58 63 68 73 83 93 103 113 124 134

Pd+Pf 41 52 62 73 81 90 100 109 123 141 159 177 196 214200 hd 3.44 3.82 4.12 4.39 4.62 4.83 5.01 5.19 5.50 5.78 6.02 6.25 6.45 6.64

Pd 51 58 66 73 80 86 93 100 113 126 139 153 166 179Pd+Pf 56 68 81 93 103 113 125 136 153 174 195 217 238 259

300 hd 4.15 4.59 4.94 5.24 5.50 5.74 5.96 6.16 6.51 6.83 7.11 7.37 7.60 7.82Pd 61 70 79 87 95 103 111 118 133 149 164 180 195 211

Pd+Pf 66 80 94 107 118 130 143 154 173 197 220 244 267 291400 hd 4.72 5.20 5.58 5.91 6.20 6.46 6.71 6.92 7.32 7.67 7.97 8.26 8.52 8.76

Pd 69 80 89 98 107 116 125 133 150 167 184 202 219 237Pd+Pf 74 90 104 118 131 143 157 169 190 215 240 266 291 317

500 hd 5.20 5.72 6.13 6.48 6.80 7.08 7.34 7.58 8.00 8.37 8.70 9.01 9.29 9.55Pd 76 88 98 108 118 127 137 146 164 182 201 220 239 258

Pd+Pf 81 98 113 128 141 154 169 182 204 230 257 284 311 338600 hd 5.62 6.17 6.61 6.99 7.32 7.62 7.89 8.14 8.59 8.99 9.34 9.67 9.97 10.3

Pd 83 94 106 116 127 136 147 156 176 196 216 236 256 278Pd+Pf 88 104 121 136 150 163 179 192 216 244 272 300 328 358

800 hd 6.34 6.94 7.43 7.84 8.21 8.54 8.84 9.11 9.61 10.0 10.4 10.8 11.1 11.4Pd 93 106 119 130 142 153 164 175 197 219 241 264 286 308

Pd+Pf 98 116 134 150 165 180 196 211 237 267 297 328 358 3881000 hd 6.94 7.59 8.11 8.56 8.98 9.31 9.64 9.93 10.5 10.9 11.4 11.7 12.1 12.4

Pd 102 116 130 142 155 167 179 191 215 238 262 286 311 335Pd+Pf 107 126 145 162 178 194 211 227 255 286 318 350 383 415

Note: Linear interpolation is appropriate.a The drift height (hd), maximum drift load (Pd), and maximum load at wall (Pd + Pf) are limited to hc, (hc × D), and (hr × D) respec-tively.



Table 3, Continued. (To be used with Figure 4a) Ground Snow Load (Pg) versus Balanced Snow Load (Pf), Density (D), Bal-ance Snow Load Height (hb), Drift Height (hd), Max Drift Load (Pd) and Max Load (Pd+Pf)

Metric Units:Ground Snow Load, Pg (kN/m2)

Balanced Snow Load, Pf (kN/m2)0.25 0.5 0.6 0.9 1.0 1.4 1.9 2.0 3.0 4.0 5.00.25 0.5 0.6 0.9 1.0 1.3 1.7 1.7 2.4 3.2 4.0

Density, D (kN/ cu m)Balanced Snow Load Height, hb (m)

2.3 2.4 2.5 2.6 2.6 2.8 3.0 3.1 3.5 3.9 4.40.1 0.2 0.2 0.3 0.4 0.4 0.6 0.6 0.7 0.8 0.9

UpwindDistance Wb (m)

Drift Height, hd (m)a

Max. Drift Load, Pd (kN/m2)a

Max. Load at Wall, Pd + Pf (kN/m2)a

10 hd .37 .43 .46 .51 .53 .59 .66 .67 .77 .85 .91Pd .85 1.04 1.14 1.34 1.38 1.66 1.97 2.07 2.68 3.30 4.02

Pd+Pf 1.10 1.54 1.74 2.24 2.38 2.92 3.67 3.77 5.08 6.50 8.0215 hd .49 .56 .59 .65 .67 .74 .82 .83 .91 1.03 1.11

Pd 1.13 1.35 1.47 1.70 1.75 2.08 2.45 2.53 3.18 4.03 4.89Pd+Pf 1.38 1.85 2.07 2.60 2.75 3.34 4.15 4.23 5.58 7.23 8.89

30 hd .74 .83 .86 .94 .97 1.06 1.15 1.16 1.31 1.42 1.52Pd 1.69 1.99 2.15 2.45 2.52 2.96 3.44 3.61 4.58 5.55 6.69

Pd+Pf 1.94 2.49 2.75 3.35 3.52 4.22 5.14 5.31 6.98 8.75 10.6950 hd .96 1.07 1.10 1.2 1.23 1.34 1.44 1.46 1.63 1.77 1.89

Pd 2.20 2.56 2.76 3.13 3.20 3.74 4.33 4.54 5.72 6.91 8.30Pd+Pf 2.45 3.06 3.36 4.03 4.20 5.00 6.03 6.24 8.12 10.11 12.30

100 hd 1.32 1.46 1.51 1.63 1.67 1.8 1.94 1.96 2.18 2.35 2.49Pd 3.05 3.51 3.77 4.25 4.34 5.04 5.81 6.08 7.62 9.16 10.97

Pd+Pf 3.30 4.01 4.37 5.15 5.34 6.30 7.51 7.78 10.02 12.36 14.97120 hd 1.44 1.58 1.63 1.76 1.80 1.94 2.09 2.11 2.34 2.52 2.68

Pd 3.30 3.80 4.08 4.59 4.69 5.44 6.26 6.55 8.17 9.84 11.78Pd+Pf 3.55 4.30 4.68 5.49 5.69 6.70 7.96 8.25 10.57 13.04 15.78

150 hd 1.58 1.74 1.79 1.94 1.98 2.13 2.29 2.31 2.56 2.76 2.92Pd 3.64 4.18 4.48 5.03 5.14 5.96 6.86 7.17 8.96 10.75 12.85

Pd+Pf 3.89 4.68 5.08 5.93 6.14 7.22 8.56 8.87 11.36 13.95 16.85180 hd 1.71 1.88 1.93 2.09 2.13 2.29 2.46 2.49 2.75 2.96 3.13

Pd 3.93 4.51 4.83 5.42 5.54 6.41 7.37 7.71 9.62 11.53 13.78Pd+Pf 4.18 5.01 5.43 6.32 6.54 7.67 9.07 9.41 12.02 14.73 17.78

200 hd 1.79 1.96 2.02 2.18 2.22 2.39 2.56 2.59 2.86 3.08 3.26Pd 4.11 4.70 5.05 5.66 5.78 6.68 7.58 8.03 10.01 12.00 14.34

Pd+Pf 4.36 5.20 5.65 6.56 6.78 7.94 9.38 9.73 12.41 15.20 18.34300 hd 2.11 2.31 2.38 2.56 2.61 2.80 3.00 3.03 3.34 3.59 3.8

Pd 4.86 5.54 5.94 6.65 6.79 7.84 8.99 9.40 11.70 14.00 16.71Pd+Pf 5.11 6.04 6.54 7.55 7.79 9.10 10.69 11.10 14.10 17.20 20.71

Note: Linear interpolation is appropriate.a The drift height (hd), maximum drift load (Pd), and maximum load at wall (Pd + Pf) are limited to hc, (hc × D), and (hr × D) respec-tively.



2.3.12.2 Adjacent Structures and Terrain Features

Apply a drift load to lower roofs or structures sited within 20 ft (6 m) of a higher structure or terrain feature(i.e., tanks, hills) as shown in Figure 5. Determine the drift load using the methodology of Section 2.3.12.1;apply the factor 1-(S/20) with S in ft (1-[S/6] with S in meters) to the maximum intensity of the drift Pd toaccount for the horizontal separation between structure S, expressed in ft (m). Drift loads need not be con-sidered for separations greater than 20 ft (6 m).

2.3.12.3 Sliding Snow

For lower roofs located below slippery roofs having a slope greater than 1.2° (1⁄4 on 12), or below other (non-slippery) roofs having a slope greater than 9.5° (2 on 12), consider a sliding snow surcharge load (psf) of0.4PfWs/15 where Pf is psf, and Ws is feet (sliding surcharge load [kN/m2] of 0.4PfWs/4.6 where Pf is kN/m2,and Ws is meters); except that hs needs to exceed hc. Determine hs by dividing the snow surcharge loadby the appropriate snow density (D). Note that Ws is the horizontal distance from the ridge to the eave of theupper roof. See Figure 6.

Apply sliding snow surcharge load to the balanced snow load (Pf) of the lower roof.

For consideration of the sliding snow surcharge, “slippery” roof surfaces are defined as metal (aluminum, cop-per, galvanized or enameled steel panels such as are used on all-metal buildings); rubber or plastic mem-branes; bituminous or asphalt without granular surfacing; or slate, concrete, clay tile, composite, or similarshingles without granular surfacing. Other (“non-slippery”) roof surfaces are defined as all surfaces notdefined here as “slippery”.

Sliding snow need not be considered if the lower roof is separated a distance S greater than hr, or 20 ft (6 m),whichever is less.

Fig. 5. Snow loads for lower roof of adjacent structures



2.3.12.4 Roof Projections and Parapets

Projections above lower roofs, such as high bays or higher roofs of the same building, or penthouses andmechanical equipment, can produce windward drifting on the lower roof as depicted in Figure 7. Calculatesuch drift loads on all sides of projections having horizontal dimensions (perpendicular to wind direction)exceeding 15 ft (4.6 m) using the methodology described in this section, even though the surcharge loadingshape may be quadrilateral rather than triangular. To compensate for a probable lower drift height, 75% ofthe drift height (hd) is used, based on a value of Wb taken as the maximum distance upwind from the projectionto the edge of the roof.

Compute drift loads created at the perimeter of the roof by a parapet wall using 75% of the drift height (hd),with Wb equal to the length of the roof upwind of the parapet.

Where the width of the roof projection (Wp) is 10 ft (3.0 m) or greater, consider leeward drift on the low roofadjacent to the roof projection in accordance with Section 2.3.12.1; however, if the length of the projection(perpendicular to Wp) is less than 1⁄3 of hc , leeward drift need not be considered. Leeward drift load issuperimposed on balanced snow load; it need not to be added to the windward drift load.

Fig. 6. Sliding snow load for lower roofs (upper roof snow load not shown)

Fig. 7. Snow load at roof projections



2.4 Rain-on-Snow Surcharge

For locations where the ground snow load (Pg) is 20 psf (0.96 kN/m2) or less, but not equal to zero, use a uni-form rain load surcharge of 5 psf (0.24 kN/m2) in combination with the balanced snow load, depending onthe roof slope (see Table 4). Note that the rain-on-snow surcharge load need not be used in combination withunbalanced, drifting, or sliding snow loads.

Table 4. Rain-on-Snow Surcharge Load

Rain-on-Snow SurchargeW(ft)

Roof Slope Win./ft Rise: Run Degrees % (m)

30 0.125 1⁄8: 12 0.6 1.0 945 0.1875 3/16: 12 0.9 1.6 1460 0.25 1⁄4: 12 1.2 2.1 1890 0.375 3⁄8: 12 1.8 3.1 27120 0.5 1⁄2: 12 2.4 4.2 37150 0.625 5⁄8: 12 3 5.2 46180 0.75 3⁄4: 12 3.6 6.3 55240 1.0 1: 12 4.8 8.4 73300 1.25 1 ¼: 12 6 10.5 91360 1.5 1 1⁄2: 12 7.2 12.6 110

Notes:1. For roof slopes less than those shown in the table, add a uniform design surcharge load of 5 psf (0.24 kN/m2)to the uniform design snowload.2. The 5 psf surcharge load need not be applied where the 50-year ground snow load is greater than 20 psf (0.96 kN/m2).3. The 5 psf surcharge load need not be applied where the 50-year ground snow load is zero.4. The 5 psf surcharge load is applicable to balanced snow load cases only, and need not be used in combination with drift, sliding, or unbal-anced snow load.5. W = the horizontal distance from the roof ridge or valley to the eave.

2.5 Rain Loads

2.5.1 General

Determine design rain loads in accordance with the recommendations in this section; however, ensure thegoverning design roof loads are not less than the minimum live loads or snow loads designated by the appli-cable building code, nor less than the minimum roof live loads and snow loads covered in Sections 2.2, 2.3,and 2.4 of this data sheet. Rain loads cannot be determined until the primary and secondary roof drain-age systems have been designed.

2.5.2 Bases for Design Rain Loads

2.5.2.1 Design rain loads: Design each section of the roof structure to sustain the load from the maximumdepth of water that could accumulate if the primary drainage system is blocked, including the depth of waterABOVE the inlet of the secondary drainage system at its design flow.

Determine this design rain load (load due to the depth of water [total head]) by the relative levels of the roofsurface (design roof line) and overflow relief provisions, such as flow over roof edges or through overflowdrains or scuppers. If the secondary drainage system contains drain lines, ensure they are independent ofany primary drain lines. (See Figures 8a and 8b.)

2.5.2.2 The general expression given below for the design rain load for roof supporting members is the totalhead times the weight of the water. Total head is measured from the design roof line to the maximum waterlevel (overflow discharge), as illustrated in Figures 8a and 8b. The total head includes the depths of waterfrom the design roof line to the overflow provision plus the hydraulic head corresponding to either an over-flow drain or scupper. In addition, have the roof framing designer prepare calculations substantiating thatthe roof design precludes roof instability due to ponding.

Total head = maximum water depth from design roof line to overflow discharge level, including any hydrau-lic head.



English Units:

Design rain load (psf) = total head (in.) × 5.2 ≥ 15 psf for dead-flat roofs, and 30 psf at low-point of slopedroofs.

Metric Units:

Design rain load (kN/m2) = total head (mm) × 0.01 ≥ 0.7 kN/m2 for dead-flat roofs, and 1.4 kN/m2 at low-point of sloped roofs.

2.5.2.3 Minimum design rain loads: Design structural roof support members to support at least a 3 in. (75mm) depth of water on dead-flat roofs, or at least a 6 in. (150 mm) depth of water at the low point of drainsand scuppers on sloped roofs, but not less than the total head. The actual rain load distribution to the struc-tural members will depend on any roof slope and the overflow relief provisions. These minimum rain loadsare included in the above equation.

2.5.2.4 Ponding instability: Design roofs with a slope less than ¼ in./ft (1.2 degrees) to preclude (i.e., rul-ing out in advance) instability from ponding with the primary drainage system blocked. Use the larger of therain or snow loads.

2.5.2.5 Controlled drainage provisions: Provide roofs with controlled flow drains with an overflow drainage sys-tem at a higher elevation that prevents rainwater buildup on the roof above that elevation, except for theresulting hydraulic head (see ‘‘typical roof drains’’ in Fig. 8a). Design such roofs to sustain the load of the maxi-mum possible depth of water to the elevation of the overflow drainage system, plus any load due to the depthof water (hydraulic head) needed to cause flow from the overflow drainage system. Consider roof instabil-ity due to ponding in this situation. Likewise, ensure the overflow drainage system is independent of any pri-mary drain lines.

Fig. 8a. Typical primary and overflow systems for pitched roofs



2.5.3 Designing for Stability Against Ponding

Roof instability due to ponding can be minimized or controlled in the initial roof design by any of the follow-ing methods:

a) Provide sufficient overflow relief protection to remove the water before it reaches an excessive depth.

b) Slope the roof sufficiently to ensure water will flow off the edges of the roof.

c) Provide a sufficiently stiff and strong roof to limit the amount of deflection and to withstand pondingas well as the total load.

d) Specify camber for roof supporting members (e.g., open web joists, structural shapes, and plate gird-ers of steel).

Design standards, such as the American Institute of Steel Construction (AISC) Specifications for StructuralSteel Buildings, require that roof systems be investigated by structural analysis to ensure adequate strengthunder ponding conditions, unless the roof surface is provided with sufficient slope toward points of free drain-age or other means to prevent the accumulation of water. The AISC specifications permit a reduction insafety factor to 1.25 (yield) with respect to bending stress due to ponding plus the total load supported bythe roof (i.e., design rain and dead loads). See additional information in this section stability against pond-ing.

Analyze roof framing systems according to the following recommendations (as applicable), to ensure insta-bility from ponding does not occur based on the total load (dead plus snow and rain loads) supported bythe roof framing before consideration of ponding, or by substantiating that a roof slope is sufficient.

a) Dead-flat roofs: Ensure the total load supported is the design rain load plus the dead load of the roof.An acceptable analysis method for ponding of two-way framing systems is presented in the ASD andLRFD Specifications for Structured Steel Buildings, Commentary, Chapters K2, American Institute of SteelConstruction (AISC).

b) Sloped roofs to drains or scuppers: Ensure the total load supported is the design rain loads distrib-uted locally to the low areas, plus the dead load of the roof. An acceptable analysis method, conserva-tive for sloped roofs, is the AISC method given in Part a above using an appropriate equivalent uniform load

Fig. 8b. Typical primary and overflow drainage systems for flat roofs



based on the design rain load distribution plus dead load for the total load supported. Also, if the designroof slope is less than 1⁄4 in./ft (2%), ensure it is sufficient according to Section 2.5.4.1.13.2.

c) Sloped roofs to free drainage over the roof edge: If the design roof slope is less than 1⁄4 in./ft (2%),ensure it is sufficient according to Section 2.5.4.1.13.2.

2.5.4 Roof Drainage

2.5.4.1 Conventional (Non-Siphonic) Roof Drainage

2.5.4.1.1 Positive Drainage

Design all roofs with positive drainage; however, dead-flat roofs consistent with this data sheet are acceptable.Sloping the roof surface 1⁄4 in./ft (2%) toward roof drains or scuppers or points of free drainage (roof edge)should be sufficient for positive drainage. If a slope of less than 1⁄4 in./ft (2%) is desired for positive drainage,use the analysis methods presented in Section 2.5.7.

2.5.4.1.2 Secondary Drainage

Provide secondary (overflow or emergency) roof drains or scuppers where blockage of the primary drains,if any, allows water to accumulate. This includes when roof gutters or other drains are located behind aparapet.

2.5.4.1.3 Design Rainfall Intensities

Design primary and secondary roof drainage to handle no less than the rainfall intensity (in./hr or mm/hour)based on a duration of 1-hr and frequency (MRI) of 100-years. For locations outside the United States, exceptas noted below, use the greater of the rainfall intensities determined using this data sheet or local codesand rainfall intensity maps.

Rainfall intensity maps are in Appendix C. Linear interpolation is appropriate between rainfall intensity lines.The US maps are identical to those in the 2003 and 2006 International Plumbing Codes.

Note: The rainfall intensities will not necessarily correspond along the common boundary of the Westernand Central United States because the Central and Eastern United States map is newer (1977 vs. 1961).The values expressed in inches are the most intense 60-min duration rainfalls having a 1% probability of beingexceeded in one year. This is commonly designated as the ‘‘100-year, 1-hour rainfall.’’

Calculate rainfall intensity (i) for locations in Canada by multiplying the values tabulated by the AtmosphericEnvironment Service (Ontario, Canada) in Appendix C of the 2005 National Building Code of Canada by afactor of 4. The tabulation presents the probable rainfall intensity in millimeters for a 15-min duration and a10-yr return period or ‘‘10-yr, 15-min rainfall.’’ To convert millimeters to inches, divide by 25. The 4.0 multiplieris a slightly conservative conversion from the 10-yr, 15-min rainfall basis of the Canadian Code to the100-year, 1-hour rainfall of this data sheet.

In areas outside those covered by the maps and tabulation, or in local areas of intense rainfall history, obtainthe rainfall intensities from local meteorological stations based on a 1-hr duration rainfall and a 100-yr MRI.Reasonable, but not exact, multiplication factors for converting a 1-hr duration rainfall of 30-yr and 50-yrMRI to a 100-yr MRI are 1.2 and 1.07, respectively.

2.5.4.1.4 Design Drainage Area

Use the roof area along with one-half (1⁄2) the area of any vertical walls that drain to the roof area in sizingdrains and determining roof loads and stability from ponding.

2.5.4.1.5 Roof Loads

The roof primary and secondary drainage systems must be designed before the roof loads can be determined.



2.5.4.1.6 Existing roofs

Provide existing roofs (especially lightweight roof constructions) that have severely inadequate primarydrainage and no overflow relief protection with additional drainage provisions. Determine the need for overflowdrainage in conjunction with an evaluation of existing conditions.

2.5.4.1.7 Roof Drains and Scuppers

Roof drains may be used for conventional, or controlled-flow drainage systems. Roof drains and scuppersmay be used separately or in combination for primary or overflow drainage systems. The following sections,when referring to drains, apply to conventional or controlled-flow drainage systems.

2.5.4.1.7.1 Quantity

Provide at least two roof drains or scuppers for total roof areas of 10,000 ft2 (930 m2) or less. For largerroof areas, provide a minimum of one drain or scupper for each 10,000 ft2 (930 m2) of roof area. The roofarea may be increased to 15,000 ft2 (1400 m2) with a minimum drain diameter of 6 in. (150 mm) or scupperwidth of 8 in. (200 mm).

2.5.4.1.7.2 Drain Sizes

Provide roof drains and vertical leaders in sizes of 4 to 8 in. (100 to 200 mm) diameter inclusive, exceptfor areas less than 2500 ft2 (230 m2), such as canopies, where 3 in. (75 mm) diameter drains may be used.It is usually impractical to use larger than 8 in. (200 mm) diameter drains because of drainage area limitationsand drain flow restrictions imposed by drainage piping and/or water buildup loads.

2.5.4.1.7.3 Drain Strainers

Provide strainers extending a minimum of 4 in. (102mm) above the roof surface over all roof drains. Usestrainers with an available inlet area not less than one and one-half times the area of the conductor or leaderconnected to the drain. Flat-surface strainers with an inlet area not less than two times the area of theconductor can be used on flat decks, including parking decks and sun decks.

2.5.4.1.7.4 Placement

The placement of (primary) roof drains or scuppers are influenced by the roof structure’s support columnsand walls, expansion joints, roof equipment, and other projections. When possible, locate roof drains atmid-bay low points, or within 20% of the corresponding bay spacing from the low points in each direction.If roof drains or scuppers are located at points of little deflection, such as columns and walls, slope the roofsurface toward them at least 1⁄8 in./ft (1%) to compensate for minimum deflections at these locations. Ingeneral, do not locate interior (non-perimeter) drains more than 50 ft (15 m) from the roof perimeter, nor morethan 100 ft (30 m) apart. Exception: Distances of 75 ft (23 m) from the perimeter and 150 ft (46 m) apart,may be used with a minimum drain diameter of 6 in. (150 mm). Place primary scuppers level with the roofsurface in a wall or parapet as determined by the roof slope and the contributing area of the roof, but notlocated more than 50 ft (15 m) from a roof juncture, nor more than 100 ft (30 m) apart along the roof perimeter,except 60 ft (18 m) and 125 ft (38 m), respectively, may be used with a minimum scupper width of 8 in.(200 mm). Careful consideration of the above during the design phase is essential to provide adequate anduniform drainage of each roof section.

2.5.4.1.7.5 Secondary Drainage

Provide secondary drainage for both dead-flat and sloped roofs to prevent any possibility of water overload.The overflow relief provision establishes the maximum possible water level based on blockage of the primarydrainage system. Ensure the provision is in the form of minimal height roof edges, slots in roof edges,overflow scuppers in parapets or overflow drains adjacent to primary drains (see Figures 8a and 8b). Ensurethe overflow relief protection provides positive and uniform drainage relief for each roof section, with drainageareas preferably not exceeding those of the primary drainage or the drainage area limits in Section2.5.4.1.7.1. Do not consider flow through the primary drainage system when sizing overflow relief drains andscuppers.



Ensure the inlet elevation of overflow drains and the invert elevation (see sketches in Table 6) of overflowscuppers are not less than 2 in. (50 mm) nor more than 4 in. (100 mm) above the low point of the (adjacent)roof surface unless a safer water depth loading, including the required hydraulic head to maintain flow, hasbeen determined by the roof-framing designer.

2.5.4.1.7.5.1 Secondary drainage discharge: Discharge roof overflow drain or scupper drainage systemsusing vertical leaders, conductors, or piping separate from the primary drainage system and to an above-grade location normally visible to building occupants. Discharge to points of free drainage, such asover-the-roof edges or through relief openings atop conductors, if this isn’t practical.

2.5.4.1.8 Scuppers and Gutters

Use three-sided channel-type roof scuppers whenever possible. For parapet walls, use the four-sidedperimeter, closed-type scuppers (see sketch with Table 6). Provide scuppers and leaders or conductors withminimum dimensions of 6 in. (150 mm) wide by 4 in. (100 mm) high and 5 in. (125 mm) diameter or equivalent,respectively. Ensure the height of the closed-type scupper is at least 1 in. (25 mm) higher than the estimatedwater buildup height (hydraulic head) developed behind the scupper (see Table 6).

Provide a watertight seal between gutters and the underside of the roof to ensure that rainwater will not enterthe building, nor breach the building’s weather tight envelope, due to wind-driven rain or gutter overflow.

2.5.4.1.9 Downspouts

Provide downspouts that are protected or truncated above the highest expected level of snow banks andpotential impacting objects (truck docks, etc.) or are of open-channel design.

2.5.4.1.10 Inspection

Inspect roofs and their drainage inlets after roof construction, prior to the start of the rainy or tropical cycloneseasons, and following storms, or at least every three months. Clear obstructions or accumulations of foreignmatter as frequently as necessary.

Inspects gutters to ensure that they are properly sealed at the underside of roofing to prevent rainwater fromentering the building.

2.5.4.1.11 Drainage System Sizing

Determine the rainfall intensity for a given location using Section 2.5.4.1.3 and Appendix C, then calculatethe number and sizes of roof drains and/or scuppers for the primary drainage system, as well as the sizes ofvertical leaders or conductors and horizontal drainage piping as follows:

Secondary drain sizing: Where provided, size secondary drains at least equivalent to the maximum capacityof the primary roof drains or scuppers as stated in Tables 5 and 6. For example, if the primary drainage systemconsists of six 6 in. (150 mm) drains each flowing 540 gpm (2044 L/min) and scuppers are used for thesecondary drainage system, then the maximum drainage capacity of all scuppers should be equivalent tothe 3,240 gpm (12,260 L/min) maximum drainage capacity of the primary roof drains.

Total scupper capacity = 6 x the 540 gpm (2044 L/min) maximum capacity of each 6 in. (150 mm) drain perTable 5 = 3,240 gpm (12,260 L/min)

1. Sizing Conventional Roof Drains/Vertical Leaders and Scuppers

a. Determine the total number of roof drains or scuppers needed:

Equation 1.1 English Units

(for 6 in. dia. drains and8 in. wide scuppers per Section 2.5.4.1.7.1)

n = A10,000

; or n = A15,000

Where n = number of drains needed (nearest higher whole no. ≥ 2)A = total roof drainage area (ft2)



Equation 1.2 Metric Units

(for 150 mm dia. drainsand 200 mm wide scuppers per Section 2.5.4.1.7.1)

n = A930

; or n = A1400

Where n = Number of drains needed (nearest higher whole no. ≥ 2)A = Total roof drainage area (m2)

b. Determine the flow rate needed per roof drain, leader, or scupper:

Equation 2.1 English Units

Q = 0.0104 × i × A (See Note below)n

Where Q = drain, leader or scupper flow needed (gpm)i = rainfall intensity (in./hr), Section 2.5.4.1.3A = total roof drainage area (ft2)n = number of drains needed (Equation 1.1)

Equation 2.2 Metric Units

Q = 0.0167 × i × A (See Note below)n

Where Q = drain, leader or scupper flow needed (dm3/min)i = rainfall intensity (mm/hr), Section 2.5.4.1.3A = total roof drainage area (m2)n = number of drains needed (Equation 1.2)

Note: The above coefficients (0.0104 or 0.0167) times ‘‘i’’ convert the rainfall intensity to an (average)flow rate per unit area (see Table 7); however, these coefficients may vary for controlled drainagesystems (see Sizing Controlled Roof Drain/Vertical Leaders below).

c. Determine the size needed for roof drains, leaders, or scuppers:

Drains and vertical leaders

Apply the flow, Q, needed per drain or vertical leader to Table 5 and select a drain or vertical leader diameterthat provides adequate flow capacity.

Scuppers

Apply the flow, Q, needed per scupper to Table 6 and select a scupper type and size that provides adequateflow capacity.

2. Sizing Controlled Roof Drains/Vertical Leaders

a) Use the methodology in this section for controlled drainage systems by converting the rainfall intensityto the design peak flow rate rather than to the (average) flow rate.

b) The design peak flow rate is usually approximated at twice the average flow rate for a controlled drainage system.



Table 5. Flow Capacity for Roof Drains and Pipinga

English UnitsDiameter of Drain or

Pipe(in.)

Roof Drains andVertical Leaders

(gpm)

Horizontal Drainage Piping, gpm Slopes (in. per ft)

1⁄8 Slope 1⁄4 Slope 1⁄2 Slope

3 90 34 48 694 180 78 110 1575 360 139 197 2786 540 223 315 4468 1170b 479 679 95810 — 863 1217 172512 — 1388 1958 277515 — 2479 3500 4958

Metric UnitsDiameter of Drain or

Pipe(mm)

Roof Drains andVertical Leaders

(L/min)

Horizontal Drainage Piping, L/min Slopes (percentages)

1 Slope 2 Slope 4 Slope

75 340 130 180 260100 680 295 415 595125 1360 525 745 1050150 2040 845 1190 1690200 4420b 1815 2570 3625255 — 3265 4605 6530305 — 5255 7410 10,500380 — 9385 13,245 18,770

a To ensure these flow capacities are achieved, roof drains must be placed at mid-bay or the roof surfaces must be sloped toward the roofdrains).

b Design flow of this capacity is impractical; water must build up approximately 4.5 in. (113 mm) to achieve this flow.



Table 6. Hydraulic Head Versus Flow Capacity for Roof Scuppers(Depth of water over invert versus flow of water through scupper)

English UnitsScupper Flows (gpm)

WaterBuildup

H, in.

Channel Type Closed Typeh ≥ H Width b, in. Height h = 4 in. Height h = 6 in.

Width b, in.6 8 12 24 6 8 12 24 6 8 12 24

1 18 24 36 72 (see channel type) (see channel type)2 50 66 100 2003 90 120 180 3604 140 186 280 5605 194 258 388 776 177 236 354 7086 255 340 510 1020 206 274 412 8247 321 428 642 1284 231 308 462 924 303 404 606 12128 393 522 786 1572 253 338 506 1012 343 456 686 1372

Metric UnitsScupper Flows L/min

WaterBuildupH, mm

Channel Type Closed Typeh ≥ H Width b, mm Height h = 100 mm Height h = 150 mm

Width b, mm150 200 300 500 150 200 300 500 150 200 300 500

25 63 84 126 210 (see channel type) (see channel type)50 178 237 356 59575 327 437 656 1093

100 505 673 1009 1682125 705 940 1411 2351 642 856 1284 2141150 927 1236 1854 3090 749 998 1497 2495175 1168 1558 2337 3894 841 1121 1681 2802 1105 1474 2211 3684200 1427 1903 2855 4758 923 1230 1846 3076 1249 1665 2498 4163

Notes: Whenever h ≥ H for a closed-type scupper, the scupper flows under channel-type scuppers are appropriate.Interpolation is appropriate.



Table 7. Conversion of Rainfall Intensity to Flow Rate and Rain Load per Unit Area

English UnitsRainfall Intensity (in./hr) Flow Rate (gpm/ft2) Rain Load/hr (psf)

1.0 .0104 5.21.5 .0156 7.82.0 .0208 10.42.5 .0260 13.03.0 .0312 15.63.5 .0364 18.24.0 .0416 20.84.5 .0468 23.45.0 .0520 26.05.5 .0572 28.66.0 .0624 31.27.0 .0728 36.48.0 .0832 41.69.0 .0936 46.8

10.0 .1040 52.0Metric Units

Rainfall Intensity (mm/hr) Flow Rated (L/min per m2) Rain Load/hr(kilonewtons [kN] per m2)

25 0.42 .2530 0.5 .2935 0.58 .3440 0.67 .3945 0.75 .4450 0.83 .4955 0.92 .5460 1.0 .5970 1.2 .6980 1.3 .7990 1.5 .88100 1.7 .98200 3.3 1.96300 5.0 2.94

Note: Interpolation is appropriate.

Table 8. Hydraulic Head versus Roof Drain Flow

Hydraulic Head(Approx.Water Depth Over Inlet)

Drain Diameter4 in. (100 mm) 6 in. (150 mm) 8 in. (200 mm)

Approximate Flow in gpm (L/min)1.0 in. (25 mm) 80 (300) 100 (380) 125 (470)1.5 in. (38 mm) 120 (450) 140 (530) 170 (640)2.0 in. (50 mm) 170 (640) 190 (720) 230 (870)2.5 in. (63 mm) 180 (680) 270 (1020) 340 (1290)3.0 in. (75 mm) 380 (1440) 560 (2120)3.5 in. (88 mm) 540 (2040) 850 (3220)4.0 in. (100 mm) 1100 (4160)4.5 in. (113 mm) 1170 (4430)

Note: Interpolation is appropriate.



c) The peak flow rate is the limited (controlled) flow rate required to maintain a predetermined depth ofwater on a roof and drain the roof within a 24-hour or 48-hour period. It varies according to the controlleddrainage design criterion, rainfall intensity, and roof slope configuration.

3. Sizing Horizontal Drainage Piping

a) Determine the flow, Qp, needed per horizontal drainage pipe section:

Qp = Q times the number of drains serviced by the pipe section.

b) Determine the size of horizontal drainage piping needed:

Apply the flow, Qp, needed per pipe section to Table 5 and select the pipe diameter and slope that provideadequate flow capacity.

2.5.4.1.12 Rain Loads with Drains and/or Scuppers

2.5.4.1.12.1 Hydraulic head: Determine the water depth needed to cause flow out of overflow drainagesystems as follows:

a) Roof edges: Ignore the negligible hydraulic head needed to cause flow across a roof and over its edges.

b) Overflow roof drains: Use Table 8 with the needed flow rate Q (Section 2.5.4.1.11), under anappropriate drain diameter and determine the approximate depth of water over the drain’s inlet (byinterpolation when necessary).

c) Overflow roof scuppers: Use Table 6 with the needed flow rate, Q (Section 2.5.4.1.11) under anappropriate scupper type and size, and determine the approximate depth of water over the scupper’s invert(by interpolation when necessary).

2.5.4.1.13 Roof Slope

2.5.4.1.13.1 Roofs with interior drains: To ensure the points of maximum sag are no lower than the roofsurface between these points and the drains of roofs with interior drainage provide a positive drainage slopeof at least 1⁄4 in./ft (2%). In Figure 9 this is illustrated in the sloped roof detail where ponding occurs locallyat the origin, whereas in the flat roof detail ponding occurs in every bay.

If a slope less than 1⁄4 in./ft (2%) is desired, use deflection analysis to determine the needed slope. If watermust flow across one bay into another, relatively complicated two-way deflection analysis is involved. Therecommendations in Section 2.5.4.1.13.2 for roof slope with edge drainage are appropriate. Have the roofframing designer prepare calculations according to these recommendations, or other appropriate method, tosubstantiate that the design slope is sufficient to prevent roof instability due to ponding.



2.5.4.1.13.2 Roofs with edge drainage: If interior drains are not provided and drainage is accomplished bycausing the water to flow off the perimeter of the roof, sufficient roof slope is vital; at least 1⁄4 in./ft (2%). Underthese circumstances, sufficient slope is needed to overcome the deflections caused by the dead load of theroof plus the weight of the 1-hour design storm less the effect of any specified camber. This is achieved whenthe actual downward pitch of the roof surface exceeds the upward slope for all deflected roof framing at ornear their downward support column (or wall) (see Fig. 10).

Fig. 9. Flat and sloped roofs with interior roof drains

Fig. 10. Sloped roof with roof edge drainage



If a design roof slope (Sd) less than 1⁄4 in./ft (2%) is desired, have the roof framing designer prepare calcu-lations according to the following recommendations or other appropriate method, to substantiate that thedesign slope is sufficient to prevent roof instability due to ponding:

a) Ensure the actual slope (Sa) under the dead load of the roof less the upward camber, when speci-fied is at least 1⁄8 in./ft (1%).

b) Ensure the actual slope (Sa), from the perimeter of the roof, under the dead load of the roof plus 1-hourof rain load (see Table 7), less the upward camber, when specified is greater than zero (i.e., upward posi-tive slope, not flat).

c) Ensure all primary and secondary members perpendicular to the roof edge, for the entire roof slope,have actual slopes (Sa), calculated by the roof designer, meeting the slope criteria of (a) and (b) as fol-lows:

English Units:

Sa (%) = Sd (%) + 240 × (Camber) − (D.L.) L ≥ 1%L 1.44 × 24 × E × I

3

Sa (%) = Sd (%) + 240 × (Camber) − (D.L. + 5.2 × i) L ≥ 0%L 1.44 × 24 × E × I

3

Where: Sa and Sd = the actual and design roof slopes in percent, respectively.D.L. = the roof’s dead load in psfCamber = upward camber in inches when it is specified (not optional) by fabrication specifications(see Part e).I = rainfall intensity in in./hrL = span length of member in inchesE = modulus of elasticity of members material, psiI = effective moment of inertia of member, (in.)4 per inch of (tributary loaded) roof width

To convert roof slope (percent) to in./ft multiply percent by 0.12

Metric Units:

Sa (%) = Sd (%) + 0.24 × (Camber) − (D.L.) L ≥ 1%L 24 × E × I

3

Sa (%) = Sd (%) + 0.24 × (Camber) − (D.L. + 0.01 × i) L > 0%L 24 × E × I

3

Where: Sa and Sd = the actual and design roof slopes in percent, respectively.D.L. = Roof’s dead load in kN/m2

Camber = upward camber in mm when it is specified not optional by fabrication specifications (seePart e).I = rainfall intensity, in mm/hrL = span length of member in metersE = modulus of elasticity of members material, in kN/m2

I = effective moment of inertia of member, in (m)4 per meter of (tributary loaded) roof width

d) If secondary members are parallel to relatively stiff perimeter walls (e.g., masonry or metal panel walls),increase the actual roof slope to compensate for maximum deflection (adjusted for any specified cam-ber) of the secondary member closest to the wall. Adjust the actual slope computed in the equations ofPart c above by a decrease as follows:

Sa Decrease (%) = – (Max. Deflection of secondary member) 100(Distance secondary member from wall)

Where: deflection and distance are in the same units (e.g., in. or mm)



e) The following are cambers specified in the Standard Specifications of the Steel Joist Institute (SJI)for LH-Series (Longspan) and DLH-Series (Deep Longspan) Joists and Joist Girders:

Top Chord Length ft (m) Approximate Camber in. (mm)20 (6) 1⁄4 (6)30 (9) 3⁄8 (10)

40 (12) 5⁄8 (16)50 (15) 1 (25)60 (18) 11⁄2 (38)

>60 (>18) See SJI Specifications

Do not assume the above cambers for K-Series (Open Web) Joists because they are optional with themanufacturer.

2.5.4.2 Siphonic Roof Drainage

2.5.4.2.1 Restrictions

2.5.4.2.1.1 For roofs with internal drains distributed throughout the roof , do not use siphonic roof drainagein hurricane-prone, tropical cyclone-prone, or typhoon-prone regions as defined in FM Global Data Sheet1-28. This recommendation does not apply to roofs with siphonic drains located only in eave (perimeter)gutters or valley gutters.

2.5.4.2.1.2 Do not use siphonic roof drainage for roofs that will be prone to debris accumulation - such asroofs with nearby or overhanging vegetation where leaves, pine needles, or other vegetation is prone tosubstantially restrict roof drains flows or clog the siphonic piping system. Keep vegetation at least 50 ft (15 m)offset horizontally from the roof perimeter and no higher than the elevation of the lowest roof parapet. Ensurethat a program is in place to control vegetation.

2.5.4.2.1.3 Do not use siphonic roof drainage for gravel covered or stone ballasted roof, or for vegetated(green) roofs.

2.5.4.2.2 Design Rainfall Intensity, Duration, and Frequency

Rainfall intensity (i) is the rate that rainfall accumulates over time, is frequently expressed in inches ormillimeters per hour (in/hr or mm/hr), and is a function of both duration (minutes or hours) and frequency(or return period, in years) for a given location and climate.

For example, if the 100-year 2-minute rainfall is 10-inches (254 mm) per hour, then:

Intensity (i) = 10 in./hr (254 mm/hr)Duration (D) = 2 minutesFrequency (F) = 100 years

2.5.4.2.2.1 Determine the flow rate (Q [gpm or liter/min]) needed per roof drain, leader or scupper in thesame manner as for gravity roof drainage, but with any adjustments as noted to the rainfall intensity (i).

2.5.4.2.3 Acceptable Drainage Designs and Design Assumptions

2.5.4.2.3.1 Acceptable Design Options

Note that the recommendations in Section 2.5.4.2.3.2, General Design Assumptions and Requirements, applyto all acceptable design options.

Option 1:

• Primary siphonic drainage designed for the 2-year 2-min rainfall intensity.

• Secondary conventional (non-siphonic) drainage designed for the 100-year 15-min rainfall intensity,with primary drainage completely blocked.

Option 2:

• Primary siphonic drainage designed for the 100-year 2-min rainfall intensity.



• Secondary siphonic drainage designed for the 100-year 2-min rainfall intensity, with primary drainagecompletely blocked.

• Roof structure designed to adequately support the 100-year 24-hour rainfall depth, unless depthreductions are appropriate where rainwater can freely overflow the roof area by gravity alone (e.g.,at roof perimeter without parapets). This rain load may be considered an extreme design load, andtherefore the use of a lower than normal rain load factor (partial safety factor), or net safety factor,is appropriate, provided that the resultant safety factor against structural material yielding orfracture/crushing (whichever occurs at the lower load) is not less than 1.25 when considering totalload. Include ponding instability analysis under these conditions.

2.5.4.2.3.2 General Design Assumptions and Requirements

2.5.4.2.3.2.1 The design life of the drainage systems should not be less than the design life of the building,nor less than 50 years.

2.5.4.2.3.2.2 Primary and secondary drainage must be completely independent systems for all acceptableroof drainage options.

2.5.4.2.3.2.3 For secondary gravity drainage and scupper details (minimum sizes, inlet elevations) – followthe recommendations in Section 2.5.4.1 (the conventional drainage section) of DS 1-54.

2.5.4.2.3.2.4 The siphonic drainage system must be designed to operate properly at all flow rates and rainfallintensities, up the maximum design flow rate and rainfall intensity. Ensure that the water depths on the roofor in the roof gutters will not exceed depths occurring at the maximum design flow rate and rainfall intensity.

2.5.4.2.3.2.5 All secondary siphonic drainage systems should be designed to operate properly at the designrainfall intensity based on the following assumptions:

a) All secondary siphonic roof drains are operating as designed (no clogging or blinding).b) At least one secondary siphonic drain per roof, but not less than 5% of the total secondary drains on

a roof, are completely clogged or blinded — with the blocked or blinded secondary drains arranged toplace the most demand on the roof drainage and roof structure.

2.5.4.2.3.2.6 Do not credit any temporary storage of water on roofs or in gutters for the siphonic drainagedesign.

2.5.4.2.3.3 Roof Load

Arrange the secondary drain high enough above the primary drain so that water will reach a sufficient depthto ensure the primary drainage system operates properly but not so high that water reaches a depth thatwill overload the roof structure.

2.5.4.2.4 Roof Slope, Positive Drainage, and Stability against Ponding

Follow the recommendations for gravity drainage in Section 2.5.4.1 except as noted in Section 2.5.4.2.1(Restrictions in hurricane-prone locations).

2.5.4.2.5 Roof Drains

2.5.4.2.5.1 Quantity (minimum number of drains per roof area): Follow the recommendations for gravitydrainage in Section 2.5.4.1.

2.5.4.2.5.2 Drain Strainers (Debris Guards): Provide domed drain strainers extending at least 4-inches (100mm) above the roof surface for all siphonic roof drains, including those placed in roof gutters. Ensure thatthe open area of the strainer is at least three-times (3x) the cross-sectional area of the drain outlet or tailpipe,whichever is larger. Ensure that the hydraulic performance properties for the roof drain account for thepresence of the drain strainers.

2.5.4.2.5.3 Drain Baffle (Anti-Vortex Plate): All siphonic drains must have a baffle to prevent air entrainmentinto the siphonic system and allow for full-bore siphonic flow. Ensure that the baffle is clearly and permanentlymarked with a warning not to remove the baffle.

2.5.4.2.5.4 Sump Bowl or Drainage Basin: Roof drains on flat and low sloped roof (2% slope or less) shouldhave a sump bowl or drainage basin to allow for siphonic flow while minimizing water depth on the roof.



2.5.4.2.5.5 Provide roof drains with the manufacturer name and model number, drain outlet size (in2 [mm2]),and hydraulic resistance coefficient (e.g., “K” value), clearly and permanently marked on the drain body whereit will be legible in its installed condition to an observer on the roof.

2.5.4.2.6 Design Validation

2.5.4.2.6.1 Siphonic design and analysis must be performed by a plumbing engineer licensed to practice inthe project location. The design calculations, including computerized calculations and results, must be signedand stamped by the licensed plumbing engineer.

2.5.4.2.6.2 The hydraulic properties and performance of manufactured roof drains used in the siphonic systemmust be based on physical test results from a testing program established in a nationally-recognized standard(such as ASME A112.6.9) and tested by a laboratory that has been verified to be qualified to perform thetesting. Using roof drains with hydraulic performance based on calculation alone — or based on calculatedhydraulic performance taken from test results of a different, albeit similar, roof drains — is not acceptable.

2.5.4.2.7 Disposable (Available) Head

2.5.4.2.7.1 Use the Design Disposable Head (HD), also known as the Design Available Head, as the verticaldistance in ft (m) from the inlet (rim) of the roof drain to highest elevation (i.e., least vertical distance) of:

a) Grade elevation at discharge inspection chamber(s) or manhole(s)b) Flood elevationc) Elevation of siphonic break (for discharge above grade)

Refer to Figure 12, Elevation View of Siphonic System and Disposable (Available) Head

2.5.4.2.7.2 Use the Theoretical Disposable Head (HT) as the vertical distance in ft (m) from the water leveldirectly upstream of the roof drain to the centerline of siphonic discharge pipe at or below grade.

2.5.4.2.7.3 Ensure that either HT or HD, whichever provides for the more demanding condition, has beenused when determining the properties or performance of the siphonic drainage system. For example, whendetermining if the siphonic drainage system has adequate capacity to drain the roof based on the designrainfall intensity (i), or to determine the maximum depth of water build-up on the roof based on the designrainfall intensity, use HD. However, when determining the minimum pressure or maximum velocity to compareto allowable values, use HT.

2.5.4.2.7.4 Ensure that HD is at least 10% greater than the sum of the residual velocity head (at the lastsection of siphonic pipe just before the point of discharge) and the head losses.

2.5.4.2.7.5 Ensure that the calculated head losses account for pipe roughness values for both new and agedconditions, and that those roughness values which results in the most demanding condition are used.

2.5.4.2.7.6 Ensure that the designer’s calculated imbalance in head at the design flow rate between anytwo roof drains with a common downpipe (stack) and the point of discharge is not greater than 1.5 ft (0.46 m),or 10% of HD, whichever is less.

2.5.4.2.8 Minimum and Maximum Pressure

2.5.4.2.8.1 Operating Pressure

2.5.4.2.8.1.1 The operating pressure (gage) should not exceed 13 psig (90 kPa), or 30 ft (9.2 m) of watercolumn head pressure.

2.5.4.2.8.1.2 The operating pressure (gage) should be no less than:

(3.5 psi [24.2 kPa]) – (local atmospheric pressure [Patm] accounting for site elevation)

For example:

a) At sea level Patm = 14.7 psi (101.6 kPa), therefore the minimum operating gage pressure is:3.5 psi (24.2 kPa) – 14.7 psi (101.6 kPa) = -11.2 psig (-77.4 kPa)

b) At 3000 ft (915 m) above sea level, Patm = 13.2 psi (91.1 kPa), therefore the minimum operational gagepressure is:

(3.5 psi [24.2 kPa]) – 13.2 psi (91.1 kPa) = -9.7 psig [-67.0 kPa]



2.5.4.2.8.1.3 Minimum operating pressures are intended to prevent cavitation, air infiltration at pipe fittingsand joints, and pipe overload (buckling or collapsing).

2.5.4.2.8.1.4 See the recommendation for maximum velocity which relates to the minimum operationalpressure limits above.

2.5.4.2.8.2 Rated Pressure for Air Infiltration

Pipe joints and fitting shall be rated to prevent air infiltration when subjected to negative (gage) pressure of-12.3 psig (-85.0 kPa) for 1-hour.

2.5.4.2.9 Minimum and Maximum Velocity

2.5.4.2.9.1 At the design flow rate, ensure that the velocity in tailpipes and horizontal collector pipes is atleast 3.3 ft/sec (1 m/sec).

2.5.4.2.9.2 At the design flow rate, ensure that velocity in downpipes 6-inch (150 mm) or smaller is at least7 ft/sec (2 m/sec).

2.5.4.2.9.3 The maximum velocity in the siphonic system should be based on maintaining allowable minimumpressures (maximum negative gage pressures) in the system. Generally, the velocity at design flow shouldnot exceed 20 ft/sec (6 m / s) for the given minimum operational pressure.

2.5.4.2.9.4 Ensure that the velocity at the discharge pipe is limited to no more than approximately 10 ft/sec(3 m/sec). This can be accomplished by increasing the pipe diameter to break the siphonic flow. The increasein pipe diameter should be at a point at least 10 pipe (siphonic) diameters from the discharge point.

2.5.4.2.10 Priming

The siphonic drainage system should be designed to prime (i.e., to begin full-bore siphonic flow) at not morethan 1⁄2 the duration associated with the design rainfall intensity. A reasonable estimation can be made bydetermining the time required to fill the siphonic system based on the following equation:

Tf = 1.2(Vp)(qt) ≤ 60 seconds

Where:Tf = time to fill the system (seconds)qt = the flow capacity (cfs or liter/sec) of all the contributing tailpipes when assumed to be acting siphonically,

but also independently, and discharging to atmospheric pressure at the collector pipe.Vp = the volume (cubic feet or liters) of the downpipe (to the point of theoretical siphonic discharge) and

the contributing collector pipes.

2.5.4.2.11 Siphonic Discharge

2.5.4.2.11.1 Discharge the siphonic drainage system to the open atmosphere – either to a below-gradeinspection chamber (manhole), or to an above grade trench or swale – to break the siphonic action.

2.5.4.2.11.2 Below-Grade Inspection Chamber (Manhole): Provide a vented cover for the chamber (manhole)that is at least 50% open area, or where the total open area of the vented cover is not less than three times(3x) the cross-sectional area of the siphonic discharge pipe, whichever is greater.

2.5.4.2.11.3 Keep the manhole cover clear or snow, ice and debris. Provide bollards or similar protectivedevices to keep materials, vehicles, etc from blocking the manhole cover.

2.5.4.2.11.4 Avoid the use of vermin guards on discharge pipes since they could collect debris and blockproper siphonic flow. The preferred alternative is to conduct frequent visual inspections to ensure thatdischarge pipes remain free of debris. See the Inspection and Maintenance section for additional details.

2.5.4.2.12 Pipe Strength, Details and Materials

2.5.4.2.12.1 Piping and Fittings – General

2.5.4.2.12.1.1 Use metal pipe — such as cast iron, galvanized steel, stainless steel, or copper — rather thanplastic pipe for better long-term durability and performance. Ensure that pipe and associated joints, fittings,couplings, etc meet or exceed nationally recognized plumbing materials standards such as those by ASTM,CSA, BS, or DIN.



2.5.4.2.12.1.2 If plastic pipe cannot be avoided, then use Schedule 40 pipe or better (or an SI equivalent,based on the minimum ratio of pipe wall thickness to mean pipe diameter – See Table 10) for all pipingcomponents of the siphonic drain system. Ensure that plastic pipe (such as ABS, HDPE, or PVC) meets orexceeds applicable, nationally recognized plumbing materials standards such as those by ASTM, CSA, BS,or DIN.

2.5.4.2.12.1.3 If plastic pipe is used, then take care to address the issues associated with thermal expansion,expansion joints, and pipe supports and restraints – and provide the necessary detailing to prevent damage.

2.5.4.2.12.2 Expansion Joints

Avoid the use of expansion joints in siphonic systems wherever possible since proper connection detailingand adequate long-term performance can be difficult to achieve. If the use of expansion joints cannot beavoided, then ensure that all of the following are met:

a) Thermal expansion and contraction are based on temperature extremes associated with the localclimate, building type, and location of building expansion joints; and

b) Expansion joints are rated, with a minimum safety factor not less than 3.0, for both the maximum andminimum siphonic piping operating pressures, and with a critical buckling strength no less than thatrequired of the adjacent siphonic piping; and

c) The expansion joint and connections have smooth inner bores to prevent the accumulation of debris/sediment and to avoid cavitation.

2.5.4.2.12.3 Tailpipe

To initiate adequately rapid priming, maintain full-bore siphonic flow, and reduce the likelihood of cavitation,ensure that:

a) The diameter of the tailpipe is not greater than the diameter of the roof drain outlet.

b) Pipe increasers are used only in the horizontal portion of the tailpipe, not in the vertical portion; andthat only eccentric (not concentric) increasers are used with the crown (top) of the pipes set flush and themaximum offset at the pipe invert.

c) 90-degree bends are used where transitioning from the vertical to the horizontal portion of the tailpipe(45-degree bends, or substantial slopes in the horizontal portion of the tailpipe, are not acceptable).

2.5.4.2.12.4 Horizontal Collector Pipe

Ensure that all reducers or increasers are eccentric (not concentric) with the crown (top) of the pipe set flushand the offset at the pipe invert.

2.5.4.2.12.5 Downpipe (Stack)

2.5.4.2.12.5.1 Ensure that the downpipe diameter is no greater than the diameter of the horizontal collectorpipe.

2.5.4.2.12.5.2 At the top of the downstack, where the collector pipe connection is made, use either two (2)45-degree bends, or a 90-degree bend with a minimum centerline bend radius equal to the pipe diameter.

2.5.4.2.12.5.3 If a reducer is used just after an elbow, use an eccentric reducer with the pipes set flush at theoutside radius of the elbow.

2.5.4.2.12.6 Minimum Pipe Size

Use pipe with an inside diameter of at least 1.6 in. (40 mm).

2.5.4.2.12.7 Critical Buckling Strength of Pipe (Pcrit)

All pipe sections used in siphonic systems must withstand a resultant (net) critical buckling pressure of atleast three atmospheres based on all of the following conditions:

a) Standard atmospheric pressure at sea level

b) An assumed minimum out-of-roundness (maximum diameter – minimum diameter), or ovality, of one-halfthe pipe wall thickness



c) Buckling strength based on the creep modulus of elasticity (Ec)

d) Assumed operating temperatures from 40°F (4°C) to 90°F (32°C)*

That is, ensure that Pcrit ≥ 3 atm (44.1 psi [304.8 kPa]) when considering the conditions listed above.

*For piping that will be heat traced, ensure that the pipe temperature will be within these assumed operatingtemperatures.

2.5.4.2.12.8 Pipe Supports and Bracing

2.5.4.2.12.8.1 Provide pipe supports and bracing as needed based on engineering analysis when accountingfor all applicable conditions – including, but not limited to: Gravity loads, deflections, and material creep;siphonic pipe pressures; operational vibrations and fatigue; thermal expansion/contraction; and seismicbracing when located in an active earthquake zone based on FM Global Data Sheet 1-2, Earthquakes (50-,100-, 250-, and 500-year zones).

2.5.4.2.12.8.2 For plastic pipe, provide pipe supports as described in the previous paragraph, but also ensurethat the pipe supports and bracing conform to the following minimum requirements:

a) Provide pipe supports every 4 ft (1.2 m) or lessb) Provide pipe supports at every change in direction (e.g., at pipe elbows)c) Provide lateral bracing at every 30 ft (9.1 m) or lessd) Provide lateral bracing at every change in pipe direction

2.5.4.2.13 Icing, Freeze-up, and Impact and Environmental Damage

2.5.4.2.13.1 Keep roof drains free of ice and snow.

2.5.4.2.13.2 Use heat tracing on siphonic drain bodies and outlets exposed to freezing temperatures.

2.5.4.2.13.3 Where siphonic drainage piping is used in an unheated building or is installed at the exteriorof a building (e.g., downpipe attached to the building façade), and is exposed to freezing temperature, installheat tracing at all the exposed piping.

2.5.4.2.13.4 Use only noncombustible metallic materials for drain components that are to be heat traced.

2.5.4.2.13.5 Position above-grade secondary discharge pipes above the maximum expected snow level(including drift) and take special precaution to protect them from crushing or impact (such as protectivebollards) when exposed to car parks or storage areas.

2.5.4.2.13.6 Protect exposed siphonic drainage piping from ultra-violet radiation and other environmentalsources of degradation.

2.5.4.2.14 Testing and Handover

2.5.4.2.14.1 The siphonic system must be verified as being clean and free of debris. Since it is very difficultto perform an in-situ operational test of the siphonic system, other means such as video verification can beused to ensure that the system is not clogged and will operate as designed.

2.5.4.2.14.2 Verify that all roof drains have baffles (anti-vortex plates) and securely attached debris guards.

2.5.4.2.14.3 Pressure test the siphonic system to 50% greater than the maximum pressure at designconditions but not less than 13 psig (89.9 kPa) or 30 ft (9.0 m) water column. Ensure the system holds thetest pressure of at least 1-hour.

2.5.4.2.15 Inspection and Maintenance

Ensure that facilities personnel visually inspect the roof drains and discharge pipes at least once every 3months and keep a written log of the inspections. Inspect each roof drain to ensure that the debris guard andbaffle plate is intact and that there is no debris clogging the opening around the baffle plate. Inspect eachdischarge pipe to ensure that the pipe is free of debris. Facilities personnel should remove any scattereddebris on the roof that could clog or otherwise degrade the performance of the roof drains.



Fig. 11. Diagram of Siphonic Roof Drain System



Fig. 12. Elevation View of Siphonic System and Disposable (Available) Head



Fig. 13. Siphonic Roof Drain [photo courtesy of Jay R. Smith Mfg. Co.]

Fig. 14. Siphonic Roof Drain for Gutters (without dome strainer or debris guard)[photo courtesy of Jay R. Smith Mfg. Co.]



Table 9. Rainfall Intensity Conversion Rates

Rainfall Intensity (i)in./hr mm/min l/sec-m2

1.4 0.6 0.012.1 0.9 0.0152.8 1.2 0.023.5 1.5 0.0254.3 1.8 0.035.0 2.1 0.0355.7 2.4 0.046.4 2.7 0.0457.1 3.0 0.057.8 3.3 0.0558.5 3.6 0.069.2 3.9 0.0659.9 4.2 0.07

10.6 4.5 0.07511.3 4.8 0.08

Note: (l/sec-m2 ) x 141.7 = in./hr(mm/min) x 2.362 = in./hr

where l = liter

Table 10. Schedule 40 Pipe Dimensions and Geometric Properties

Schedule 40 (Standard Weight) PipeNominal Size Wall Thickness (t) Inside Diameter

(DI)Mean Diameter (DM) t/DM (t/DM)3

x1000(inch) (mm) (inch) (mm) (inch) (mm) (inch) (mm)1.5 38 0.145 3.7 1.610 40.9 1.755 44.6 0.0826 0.56402 51 0.154 3.9 2.067 52.5 2.221 56.4 0.0693 0.3334

2.5 64 0.203 5.2 2.469 62.7 2.672 67.9 0.0760 0.43853 76 0.216 5.5 3.068 77.9 3.284 83.4 0.0658 0.2845

3.5 89 0.226 5.7 3.548 90.1 3.774 95.9 0.0599 0.21474 102 0.237 6.0 4.026 102.3 4.263 108.3 0.0556 0.17185 127 0.258 6.6 5.047 128.2 5.305 134.7 0.0486 0.11506 152 0.28 7.1 6.065 154.1 6.345 161.2 0.0441 0.08598 203 0.322 8.2 7.981 202.7 8.303 210.9 0.0388 0.0583

10 254 0.365 9.3 10.020 254.5 10.385 263.8 0.0351 0.043412 305 0.375 9.5 12.000 304.8 12.375 314.3 0.0303 0.0278Nominal Size Cross Sectional (Open) Area

(inch) (mm) (in2) (ft2) (mm2) (m2)1.5 38 2.03 0.0141 1313 0.00132 51 3.35 0.0233 2164 0.0022

2.5 64 4.79 0.0332 3087 0.00313 76 7.39 0.0513 4767 0.0048

3.5 89 9.88 0.0686 6375 0.00644 102 12.72 0.0884 8209 0.00825 127 20.00 0.1389 12900 0.01296 152 28.88 0.2005 18629 0.01868 203 50.00 0.3472 32259 0.0323

10 254 78.81 0.5473 50848 0.050812 305 113.04 .7850 72929 0.0729



Table 11. Standard Atmospheric Pressure at Various Elevations

Elevation AboveSea Level

PressureHead* Pressure

(ft) (m) (ft) (m) (psi) (kPa) (atm)0 0 34.0 10.37 14.7 101.6 1.00

1500 457 32.2 9.82 14.0 96.2 0.953000 915 30.5 9.30 13.2 91.1 0.904500 1372 28.8 8.78 12.5 86.0 0.856000 1829 27.2 8.29 11.8 81.3 0.807500 2287 25.7 7.84 11.1 76.8 0.76

*Pressure Head is feet (ft) or meter (m) of water column, with an assumed water density of 62.4 Lb/ft3 (999.6 kg/m3).Linear interpolation is appropriate.

2.6 Other Roof Loads and Roof Overloading

2.6.1 Reinforce existing roofs that are overloaded and subject to collapse from snow loading. Wherereinforcing is impractical, use snow removal teams (as part of the emergency response team) to removeexcessive snow. Determine the safe maximum snow depth for the roof areas. Have snow removal teams clearsnow from the roofs when one-half of the safe maximum snow depth is reached.

2.6.2 Evaluate and analyze existing roofs that have roof-mounted or roof-suspended equipment andstructures added or modified. Include the supporting roof framing, columns and bearing walls in the analysis.Ensure the analysis and design of any needed reinforcing is performed by a qualified structural engineer.

2.6.3 Design suspended or otherwise supported ceilings that allow access for maintenance workers forappropriate concentrated and uniform live loads based on the anticipated maintenance work.

2.6.4 Indirect roof overloading: The overloading and collapse of the primary vertical support elements of theroof structure, such as columns and bearing walls, is another cause of roof collapse.

Columns adjacent to traffic aisles for fork-lifts and other trucks are vulnerable to upset if not adequatelyprotected from impact. Ensure the base plates of these columns are anchored to their foundations with aminimum of four (4) 1 in. (25 mm) diameter anchor bolts, and protected with concrete curbing, steel guardrails, or concrete-filled pipe bollards to resist and/or prevent impact loads from vehicles.

Ensure walls, particularly masonry walls, are not be laterally loaded as a result of having bulk materials (e.g.,sand, salt, grain) or rolled products (e.g., carpets or paper) placed against them, unless the wall and roofstructure are designed to resist the resulting lateral loads. Likewise, ensure rack storage structures or verticalstays for confining rolled products in storage are not secured to the roof-framing system unless the framingand bracing systems are designed to resist the resulting laterally-induced loads.

2.7 Use of Eurocode

For use by European Committee for Standardization (CEN) member nations that have adopted, and complywith, the Eurocode as the national standard.

2.7.1 Eurocode for Snow Loads

Eurocode 1 (Eurocode 1, Actions on Structures, Part 1-3: General Actions — Snow Loads [EN 1991-1-3:2003]) may be used in CEN (European Committee for Standardization) member nations for snow loaddetermination where it has been approved as the national standard, provided that the followingrecommendations are adhered to:

2.7.1.1 Snow Density

a) For locations where the 50-year ground snow load is greater than 1.8 kN/m2 (38 psf):

In Equation 5.8, Section 5.3.6, Roof Abutting and Close to Taller Construction Works (leeward drifts on lowerroofs) of Eurocode 1, use an upper limit bulk weight snow density of no less than 3 kN/m3 (18.9 lb/ft3).

In Equation 6.1, Section 6.2, Drifting at Projections and Obstructions (windward drift at projection or parapet)of Eurocode 1, use an upper limit bulk weight snow density of no less than 3 kN/m3 (18.9 lb/ft3).



b) For locations where the 50-year ground snow load is less than or equal to 1.8 kN/m2 (38 psf), use anupper limit bulk weight snow density of not less than 2 kN/m3 (12.6 lb/ft3) for windward and leeward snowdrifts as recommended in Eurocode 1 (see Sections 5.3.6 and 6.2 of Eurocode 1).

2.7.1.2 Hip and Gable (Pitched) Roofs

For hip or gable sloped roofs (pitched roofs) of lightweight construction (metal roof, insulated steel deck,boards-on-joists, plywood diaphragm, and similar constructions) with slopes greater than or equal to 5degrees or slopes less than 60 degrees (5 ≤ Θ <60); apply a factor of 1.25 to the pitched roof shape coefficient(1.25 µ1) to determine the uniform snow drift on the leeward (downwind) roof slope; see Case ii orCase iii in Figure 5.3 of Eurocode 1. Note that the shape coefficient for the windward (upwind) roof sloperemains as recommended in Eurocode 1 without change (0.5 µ1) for all roof slopes.

2.7.1.3 Ground Snow Load Maps

Use the ground snow load maps in the appropriate National Annex to Eurocode 1 provided the maps arebased on 50-year mean recurrence interval (50-year MRI) ground snow load (Pg) and account for regionalconditions.

2.7.1.4 Load Classification

Consider roof snow and snow drift loads based on 50-year ground snow loads to be characteristic valuesof Variable Actions for use in Persistent/Transient design situations.

2.7.1.5 Coefficients

As recommended in Eurocode 1, use an exposure coefficient (Ce), and thermal coefficient (Ct), of not lessthan 1.0.

2.7.2 Eurocode for Roof Live Load (Imposed Load)

2.7.2.1 Minimum Roof Live Load

a) Where the dead load (characteristic value of a Permanent Action) of the roof is greater than or equal to1.5 kN/m2 (31 psf), use a minimum roof live load (characteristic value of a Variable Action) of not less than0.6 kN/m2 (12 psf) applied uniformly over the entire roof; however, where the National Annex recommendsa minimum roof live load larger than 0.6 kN/m2 (12 psf), use the larger value. Do not reduce the roof live loadto less than 0.6 kN/m2 (12 psf) for any reason, regardless of tributary area or number of building stories.

b) Where the dead load (characteristic value of a Permanent Action) of the roof is less than 1.5 kN/m2

(31 psf), use the roof live load provisions of this data sheet (see Section 2.2).

Note that for purposes of foundation design only (e.g., footings, grade beams, piles, and caissons), the useof roof live (imposed) loads and live reduction techniques as recommended in the Eurocode are acceptablewithout revision or exception; that is, the recommendations in Section 2.2.1 of this data sheet may be waivedfor the purposes of foundation design.

2.7.2.2 Load Classification

Consider roof live loads (imposed loads) to be Variable Actions for use in Persistent/Transient designsituations.

2.7.3 Eurocode for Rain Loads

This section is applicable only to conventional (non-siphonic) roof drainage. For siphonic roof drainagerecommendations, refer to Section 2.5.4.2 Siphonic Roof Drainage.

European Standards EN 12056-1 Gravity Drainage Systems Inside Building – Part 1: General andPerformance Requirements, and EN 12056-3 Gravity Drainage Systems Inside Buildings – Part 3: RoofDrainage, Layout, and Calculation, may be used in CEN (European Committee for Standardization) membernations for rain load and roof drainage, with the following exceptions and changes:

2.7.3.1 Use design rainfall intensity based on adequate statistical data from a nationally recognized sourceor agency, where the rainfall intensity is based on frequency (return period or recurrence interval, in years)and duration (in minutes). If there is any doubt regarding the adequacy or validity of the rainfall data, then applythe risk factors from Table 2 of Section 4.2 of EN 12056-3 with the following restrictions:



For eave gutters, use a risk factor of at least 1.5For all other cases, use a risk factor of at least 3.0

2.7.3.2 Effective Rainfall Intensity

2.7.3.2.1 Ensure that the primary drainage system is adequate for the 100-year 60-min rainfall intensityassuming that the secondary drainage system is completely blocked.

2.7.3.2.2 Ensure that the secondary drainage system is adequate for the 100-year 60-min rainfall intensityassuming that the primary drainage system is completely blocked.

2.7.3.2.3 Ensure that the secondary drainage system capacity is not less than the primary drainage systemcapacity.

2.7.3.3 Do not use the following sections from EN 12056-3:

6.2 Siphonic Systems6.3 Drains6.4 Connection to Sanitary Pipework7 Layout

2.7.3.4 Follow the recommendations in Sections 2.5.2 through Section 2.5.4.1.13 of this data sheet, whichinclude, but are not limited to, independence of primary and secondary drainage systems, minimum designrain depths, ponding instability requirements, roof slope requirements, minimum roof drain quantities(maximum roof drainage area per drain), drain placement, minimum drain sizes, drain strainers (debrisguards), inlet elevations of secondary drains or scuppers relative to primary drain inlet elevations, anddownspout recommendations (height above snow level, impact protection, freeze-up protection).

2.7.3.5 Design Rainfall Intensities for Several Nations

Germany: Use DIN EN 12056 and DIN 1986-100, including their rain intensity maps and tables, to determinethe rainwater runoff for the primary and secondary drainage systems. These require independent secondarydrainage systems for all flat roofs and roofs with internal drains with discharge to a free unobstructedlocation.

United Kingdom: Use BS EN 12056-3:2000, including their rainfall intensity maps, to determine rainwaterrunoff for the primary and secondary drainage systems.

France, Netherlands, and Switzerland: Use EN 12056-3:2000 with applicable rainfall intensity maps subjectto the following minimum rainwater intensities noted in EN 12056-3:2000, Annex B, to determine the rainwaterrunoff for the primary and secondary drainage systems. Minimum intensities: France – 0.05 L/s/m2;Netherlands and Switzerland – 0.03 L/s/m2.

2.8 Use of ASCE 7 for Snow Loads

The provisions in Chapter 7 of ASCE 7-02 or ASCE 7-05 (ASCE/SEI 7-02 or 7-05, Minimum Design Loadsfor Buildings and Other Structures) may be used for the determination of snow loads provided the followingrecommendations are adhered to:

2.8.1 Factors

Importance factor (I) not less than 1.1Exposure Factor (Ce) not less than 1.0Thermal Factor (Ct) not less than 1.2 for unheated structures or structures intentionally kept below freezing;and not less than 1.1 for other buildings.

2.8.2 Hip and Gable Roofs

For unbalanced snow load on hip and gable roofs, use the provisions of ASCE 7-02 with the minimum factorsgiven in Section 2.8.1.



2.9 Plan Review and Submissions

2.9.1 General

During initial design of buildings insured by FM Global, have the building designer submit the followinginformation to the appropriate FM Global Operations office for confirmation that the design loads and drainageof each roof are in accordance with the recommendations in this data sheet (if the design does not followthe guidelines of this data sheet, proposed exceptions should be identified and compared):

a) Roof framing and drainage system plans, sections and details

b) The applicable building and plumbing codes and/or standards

c) Identification, where a minimum roof live load of 20 psf (1.0 kN/m2) governs, of reductions taken inthe minimum roof live load for any primary or secondary members and their respective design dead andlive loads

d) The ground snow load, the mean recurrence interval (MRI), snow density, and the source, if differentfrom the recommendations in this data sheet

e) The balanced, unbalanced, drift, and sliding surcharge snow loads and drift length, and rain-on-snowsurcharge loads as appropriate for the roof configurations, showing loading diagrams and denoting anydifferences from the recommendations in this data sheet. Focus on areas such as low roofs at roof steps,and roof projections, where substantial snow drifting can occur; verify that more substantial roofconstruction is provided at these areas.

f) All roof drainage (both conventional [non-siphonic], and siphonic):

• The rainfall intensity for the recommended duration (e.g., 60-minute, 15-minute, or 2-minute),frequency (mean recurrence interval [MRI], for example 100-year or 2-year), and the source, ifdifferent from the recommendations in this data sheet

• Primary drains and/or scuppers: type, size, maximum drainage area and flow rate, roof surface slopeto drainage point or dead-flat, and whether drains are located at mid-bay

• Overflow drainage provisions: whether over the roof edge, or overflow scuppers or drains; type, size,maximum drainage area and flow rate for scuppers and drains; height to roof edge, invert (scuppers)or inlet (drains) from the (adjacent to) design roof line; and roof surface slope to overflow point ordead-flat

• For conventional (non-siphonic) systems, maximum hydraulic head and total head for primary andoverflow drains and scuppers; hydraulic head versus discharge rates for specific drains or scuppersto be used

• Maximum design rain load for dead-flat roofs and for the low points of sloped roofs

• Analysis method for dead-flat roofs and source used to substantiate that the roof is stable basedon the design rain load and ponding recommendations in this data sheet.

• Roof slope for roofs with drainage over the edge or sloped to drains or scuppers. If the slope is lessthan 1⁄4 in./ft (2%), substantiate with calculations that the design slope is sufficient based on Sections2.5.2.4, 2.5.3, and 2.5.4.1.13.

g) Siphonic drainage:

• Siphonic drainage calculations and construction documents stamped and signed by a licensedprofessional engineer.

• Verification that the hydraulic properties and performance of the manufactured roof drains have beendetermined based on physical test results, from a qualified testing laboratory, in conformance witha nationally recognized standard (such as ASME A112.6.9).

• Verification that the primary and secondary drainage systems are independent of each other.

• Verification that all piping has adequate critical buckling strength for the full range of assumedoperating temperatures.



• Verification that expansion joints (if used) are properly detailed, are rated with an adequate safetyfactor, and have adequate critical buckling strength for the full range of assumed operatingtemperatures.

• Verification that the design disposable head (HD) is at least 10% greater than the sum of the residualvelocity head and the head losses.

• Verification that the minimum and maximum operating pressures are in accordance with therecommended ranges.

• Verification that adequate priming is provided, as indicated by the calculated time to fill (Tf).

• Ensure that HD is appropriate based on the elevations differences between the roof height anddischarge points.

• Ensure that the roof drain strainers (debris guards), and vented inspection chamber covers, haveadequate open areas.

• Ensure that the minimum pipe size (inside diameter) is not less than of 1.6 in. (40 mm).

• Ensure that adequate pipe supports and bracing are provided.

2.9.2 Other Codes and Standards

When submitting a project that is in conformance with Section 2.1 of this data sheet, in addition to therecommendations in Sections 2.7 and 2.8, include the following:

a) Edition/version of code and standard, including the date [for example, the 2005 edition of ASCE 7 (ASCE7-05)]; the load classification (e.g., Variable Action for Persistent/Transient design for Eurocode); the loadsfactors; and applicable coefficients (e.g., Ce, Ct, and I). This information will typically be located on thegeneral notes or structural notes sheet(s) of the construction drawings.

b) For Eurocode-specific projects, verify the minimum roof dead load of 1.5 kN/m2 (31 psf) if a reducedroof live (imposed) load is used per Section 2.7.2.1.a.

3.0 SUPPORT FOR RECOMMENDATIONS

3.1 General

3.1.1 Use of Other Codes and Standards

3.1.1.1 Eurocode

The thirty CEN (European Committee for Standardization) member nations include the following (as of late2007): Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany,Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, United Kingdom. Consult the NationalAnnex for additional guidelines and provisions.

3.1.1.1.1 Snow Loads

For the snow load provisions, refer to Eurocode 1 – Actions on Structures – Part 1-3: General Actions – SnowLoads (EN 1991-1-3). EN 1991-1-3 uses 50-year ground snow loads, and a recommended minimum snowdensity of 2 kN/m3 (12.6 pcf)

3.1.1.1.1.1 Return Period for Ground Snow Loads

Eurocode 1 ground snow maps are based on a return period of 50 years. However, there may be acountry-specific code or annex that uses a lesser return period; in these cases, an appropriate factor shouldbe applied to obtain equivalent 50-year ground snow loads (see Section 2.3.3).

3.1.1.1.1.2 Design Situations and Load Combinations

Eurocode 1 (and Eurocode 0, Basis of Structural Design) allow for several types of Design Situationclassifications for load combinations that include snow loads. The recommendation to consider 50-year snowand



snow-drift loads to be characteristic values of Variable Actions for use in Persistent/Transient designsituations, not Accidental or Exceptional design situations, will provide an acceptable design condition.

3.1.1.1.1.3 Exposure and Thermal Coefficients

The use of exposure coefficient (Ce) and thermal coefficient (Ct) not less than 1.0 (the recommended minimumvalues in Eurocode 1) should adequately address most reasonable unexpected design conditions; forexample, where power and heating are lost in a building, or where a building becomes more sheltered dueto future adjacent development.

3.1.1.2 ASCE 7 (ASCE/SEI 7, Minimum Design Loads for Buildings and Other Structures) with exceptions

ASCE 7-02 and 7-05 ground snow loads (Pg) are based on a 2% annual probability of being exceeded(50-year MRI). The recommendations to use several minimum factors (Importance Factor of not less than1.1, Thermal Factor of not less than 1.1, and Exposure Factor of not less than 1.0), when allowing the useof ASCE 7-02 or ASCE 7-05 for the determination of snow loads will ensure the balanced snow loads willbe adequate and sufficiently similar to the specific design snow loads recommended in this data sheet.

3.1.2 Rainfall Intensity, Duration, and Frequency used for Roof Drainage

Intensity (i)

Intensity (i) is the rainfall rate, typically recoded as in./hr, mm/hr, or liter/sec-m2. See Table 9 for conversionrates.

Duration

Duration is the time over which the peak rainfall intensity is averaged. Duration for roof drainage is typicallyrecorded in 2-min, 5-min, 15-min, or 60-min time intervals. Durations as much as 24 hours to 96 hours canbe used for site/civil drainage analysis associated with flood events. The shorter the duration, the higherthe rainfall intensity (2-min > 5-min > 15-min > 60-min intensity) for a given frequency.

Frequency

Frequency is the same as the return period or MRI (mean recurrence interval) of the event. For example,the “100-year event” has a probability of annual exceedance of approximately 1%, while the 5-year event hasa probability of annual exceed of approximately 20%.

3.1.3 Siphonic Drainage

Conventional (atmospheric, or non-siphonic) roof drainage systems use the hydraulic head above the roofdrain, which is typically no more that several inches, to create flow through the roof drain, and slopedhorizontal piping to maintain flow to the vertical leaders or downpipes. A siphonic drainage system uses thehead of the entire drainage system – in theory, from the elevation of the water directly upstream of the roofdrain, to the discharge point at or below the grade elevation – which can be many ft (m), as the energy todrive drainage flow. A siphonic system’s horizontal runs of the piping generally are not sloped.

A conventional drainage system operating at capacity could have roughly 20% to 30% of the cross-sectionalarea of the piping filled with water; however, a siphonic drainage system operating at capacity will be closeto 100% full (full-bore flow).

Since siphonic systems use the energy associated with the head of the entire drainage system, designvelocities are achieved without pitching or sloping the horizontal pipe runs. Siphonic systems operate withfull-bore flow velocities of roughly 10 ft/sec to 20 ft/sec (3 m/s to 6 m/s), while gravity systems operate witheffective velocities of roughly 2 ft/sec to 5 ft/sec (0.6 m/s to 1.5 m/s).

Conventional drainage systems operate at or near atmospheric pressure (gage pressure near zero). However,siphonic systems experience pressures less than atmospheric pressure – so that the operating negativegage pressure can be substantial. These negative operating pressures present a much more challengingtask to the design engineer and installation contractor, as compared to a gravity drainage system, due to theconcerns with air infiltration, pipe buckling and crushing, and overall performance and design sensitivity.

The Design Disposable Head (HD) is based on the reasonable assumption the manhole or inspection chamberwill experience surcharge from surface flow or site / storm drainage quite often over the life of the building.



4.0 REFERENCES

4.1 FM Global

Data Sheet 1-35, Green Roof Systems

Data Sheet 1-55, Weak Construction and Design

4.2 Others

American Institute of Steel Construction (AISC). Allowable Stress Design Specification for Structural SteelBuildings, Commentary K, Chapter K2.

American Institute of Steel Construction (AISC). Load and Resistance Factor Design Specification for Struc-tural Steel Buildings, Commentary K, Chapter K2.

American Society of Civil Engineers (ASCE). Minimum Design Loads for Buildings and Other Structures.ASCE/SEI 7-02 and 7-05.

European Committee for Standardization (CEN). Eurocode 0, Basis of Structural Design. EN 1990:2002 with2005 Amendment.

European Committee for Standardization (CEN). Eurocode 1, Actions on Structures Part 1-1: General Actions:Densities, Self-weight, Imposed Loads for Buildings. EN 1991-1-1:2002.

European Committee for Standardization (CEN). Eurocode 1, Actions on Structures Part 1-3: General Actions:Snow Loads. EN 1991-1-3:2003. EN 12056-1.

European Committee for Standardization (CEN). Gravity Drainage Systems Inside Buildings, Part 3: RoofDrainage, Layout and Calculation. EN 12056-3:2000.

International Code Council (ICC). International Plumbing Code. 2003 and 2006 editions.

Steel Joist Institute (SJI). Standard Specifications for LH-Series (Longspan), DLH-Series (Deep Longspan)Joists and Joists Girders and K-series (Open Web) Joists.

APPENDIX A GLOSSARY OF TERMS

The following discussion of terms will facilitate use of this data sheet. When using building and plumbingcodes, use the interpretations they provide.

A.1 Roof Loads and Drainage

A.1.1 Controlled Roof Drains

The design of controlled roof drains is similar to conventional roof drains. The difference is that controlleddrains are equipped with restrictive devices to accurately set the flow characteristics to the controlled drain-age requirements. The purpose of controlling roof drains is to have the roof serve as a temporary storage res-ervoir of rainwater (e.g., to prevent flooding of storm sewers).

A.1.2 Design Roof Line

The design roof line is an imaginary line established during the design stage as either dead-flat or slopedby setting elevations at points of support (i.e., columns or walls) for roof framing members. The design roofline is not the actual roof line because framing members sag under the dead weight of the roof system, andsag additionally under super-imposed live loads such as snow and rain. (See Figs. 8a and 8b.)

A.1.3 Ponding and Ponding Cycle

Ponding refers to the retention of water due solely to the deflection of relatively flat roof framing (see Figs.8a and 8b). The deflection permits the formation of pools of water. As water accumulates, deflection increases,thereby increasing the capacity of the depression formed. This phenomenon is known as the ‘‘ponding cycle.’’The amount of water accumulated is dependent upon the flexibility of the roof framing. If the roof framingmembers have insufficient stiffness, the water accumulated can collapse the roof.



A.1.4 Dead Load

The dead load of the roof is the weight of its permanent or fixed components, including supporting mem-bers, deck, insulation, roof covering, gravel, and suspended or supported ceilings or equipment, such as heat-ers, lighting fixtures, and piping, which were anticipated at the time of design.

In some cases, dead loads that were not anticipated are added to existing buildings, or an allowance forfuture dead loads was included in the design dead load. For the purposes of this data sheet, any portion ofthe dead load exceeding the design dead load should be subtracted from the design snow, rain, or live load;any unused portion of the design dead load may be added to the design snow, rain, or live load.

Dead load is normally expressed in pounds per square foot (lb/ft2 or psf), kilonewtons per square meter (kN/m2) or kilopascal (kPa).

A.1.5 Live Load

The live load of the roof is the weight allowance for temporary or movable loads, such as constructionmaterials, equipment, and workers. In some cases, where roofs are accessible to building occupants or thegeneral public, and where is it possible for people to congregate (such as a balcony, or rooftop deck or ter-race) an occupancy live load (e.g., 100 psf [4.8 kPa] for balcony or assembly areas) is required to be con-sidered as part of the total design load. In other cases, for example the top level of an exposed (uncovered)parking garage, an applicable vehicle live load must be considered. In cases where an occupied or acces-sible interior floor level or walkway (e.g., catwalk or maintenance platform) is be suspended from the roof fram-ing, the live load of the occupied level (e.g., 60 psf [2.9 kPa] for an elevated walkway) must be considered.For occupancy or vehicle live loads, most codes and standards allow for reduction in the live load basedon a function of the tributary area for each structural member, or for a reduction in live load as part of thetotal design load combination. However, the reduction of minimum roof live load (typically 20 psf [1.0 kN/m2])are only allowed when permitted by the local building code, and the reduced roof live load used for design pur-poses must not be less than that recommended in this data sheet.

The live, snow, or rain load represents the superimposed weight that the roof system can support, within allow-able design parameters, beyond its own dead load. In cases where re-roofing materials or equipment or struc-tures that were not included in the design dead load are added to the roof system, their weight should besubtracted from the design rain or snow load.

Most building codes and design standards permit reductions in minimum roof live loads, excluding snow orrain loads, based on the tributary loaded areas supported by roof members (joists, beams, etc.). This datasheet restricts live load reductions for lightweight roof constructions. Usually the minimum roof live load is 20psf (1.0 kN/m2) with a reduction to 12 psf (0.6 kN/m2) for members supporting a tributary area equal to orgreater than 600 ft2 (56 m2) and with reduced roof live loads values based on a linear relationship for tribu-tary areas from 200 ft2 (19 m2) to 600 ft2 (56 m2). For example, for a tributary area of 400 ft2 (37 m2), thedesign roof live load (reduced) is 16 psf (0.8 kN/m2). This means that roofs assumed to have a 20 psf (1.0kN/m2) live load capacity, as commonly stated on the roof plan drawings, may actually only have an effec-tive load capacity of 12 psf (0.6 kN/m2). Usually, only the design calculations identify whether live load reduc-tions have been taken. When code guidelines for live load reductions are followed, the practical result isthe construction of very flexible roofs, highly susceptible to ponding and frequently unable to resist rain orunbalanced snow (drifts) loads. It is likely that live load reductions have been applied to minimum design liveloads even in new construction, when rain loads due to drainage system blockage are not considered orappropriately understood.

Live load is usually expressed in pounds per square foot (lb/ft3 or psf), kilo-newtons per square meter (kN/m2) or kilo-pascal (kPa).

A.1.6 Total Load

The total load of the roof is the combination of the dead load plus snow, rain, or live loads, excluding windand earthquake loads. The design total load should be effectively resisted by each of the structural mem-bers of the roof system. Building codes and design standards establish allowable (design) working stressesand deflection limits, and these may only be exceeded when considering dead and snow, rain, or live loadsin combination with wind or earthquake loads.



A.1.7 Tributary Loaded Area (TA)

The TA is that area of the roof supported by a roof (supporting) member. Tributary loaded areas for typicalprimary and secondary members are illustrated in Figure 15. For secondary members, such as joists, the TAis the joist length times the joist spacing. For primary members, such as beams, girders, or trusses usuallysupporting uniformly spaced joists, the TA is the beam or truss length times its spacing. As a rule of thumb,the TA for primary members is the area of a bay (a layout of four columns constitutes a bay) or precisely theproduct of the average column spacing in each direction. An exception to the rule of thumb is constructionwith members framed along exterior column lines or along double column lines at expansion joints; then theTA is the member length times one-half the member spacing plus the roof overhang beyond the columncenterline.

A.1.8 Roof Strength

Roof strength is the measure of a roof assembly and supporting system’s ability to support loads. Total roofstrength is the measure of the roof system’s ability to support the design dead load plus snow, rain, or liveloads without exceeding the allowable design parameters. Roof strengths are expressed in psf, kN/m2 or kPa.

Steel roof deck manufacturers often provide allowable uniform total load tables in their catalogs. This canbe misleading since the strength of members supporting the deck is governed by the design total load andnot the load capacity of the roof deck. The supporting members, because of this difference, will usuallycollapse well before a failure of the deck occurs. The primary determinant of roof strength, therefore, is theroof supporting members with appropriate adjustment for any live load reductions.

A.1.9 Safety Factor

The safety factor of a structural member can be defined as the ratio of its strength to its maximum anticipateddesign stress (working stress). In steel design using ‘‘elastic-design methods,’’ a design stress equal totwo-thirds of the minimum yield stress of the material, is often used. This results in a safety factor for yieldequal to 1/0.67 or 1.5. Although the initiation of yield may not entail fracture, once the yield stress in bending

Fig. 15. Typical tributary loaded areas for primary and secondary members



is reached, the joists and beam will start to deflect significantly (plastic deformation), thereby increasing thepotential for substantial ponding and catastrophic failure.

While it is helpful to recognize this safety margin, it is equally important to understand that safety factorsare provided for the many uncertainties associated with materials, design, fabrication, installation, andunpredicted loads in excess of design values. The building designer should not compromise or use any portionof the safety margin for design purposes except when permitted for ponding analysis and wind or earthquakeload combinations.

Loads in ‘‘excess’’ of design values may occur when based on this data sheet, which establishes designvalues that reduce the risk of load-induced collapse to an acceptably low limit. The implications of such‘‘excess’’ loads, however, should be considered. For example, if a roof is deflected at the design snow loadso that slope-to-drain is eliminated, ‘‘excess’’ snow load may cause ponding and perhaps progressive failure.The rain-load to dead-load or snow-load to dead-load ratios of a roof structure are an important considerationwhen assessing the implications of ‘‘excess’’ loads. If the design rain or snow load is exceeded, thepercentage increase in total load is greater for a lightweight structure (all metal, insulated steel deck, orboards-on-joists roof constructions) than for a heavy structure (concrete deck or plank-on-timberconstructions). Thus, the lower the safety margin (expressed as a load), the higher the probability for roofcollapse due to snow or rain ‘‘excess’’ loads. This fact is supported by loss history.

APPENDIX B DOCUMENT REVISION HISTORY

January 2011. Minor editoriAl changes were made. A note was added to the China snow maps and tablesto eliminate uncertainty regarding rounding/converting.

September 2010. The following changes were made for this revision:

• Added recommendations for Siphonic roof drainage, including new plan review guidance.

• Added recommendations for using Eurocode provisions for roof drainage.

• Added recommendations for ground snow loads in China.

• Added background and guidance on rainfall intensity, duration, and frequency (i-D-F).

January 2009. Minor editorial changes were made for this revision.

July 2008. Completely revised. The following outlines the major changes:

Added section that allows the use (with exceptions and changes) of Eurocode 1 for snow loads and rooflive loads.

Added section that allows the use (with exceptions and changes) of ASCE 7 for snow loads.

Added updated ground snow load maps for the contiguous United States and ground snow load for Alaska.

Added ground snow load tables for select cities in Korea and Japan (Tables 10 and 11, respectively).

Added recommendations for rain-on-snow surcharge, intersecting snow drifts, drift distribution on dome roofs,and snow/ice load at overhanging eaves.

Added flow chart for the use of live load reduction.

Revised snow drift loads for hip and gable roofs, valley roofs, and roof projections.

Revised sliding snow surcharge on low roofs.

Revised the definition of live load to exclude variable loads such as snow and rain loads.

Accepted using Eurocode EN 12056-3:2000 and rain intensity maps and data for determining rainwater runofffor France, Germany, Netherlands, Switzerland and the United Kingdom.

September 2006. Minor editorial changes were done for this revision.

May 2006. Minor editorial changes done for this revision.

September 2004. Minor editorial changes were done for this revision.

January 2001. This revision of the document has been reorganized to provide a consistent format.

APPENDIX C SUPPLEMENTARY INFORMATION



gFig. 16a. Ground snow load (P ) in psf for Western United States.

©2008 Factory Mutual Insurance Company. All rights reserved

FM Global Property Loss Prevention Data Sheets

Roof Loads for New Construction

1-54

40

40

30

(200)20

(700)20(400)15

CS

CS

(1500)15

(1200)10

(4300)20

(4100)15

(3200)10

(200)10(100)5

(400)10(300)5

(1000)20(800)15(600)10

(2800)20

(1900)20

CS

(1800)10

(1300)5

(800)Zero

(500)5

(300)Zero

(800)5

(1500)Zero

CS

(2400)Zero

(2000)5

(1500)Zero

(2800)5

(1800)Zero

(1000)Zero

(3600)5

(2000)Zero

CS

(2000)Zero

(3000)Zero

(4000)Zero

(5000)10

(4600)5

(3500)Zero (5000)

5(3500)Zero

(3000)Zero

(3300)20 (4000)

20(3400)15

(4600)20

CS

CS

CS

(4400)15

(5000)10

(4000)5 (5400)

20

(3200)20

(4600)15

(3800)10

(4800)10

CS

(6400)15 (5000)

15

(6200)20

(5400)10

(4500)5

(3000)Zero

(2600)30

(3600)20

(4800)25

(4100)25

(5500)15

(6600)20

(6500)15

(4500)20

(6000)25

CS

(4500)20

(6000)15

(4500)10

(3600)5

(2000)Zero

(5200)20

(4500)15

(6500)15

(6400)10

(5000)5

CS(4800)15

(3600)10

(6000)10

(5000)5

(6000)15

(6500)15

(5000)10

CS

20

20

20

Zero

(3000)25

(3700)30

CS

(4400)10

(3200)5

(4500)Zero

15(5000)10

(6000)35

(6000)30

CS

(6300)15

(5400)10

(4500)5

(3000)Zero

6050

40

35

35 50

25

20

30

10

25 35

CSCS

(4500)20

(2600)30

(6000)15

(5000)10

5

Zero

15

15

20

CS

40

120° 115° 110° 105° 100°

30°

35°

40°

45°

50°

125° 120° 115° 110° 105° 100°

25°

30°

35°

40°

45°

In CS areas, site-specific Case Studies are required toestablish ground snow loads. Extreme local variationsin ground snow loads in these areas preclude mappingat this scale.

Numbers in perentheses represent the upper elevationlimits in feet for the ground snow load values presentedbelow. Site-specific case studies are required to establishground snow loads at elevations not covered.

To convert lb/sq ft to kN/m², multiply by 0.0479.

To convert feet to meters, multiply by 0.3048





100 300 Kilometers0 200 400100

Miles0 100 200 300100

gFig. 16b. Ground snow load (P ) in psf for Eastern United States.

FM Global Property Loss Prevetion Data Sheets


Roof Loads for New Construction

1-54

30CSCS

CS

40

2530

35

Zero

5

10

25

35

40

60

5040

6070

100

90

80

(700)100

(700)90

(600)80

CS

CS

(500)70

(500)60

25

20

(1000)60

60

(900)50

(800)60

(2500)20

(2600)15

40

(1800)10

(700)50

20

CS

(1000)40

15

(1000)35

Zero

(1700)30

(800)35

(1200)25

50

20

25

30

35

50

(900)30

CS

25 20

15

10

5

(2500)25

70

(500)50 (900)

50

25

25

30

35

CS

CS

20

15

CS

60

50

(1700)30

95° 90° 85° 80° 75°

25°

30°

35°

40°

45°

95° 90° 85° 80° 75° 70° 65°

25°

30°

35°

40°

45°

50°





100 300 Kilometers0 200 400100

Miles0 100 200 300100

Table 12. Ground Snow Load (Pg) for Alaskan Locations, psf (kN/m2)

LocationPg

LocationPg

LocationPg

lb/ft2 kN/m2 lb/ft2 kN/m2 lb/ft2 kN/m2

Adak 30 1.4 Galena 60 2.9 Petersburg 150 7.2Anchorage 50 2.4 Gulkana 70 3.4 St Paul 40 1.9Angoon 70 3.4 Homer 40 1.9 Seward 50 2.4Barrow 25 1.2 Juneau 60 2.9 Shemya 25 1.2Barter 35 1.7 Kenai 70 3.4 Sitka 50 2.4Bethel 40 1.9 Kodiak 30 1.4 Talkeetna 120 5.8Big Delta 50 2.4 Kotzebue 60 2.9 Unalakleet 50 2.4Cold Bay 25 1.2 McGrath 70 3.4 Valdez 160 7.7Cordova 100 4.8 Nenana 80 3.8 Whittier 300 14.4Fairbanks 60 2.9 Nome 70 3.4 Wrangell 60 2.9Fort Yukon 60 2.9 Palmer 50 2.4 Yakutat 150 7.2

Table 13. Ground Snow Load (Pg) for Locations in Korea, psf and kPa

50-Year Ground Snow Load for Select Cities in KoreaCities Location 50-yr Ground Snow

Load (kPa)50-yr Ground Snow

Load (psf)LONG (E) LAT (N)Seoul 126°58’ 37°34’ 0.85 18Incheon 126°38’ 37°28’ 0.80 17Suwon 126°59’ 37°16’ 0.70 15Cheongju 127°27’ 36°38’ 1.10 23Daejeon 127°22’ 36°22’ 1.15 24Pohang 129°23’ 36°02’ 0.90 19Daegu 128°37’ 35°53’ 0.80 17Ulsan 129°19’ 35°33’ 0.60 13Masan 128°34’ 35°11’ 1.00 21Gwangju 126°54’ 35°10’ 1.05 22Busan 129°02’ 35°06’ 0.85 18Mokpo 126°23’ 34°49’ 0.95 20Icheon 127°29’ 37°16’ 1.05 22Cheonan 127°07’ 36°47’ 0.80 17Youngju 128°31’ 36°52’ 1.10 23Gumi 128°19’ 36°08’ 1.05 22Gunsan 126°45’ 36°00’ 0.95 20Jeonju 127°09’ 35°49’ 0.80 17

Note: Snow Load is based on a snow weight density of 17.2 lb/ft3 (2.73 kN/m3)



Table 14. Ground Snow Load (Pg) for Locations in Japan, psf and kPa

50-Year Ground Snow Load for Select Cities in JapanCity Location Altitude (m) Altitude (ft) 50-yr Ground

Snow Load(kPa)

50-yr GroundSnow Load

(psf)LONG (E) LAT (N)

Sapporo 141°19.9’ 43°03.4’ 17 56 4.70 98Yamagata 140°20.9’ 38°15.2’ 153 500 2.87 60Fukushima 140°28.5’ 37°45.4’ 67 221 1.29 27

Nagano 138°11.7’ 36°39.6’ 418 1372 1.70 36Utsunomiya 139°52.3’ 36°32.8’ 119 392 0.72 15

Fukui 136°13.6’ 36°03.2’ 9 29 5.85 122Maebashi 139°03.9’ 36°24.1’ 112 368 0.96 20Kumagaya 139°23.0’ 36°08.8’ 30 98 0.89 19

Mito 140°28.0’ 36°23.0’ 29 96 0.66 14Gifu 136°45.9’ 35°23.8’ 13 42 1.21 25

Nagoya 136°58.1’ 35°09.9’ 51 168 0.55 12Kofu 138°33.4’ 35°39.8’ 273 895 1.02 21

Choshi 140°51.6’ 35°44.2’ 20 66 0.29 6Hamamatsu 137°43.4’ 34°42.4’ 32 104 0.12 3

Tokyo 139°46.0’ 35°41.0’ 7 21 0.95 20Yokohama 139°39.4’ 35°26.2’ 39 128 1.11 23Hiroshima 132°28.0’ 34°24.0’ 4 12 0.45 9

Kobe 135°10.8’ 34°41.3’ 58 189 0.25 5Osaka 135°31.3’ 34°40.7’ 23 76 0.40 8

Fukuoka 130°22.6’ 33°34.8’ 3 8 0.41 8Miyazaki 131°25.4’ 31°55.2’ 6 21 0.07 2

Note: Ground snow loads are based on the recommended unit snow weight densities provided in the guidelines of the Architectural Insti-tute of Japan (AIJ).



Table 15. Ground Snow Load (Pg) for Locations in China*

Cities by Numerical Order Cities by Alphabetical Order

City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load

(kN/m2) (psf) (kN/m2) (psf) (kN/m2) (psf) (kN/m2) (psf)

1 Urumqi 1.12 23 40 Luoyang 0.64 13 43 Anqing 0.80 17 40 Luoyang 0.64 13

2 Lhasa 0.48 10 41 Hefei 0.96 20 14 Anshan 0.80 17 3 Mohe 1.12 23

3 Mohe 1.12 23 42 Bangbu 0.80 17 42 Bangbu 0.80 17 69 Nanchang 0.80 17

4 Qiqihar 0.64 13 43 Anqing 0.80 17 36 Baoji 0.48 10 50 Nanjing 0.80 17

5 Harbin 0.80 17 44 Fuyang 0.96 20 17 Beijing 0.64 13 52 Nantong 0.48 10

6 Jiamusi 1.12 23 45 Lianyungang 0.64 13 9 Changchun 0.64 13 59 Ningbo 0.64 13

7 Ulanhot 0.48 10 46 Xuzhou 0.64 13 67 Changde 0.80 17 30 Qingdao 0.48 10

8 Hohhot 0.32 7 47 Sheyang 0.48 10 65 Changsha 0.80 17 21 Qinhuangdao 0.64 13

9 Changchun 0.64 13 48 Dongtai 0.64 13 51 Changzhou 0.64 13 4 Qiqihar 0.64 13

10 Jilin 0.64 13 49 Zhenjiang 0.64 13 73 Chengdu 0.32 7 56 Shanghai 0.48 10

11 Fushun 0.96 20 50 Nanjing 0.80 17 75 Chongqing 0.32 7 72 Shaowu 0.64 13

12 Shenyang 0.96 20 51 Changzhou 0.64 13 16 Dalian 0.64 13 12 Shenyang 0.96 20

13 Dandong 0.80 17 52 Nantong 0.48 10 13 Dandong 0.80 17 47 Sheyang 0.48 10

14 Anshan 0.80 17 53 Wuxi 0.80 17 26 Datong 0.48 10 22 Shijiazhuang 0.64 13

15 Jinzhou 0.80 17 54 Suzhou 0.64 13 48 Dongtai 0.64 13 54 Suzhou 0.64 13

16 Dalian 0.64 13 55 Kunshan 0.48 10 74 Dujiangyan 0.32 7 25 Taiyuan 0.64 13

17 Beijing 0.64 13 56 Shanghai 0.48 10 11 Fushun 0.96 20 19 Tanggu 0.64 13

18 Tianjin 0.80 17 57 Jiaxing 0.64 13 44 Fuyang 0.96 20 18 Tianjin 0.80 17

19 Tanggu 0.64 13 58 Hangzhou 0.80 17 71 Ganzhou 0.48 10 64 Tianmen 0.64 13

20 Zhangjiakou 0.48 10 59 Ningbo 0.64 13 77 Guiyang 0.48 10 7 Ulanhot 0.48 10

21 Qinhuangdao 0.64 13 60 Wenzhou 0.64 13 58 Hangzhou 0.80 17 1 Urumqi 1.12 23

22 Shijiazhuang 0.64 13 61 Jinhua 0.96 20 5 Harbin 0.80 17 28 Weifang 0.64 13

23 Xingtai 0.64 13 62 Wuhan 0.80 17 41 Hefei 0.96 20 32 Weihai 0.80 17

24 Yinchuan 0.48 10 63 Yichang 0.64 13 8 Hohhot 0.48 10 60 Wenz 0.64 13

25 Taiyuan 0.64 13 64 Tianmen 0.64 13 6 Jiamusi 1.12 23 62 Wuhan 0.80 17

26 Datong 0.48 10 65 Changsha 0.80 17 57 Jiaxing 0.64 13 53 Wuxi 0.80 17

27 Jinan 0.64 13 66 Yueyang 0.96 20 10 Jilin 0.64 13 37 Xi’an 0.48 10

28 Weifang 0.64 13 67 Changde 0.80 17 27 Jinan 0.64 13 23 Xingtai 0.64 13

29 Linyi 0.64 13 68 Jingdezhen 0.96 20 68 Jingdezhen 0.96 20 33 Xining 0.32 7

30 Qingdao 0.48 10 69 Nanchang 0.80 17 61 Jinhua 0.96 20 46 Xuzhou 0.64 13

31 Yantai 0.80 17 70 Jiujiang 0.80 17 15 Jinzhou 0.80 17 35 Yan’an 0.48 10

32 Weihai 0.80 17 71 Ganzhou 0.48 10 70 Jiujiang 0.80 17 31 Yantai 0.80 17

33 Xining 0.32 7 72 Shaowu 0.64 13 39 Kaifeng 0.80 17 63 Yichang 0.64 13

34 Lanzhou 0.32 7 73 Chengdu 0.32 7 76 Kunming 0.64 13 24 Yinchuan 0.48 10

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Table 15. Continued

Cities by Numerical Order Cities by Alphabetical Order

City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load

(kN/m2) (psf) (kN/m2) (psf) (kN/m2) (psf) (kN/m2) (psf)

35 Yan’an 0.48 10 74 Dujiangyan 0.32 7 55 Kunshan 0.48 10 66 Yueyang 0.96 20

36 Baoji 0.48 10 75 Chongqing 0.32 7 34 Lanzhou 0.32 7 20 Zhangjiakou 0.48 10

37 Xi’an 0.48 10 76 Kunming 0.64 13 2 Lhasa 0.48 10 38 Zhengzhou 0.80 17

38 Zhengzhou 0.80 17 77 Guiyang 0.48 10 45 Lianyungang 0.64 13 49 Zhenjiang 0.64 13

39 Kaifeng 0.80 17 78 Zunyi 0.32 7 29 Linyi 0.64 13 78 Zunyi 0.32 7

* Note that the loads in this table include a snow load Importance Factor (I) of 1.2. Snow load values in psf have been converted and rounded-off from snow load values in kN/m2; therefore, avoid convertingfrom psf to kN/m2 as this can result in round-off error.

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Laos

Mongolia

Kyrgyzstan

Kazakhstan

Bangladesh

Bhutan

Nepal

India

Russia

Burma(Myanmar)

Bay of Bengal

Fig. 17a, Ground Snow Load (P ) in kN/m for Western China

100 300 Kilometers0 200 400100

Miles0 100 200 300100

g2

GroundSnow Load

0.93 (19)0.80 (17)0.67 (14)0.53 (11)0.40 (8)0.27 (6)0.00 (0)

1.06 (22)

1.60 (33)1.33 (28)

Above Snow Load Valuesare at All Elevations

kN/m (PSF)2

Note: Snow load values in PSF have been convertedand rounded-off from snow load values in kN/m2;therefore, avoid converting from PSF to kN/m2 as thiscan result in round-off error.


Roof Loads for New Construction 1-54


25°

95° 100°90°85°

45°

30°

35°

40°

105°95° 100°90°85°80°75°70°

Fig. 17b, Ground Snow Load (P ) in kN/m for Eastern China

Vietnam

Taiwan

)

SouthKorea

NorthKorea

KoreaBay

EastChina Sea

Yellow Sea

GulfOf

Liaodong

PescadoreChannel

Bashi Channel

Taiw

an

Str

ait

100 300 Kilometers0 200 400100

Miles0 100 200 300100

g2

GroundSnow Load

1.60 (33)1.33 (28)1.06 (22)0.93 (19)0.80 (17)0.67 (14)0.53 (11)0.40 (8)0.27 (6)0.00 (0)

kN/m (PSF)2

Above Snow Load Valuesare at All Elevations

Note: Snow load values in PSF have been converted androunded-off from snow load values in kN/m2; therefore, avoidconverting from PSF to kN/m2 as this can result in round-off error.


Roof Loads for New Construction1-54


100° 120°115°110°105°

20°

25°

30°

35°

40°

45°

135°130°125°120°115° 140°

Fig 18. Roof Live load reduction Flow Chart/Decision Tree



PAC

IFIC

OC

EAN

Fig. 19. Rainfall intensity (i) in inches per hour for the western United States(to convert to millimeters per hour multiply by 25.4)


1.5

1.01.0

2.52.5

1.5

1.0

1.51.0

2.0

1.0

1.5

2.52.0

3.01.0

1.5 2.0

3.0

3.02.52.02.02.5

3.0

2.01.5

2.5

2.0

1.5

1.5

1.5

1.0

1.5

1.5

1.5

1.5

2.0

2.5

3.0

2.0

1.0


1-54 Roof Loads for New Construction

35°

30°

25°

115°

115°120°125°130° 105° 100°

100 Year 1 hour duration

100°

Source: U.S.Weather Bureau, Technical Paper No. 40, 1961

45°

40°

105°

35°

30°

110°

25°

50°

45°

40°

110°

Kilometers

0 100 200 300

Miles

0 200 400

Fig. 20. Rainfall Intensity (i) in inches per hour for the central and eastern United States (to convert to millimeters per hour multiply by 25.4.)

2.0

2.0

3.0

3.253.5

4.0 4.25

4.5

4.754.75

4.5

2.5

5.0

4.28

4.5

*

3.25

3.5

3.75

3.0

4.25

4.25

4.25

3.5

2.25

3.75

3.25

4.5

3.25

3.753.25

3.75

2.75

3.0

2.75

2.75

2.5

2.5

2.25

2.25

3.5

2.252.5

2.52.25

2.25

2.252.52.75

2.753.03.0

2.75

2.75

2.75

3.25

3.0

2.75

4.0

3.25

2.5

2.5

4.0

2.5

3.75

2.52.25

2.6

2.25

3.25

2.252.25

3.25

3.02.75

2.5

2.75

2.25

105° 100° 95° 90° 85°

85° 75° 70° 65°

75°

40°

80°

25°

30°

35°

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LAKE HURON

LAKE ONTARIO

LAKE SUPERIOR

Kilometers

0 100 200 300

Miles

0 200 400

100-YEAR 60-MINUTEPRECIPITATION (INCHES)

Legend:

* KEY WEST. FLORIDA VALUEREPRESENTATIVE FOR FLORIDA KEYS

100-YEAR 1-HOUR RAINFALL (INCHES)

Source: U.S.Weather Bureau, Technical Paper No.42, 1961

Fig. 21a. Rainfall intensity (i) in inches per hour for Puerto Rico (to convert to millimeters per hour multiply by 25.4.)

45

4

6.56 5.5

5

4.5

54.544.5

3.5

4.5

4

4 4 4.5 4.5

Min Val 3.4

4

4 4.5 5

5.5

5.55

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ATLANTIC OCEAN

MONA ISLAND

3.1

CULEBRA ISLAND

3.1

VIEQUES ISLAND

5.54.6

67°45 67°30 67°15 67°0 66°45 66°30 66°15 66°0 65°45 65°30 65°15

17°30

17°45

18°0

18°15

18°30

18°45

67°45 67°30 67°15 67°0 66°45 66°30 66°15 66°0 65°45 65°30 65°15

17°30

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18°30

18°45

40Kilometers

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40Kilometers

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NIHAU

MOLOKAI

KAUAI

HAWAII

OAHU

KAHOOLAWE

MAUILANAI

Fig. 21b. Rainfall intensity (i) in inches per hour for Hawaiian Islands (to convert to millimeters per hour multiply by 25.4.)

HAWAII

1.5

5

3

2.5

2

6

22.5

6

5

7

6

5.5445

4

2.5

65

3

2.5

2

2.5

2.5

2.5

43

21.5

2.5

2

4

MOLOKAI

LANAI

KAHOOLAWE

MAUI3

34

5

4

4

2.5

2

2.5

3

2.5

2.5

1.5 2.5

4

8

2.5

5

3

3

5

68

6

5

7

7

3

5.5

43

3

2.5

2.5

2.5

2.5

OAHU

3

3

36

3

33

45

6.5

2.5

NIHAU

KAUAI

7

66

55

4

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100-YEAR 1-HOUR RAINFALL (INCHES)

Source: U.S. Weather Bureau, TechnicalPaper No. 43, 1962

156°0' 155°30' 155°0'

19°0'

19°30'

20°0'

156°0' 155°30' 155°0'

19°0'

19°30'

20°0'

160° 159° 158° 157° 156° 155°

19°

20°

21°

22°

160° 159° 158° 157° 156° 155°

19°

20°

21°

22°

160°0' 159°30'

22°0'

160°0' 159°30'

22°0'

158°0'

21°30'

158°0'

21°30'

157°0' 156°30' 156°0'20°30'

21°0'

157°0' 156°30' 156°0'

20°30'

21°0'

Fig. 22. Rainfall intensity (i) in inches per hour for Alaska (to convert to millimeters per hour multiply by 25.4.)

0.8

0.6

0.5

0.5

1.2

1

1.2

1.41.4

0.4

1.4

1.4

0.80.6

1.2

1.2

1

1

1

11

1.2

1

1.2

1.2

0.6

1 1

0.8

0.8

.5

1.2

0.5

10.8

0.90

0.80

0.88

0.57

0.48

0.5

0.6

0.80.8

0.80.9

0.9

0.9

1

0.8

0.6

0.6

0.8

0.49

0.9

0.8

0.8

0.7

0.70.8

0.4

0.70.7

0.6

0.6

KingSalmon

Takotna

Ketchikan

Metlakatla

Umiat

Kaktovik

DutchHarbor

Juneau

Kotzebue

Mekoryuk

Yakutat

Shungnak

Cordova

FortYukon

Anchorage

Nome

Bethel

Gambell

Kodiak

Galena

St. PaulMeshik

Fairbanks

Chignik

Northway

ColdBay

PointLay

AdarAtka

St. George

Barrow

175° 170° 165° 160° 155° 150° 145° 140° 135° 130°

55°

60°

65°

70°

175° 170° 165° 160° 155° 150° 145° 140° 135° 130°

55°

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65°

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APPENDIX E ILLUSTRATIVE EXAMPLES AND JOB AIDS

E.1 Snow Loading Illustrative Examples

The following examples illustrate the methods used to establish design snow loads for most of the roofconfigurations discussed in this data sheet.

Example 1: Determine the balanced and unbalanced design snow loads for a proposed building forMilwaukee, Wisconsin. It has galvanized steel, insulated panels on an unobstructed gable roof, sloped 8 on12 (see Fig. E1.1).

a) Ground snow load (Pg) from Figure 16b:

Pg = 30 psf (1.4 kN/m2)

b) Flat-roof snow load (Section 2.3.5)

Pf = 0.9 Pg = 0.9 (30) = 27 psf (1.3 kN/m2)

c) Sloped-roof (balanced) snow load (Section 2.3.7):

Ps = CsPf = 0.66 (27) = 18 psf (0.9 kN/m2)

d) Sloped-roof (unbalanced) snow load — leeward (Section 2.3.9):

1.5 Ps = 1.5 (18) = 27 psf (1.3 kN/m2)

e) Sloped-roof (unbalanced) snow load – windward (Section 2.3.9)

0.3 Ps = 0.3 (18) = 5 psf (0.26 kN/m2)

Fig. E1.1. Design snow loads for Example 1



Example 2: Determine the roof snow load for a proposed (bow-string truss) curved roof building for NewHaven, Connecticut. The building has an 80 ft (24 m) clear span and 15 ft (4.6 m) rise, circular arc wood deckroof construction with insulation and built-up roofing (see Fig. E1.2).

a. Ground snow load (Pg) from Figure 16b:

Pg = 30 psf (1.4 kN/m2)

b. Flat roof snow load (Section 2.3.5): Pf = 0.9 (30) = 27 psf (1.3 kN/m2)

c. Vertical angle measured from eave to crown (see Fig. E1.2):

Tangent of vertical angle = rise = 15 = 0.3751⁄2 span 40

Vertical angle = 21°

d. Sloped-roof (balanced) snow load:

Ps = Cs Pf = 1.0 (27) = 27 psf (1.3 kN/m2)

where Cs = 1.0 (Table 2 for cold, other surface roof)

e. Unbalanced snow loads (Section 2.3.10):

Eave slope = 41° (see Fig. E1.2)

The 30° point is 30 ft (9.1 m) from the centerline (see Fig. E1.2).

Unbalanced load at crown w/slope of 30° (Fig. 2a, Case I):

0.5 Ps = .5 (27) = 14 psf (0.6 kN/m2)

Unbalanced load at 30° point (Fig. 2a, Case II):

2 Ps = 2(27) = 54 psf (2.6 kN/m2)

Unbalanced load at eave (Fig. 2a, Case II):

2 Ps (1 − eave slope −30° )40°

2×27 (1 – 41° − 30° ) = 39 psf (1.9 kN/m2)40°

Fig. E1.2. Design snow loads for Example 2



Example 3: Determine the design snow loads for the upper and lower flat roofs for a proposed building tobe located in Lansing, Michigan. The elevation difference between the roofs is 10 ft (3 m). The upper roof is200 ft (61 m) wide and the lower roof is 40 ft (12.2 m) wide (see Fig. E1.3).

a) Ground snow load (Pg) from Figure 16b:

Pg = 35 psf (1.7 kN/m2)

b) Flat-roof (balanced) snow load for either roof (Section 2.3.5)

Pf = 0.9 (Pg) = 0.9 (35) = 32 psf (1.5 kN/m2)

c) Maximum snow load at wall (lower roof) (Section 2.3.12.1):

Max. load at wall = Pd + Pf = hr × D

from Table 3, with Pg = 35 psf and Wb = 200 ft; D = 18.6 pcf and Pd + Pf = 125 psf = 10 × 18.6 = 186 psfMax snow load (lower roof) = 125 psf (6 kN/m2)

d) Drift width (Section 2.3.12.1):

Wd = 4 hd when hd ≤ hc

from Table 3, with Pg = 35 psf and Wb = 200 ft;hd = 5.01 ftWd = 4 (5.01) = 20 ft (6.1 m)

e) See Figure E.1.3 for snow loads on both roofs.

Example 4: Determine the design snow loads for the upper and lower flat roofs of the proposed building inExample 3, if the upper roof is 40 ft (12 m) wide and the lower roof is 200 ft (61 m) wide (see Fig. E1.3).

Fig. E1.3. Design snow loads for Example 3 (Leeward Drifting)



(Note: This roof configuration forms the greatest snow drift by windblown snow across the lower roof becausethe lower roof is much wider than the upper roof, see Section 2.3.12.4)

a) Items a and b from Example 3 are applicable.

b) Maximum snow load at wall (lower roof) (Section 2.3.12.4):

Max. load at wall = 3⁄4 (Pd) + Pf from Table 3, with Pg = 35 psf and Wb = 200 ft;

Pd = 93 psf, Pf = 32 psf

Max snow load (lower roof) = 3⁄4 (93) + 32 = 102 psf (4.9 kN/m2)

c) Drift width

Wd = 3⁄4 (4hd);

from Table 3, with Pg = 35 psf and Wb = 200 ft; hd = 5.01 ft

Wd = 3⁄4 (4×5.01) = 15 ft (4.6 m)

d) See Figure E1.4 for snow loads on both roofs.

E.2 Roof Drainage and Rain Loading Illustrative Examples

The following examples illustrate the methods used to establish design rain loads and roof drainage for someof the roof drainage systems discussed in the data sheet.

Example 5: A proposed building for Dallas, Texas, has a roof 168 ft (57 m) by 336 ft (102 m), with baydimensions of 28 ft (9 m) by 28 ft (9 m). Joists are spaced 5.6 ft (1.8 m) on center, normal to beams thatspan from column to column. The roof edge has a continuous cant 3-1⁄2 in. (88 mm) high, except a varyingheight parapet, 10-1⁄2 in. (267 mm) max where scuppers are shown. Size the (primary) roof drains andoverflow provisions (using roof edges or scuppers as appropriate), denoting the required hydraulic head atthe primary drainage device (drains), and the total head at the overflow provisions (roof edges or scuppers)and the design rain load to be used by the roof framing designer, when:

Fig. E1.4. Design snow loads for Example 4 (Windward Drifting)



a) The roof is dead-flat with interior roof drains (at mid-bay) and roof edge overflow relief as shown inFigure E1.5.124a.

b) The roof is sloped 1⁄4 in./ft (2%) to the low-point line where roof drains are placed. Overflow relief isby scuppers set 3-1⁄2 in. (89 mm) above the low-point line at the perimeter of the roof as shown in FigureE1.5.2.

Solution (a.) Flat Roof — Figure E1.5.1

1. Rainfall intensity (Appendix C): i = 4 in./hr (100 mm/hr)

2. Number of drains needed (Section 2.5.4.1.11):

n = A = 168 × 336 = 3.8 ≤ 8 (using eight 6 in. [150 mm] dia. drains)15,000 15,000

3. Flow rate needed per drain (Section 2.5.4.1.11):

Q = 0.0104 × i × A = 0.0104 × 4 × 168 × 336n 8

Q = 294 gpm (1110 dm3/min)

4. Roof drain size needed (Section 2.5.4.1.11):

According to Table 5, with 6 in. dia. drain, Q = 540 = 294 gpm (with 150 mm dia. drain,Q = 2040 ≥ 1110 dm3/min)

5. Hydraulic head at drain inlet (Table 8):

Hydraulic head (by linear interpolation) = 2.5 + (294 − 270) (1/2)(380 − 270)

Hydraulic head = 2.6 in. (65 mm) < 3.5 in. (88 mm) roof edge height

Therefore, overflow relief allows the needed drain flow.

6. Total head at roof edge overflow provision (See Fig. 8b):

Total head = Roof edge height

Fig. E1.5.1 Flat roof plan for Example 5



Total head = 3.5 in. (88 mm) ≥ 3 in. minimum head for dead-flat roofs (Section 2.5.2.3)

7. Design rain load (Section 2.5.2.2):

Design rain load (psf) = Total head × 5.2 = 15 psf.

Design rain load = 3.5×5.2 = 18.2 psf (0.86 kN/m2)

8. The flat roof should support the maximum depth of water of 3.5 in. (88 mm) or 18.2 psf (0.86 kN/m2)over its entirety. The roof framing designer should check the roof for instability due to ponding based on thisload. (Note: Snow load should not govern at this location.)

Solution (b.) Sloped Roof — Fig. E1.5.2

1. Items 1 through 5 from Solution (a) are applicable.

2. Number of scuppers needed (Section 2.5.4.1.11):

n = A = 3.8 ≤ 4 (using four 8 in. [200 mm] min. width scuppers)15,000

3. Flow rate needed per overflow scupper (Section 2.5.4.1.11):

Q = 0.0104 i × A = 0.0104 × 4 × 168 × 336n 4

Q = 587 gpm (2220 dm3/min)

4. Overflow scupper size needed (Sections 2.5.4.1.11):

Size of four overflow scuppers to be equivalent to the eight roof drains

Needed flow capacity Q = 8 (540) = 1080 gpm (4090 dm3/min)4

Select equivalent scupper from Table 6 for needed flow capacity Q (Part 4). Assume a scupper, 7 in. (180mm) by 24 in. (610 mm) wide with H = h. Flow capacity (under channel type) Q = 1284 = 1080 gpm (4860= 4080 L/min).

Fig. E1.5.2 Sloped roof plan for Example 5



According to Section 2.5.4.1.8, the scupper height h should be 1 in. (25 mm) higher than the (estimated)water depth H. Check Table 6 for H = 7-1 = 6 in. (150 mm) under channel type (H < h) for the needed flowrate (Part 3):Q = 1020 ≥ 587 gpm (Q = 3860 ≥ 2220 L/min)

5. Hydraulic head (H) at scupper (Table 6):

Hydraulic head (H) = 4 + 587 − 560 = 4.1 in. (103 mm)776 − 560

6. Total head at scupper overflow provision (see Fig. 8b) w/scupper set 3.5 in (88 mm) above roof surface:

Total head = hydraulic head (H) + height to scupper invert

Total head = 4.1 + 3.5 = 7.6 in. (190 mm) ≥ 6in. (150 mm) minimum head at low points for sloped roofs.(Section 2.5.2.3)

7. Design rain load at low-point line (overflow scuppers) (Section 2.5.2.2):

Design rain load (psf) = total head (max) × 5.2 ≥ 30 psf (1.5 kN/m2)

Design rain load (max) = 7.6 × 5.2 = 39.5 psf (1.9 kN/m2)

8. The sloped roof should support a maximum depth of water of 7.6 in. (190 mm) at the low-point line ofthe roof decreasing to 0.0 psf (0 kN/m2) 30.4 ft (9.27m) away from the drains up the valley of the sloped roof.The roof framing designer should check the roof for instability in the roof valley due to ponding based onthe design rain loads.

Snow and live loads must be determined and will govern for at least part of this roof.

Fig. E1.5.3 Sloped Roof Section for Example 5



Example 6: A proposed building to be located in St. Louis, Missouri, has a roof 200 ft (61 m) by 400 ft (122m) and it has six roof drains (at mid-bay). Overflow drains are placed adjacent to the primary drains andset 3 in. (75 mm) above the roof surface. The roof slopes 1⁄4 in./ft (2%) as shown in Figure E1.6. Size the pri-mary and overflow roof drains, denoting the required hydraulic head above the overflow roof drains and thetotal head and the design rain load to be used by the roof framing designer.

a) Rainfall intensity (i) (Appendix C) i = 3.3 in./hr (84 mm/hr)

b) Total number of drains (primary and overflow) needed:

n = A = 200 × 400 = 5.3 ≤ 6 (using six 6 in. dia [150 mm] drains)15,000 15,000

c) Flow rate needed per drain (primary and overflow):

Q = 0.0104 × i × A = 0.0104 × 3.3 × 200 × 400n 6

Q = 458 gpm (1730 dm3/min)

d) Size of the primary and overflow drains (as equivalent) and their hydraulic heads:

Select a 6 in. dia. drain (Table 5); Q = 540 ≥ 458 gpm (150 mm dia. drain;

Q = 2040 ≥ 1730 L/min)

Hydraulic head (Table 8 by interpolation) = 3 + (1⁄2) (458 − 380)(540 − 380)

Hydraulic head = 3.25 in. (83 mm)

e) Total head at overflow drains (Fig. 8a):

Total head = hydr. head (max) + height to overflow drain inlet

Total head (max.) = 3.25 + 3.0 = 6.25 in. (159 mm) ≥ 6 in. (150 mm) minimum head at low points for slopedroofs.

f) Design rain load at low point of roof:

Fig. E1.6. Roof plan for Example 6



Design rain load (psf) = total head (max) × 5.2 = 30 psf (1.5 kN/m2)

Design rain load (max.) = 6.25 × 5.2 ≥ 32.5 psf (1.6 kN/m2)

g) In the low areas of the roof, at the roof drains, the design rain load (max.) will be 32.5 psf (1.6 kN/m2),but it will rapidly reduce with the roof slope of 1⁄4 in./ft (2%) away from the low areas. The roof designershould check the roof for instability in the low areas due to ponding based on the design rain loads.

Example 7: A proposed building to be constructed of tilt-up walls and a plywood diaphragm roof is to be locatedin Santa Cruz, California. The roof is 150 ft (46 m) by 500 ft (152 m) and it has five roof drains near the perim-eter (at mid-bay) and five overflow scuppers in the parapet walls set 4 in. (100 mm) above the low-pointof the roof and 2 in. (50 mm) above the adjacent roof surface. The roof slopes 1⁄4 in./ft (2%) from a high-point line along one side of the building to roof drain cricket areas along the opposite side of the building (seeFig. E1.7). Size the (primary) roof drains and overflow scuppers, denoting the required hydraulic heads atthe drains and scuppers and the total head and design rain load to be used by the roof framing designer.

a) Rainfall intensity (Appendix C): i = 2 in./hr (50 mm/hr)

b) Total number of drains and scuppers needed:

n = A (using five 6 in. dia. drains and five 8 in. wide scuppers [150 mm dia. drains and200 mm wide scuppers])15,000

c) Flow rate needed per drain and scupper:

Q = 0.0104 × i × A = 0.0104 × 2 × 150 × 500n 5

Q = 312 gpm (1180 dm3/min)

d) Size of (primary) drains and their hydraulic head:

Select a 6 in. dia. drain (Table 5); Q = 540 ≥ 312 gpm (150 mm dia. drain, Q = 2040 ≥ 1180 L/min)

Hydraulic head at drains (Table 8 by interpolation)

Hydraulic head at drains = 2.5 + (1/2) (312 − 270)380 − 270

Hydraulic head at drains = 2.7 in. (68 mm)

Fig. E1.7. Roof plan for Example 7.



e) Size of overflow scuppers, equivalent to the roof drains, and their total head (similar to Fig. 8b):

Select an equivalent scupper from Table 6 with flow Q ≥ 540 gpm (2040 L/min):

A ‘‘channel’’ scupper, 4 in. (100 mm) high by 24 in. (610 mm) wide: scupper flow capacity Q = 560 ≥ 540gpm (2120 ≥ 2040 L/min).

According to Section 2.5.5.6, the scupper height h should be 1 in. (25 mm) higher than the (estimated) waterbuildup H. Checking Table 6 for h = 4-1 = 3 in. (75 mm) for the needed flow rates:

Q = 360 ≥ 312 gpm (Q = 1360 ≥ 1180 L/min)

f) Hydraulic head (H) at scupper:

Hydraulic head (H) = 2 + 312 & 200 = 2.7 in. (68 mm)360 & 200

g) Total head at low point of roof = hydraulic head (H) + height to scupper from the roof’s low-point.

Total head at low-point (max.) = 2.7 in. + 4 in. = 6.7 in. (168 mm)

Total head adjacent to scuppers = hydr. head (H) + height to scupper from adjacent roof surface.

Total head at scuppers = 2.7 in. + 2 in. = 4.7 in. (119 mm)

h) Design rain load at roof drains, low-point of roof:

Design rain load (psf) = total head (max.) × 5.2 ≥ 30 psf (1.5 kN/m2)

Design rain load (max.) at drains = 6.7 × 5.2 = 34.8 psf (1.7 kN/m2)

i) Design rain load at scuppers:

Design rain load = total head (max) × 5.2 psf (Note: minimum rain load does not apply because scup-per is not at roof’s low-point.)

Design rain load at scuppers = 4.7 × 5.2 = 24.4 psf (1.2 kN/m2)

j) In the low areas at the roof drains, the design rain load will be 34.8 psf (1.7 kN/m2), but it will rapidlydecrease with the roof slope of 1⁄4 in./ft (2%) away from the low cricket areas. The roof designer should checkthe roof for instability in the low areas due to ponding, based on the design rain loads.



E.3 Job Aids—Snow and Rain Loads and Drainage

Part 1. Roof Projections and Parapets



Part 2. Roof Rain Loads (New Contruction)



Part 3. Roof Drains and Scuppers



Part 4. Roof Drains andScuppers (Continued)



Part 5. Overflow Relief Provisions



Part 6. Overflow Relief Provisions (Continued)(Ref Sect 2.5.5.5)



Part 7. Sloped Roofs



fm global 1 54 roof loads for new construction

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