detail patterns · 2020. 3. 7. · tional slope that will cause water to drain properly from a...

PA R T

DETAILPATTERNS

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COPYRIG

HTED M

ATERIAL

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SECTION 1 � FUNCTION 3

S E C T I O N

FUNCTION1

For a building to function well, its details must function well. When designing details for abuilding, there are countless choices to make and no prescribed path toward the best solu-

tion. This portion of the book guides the detailer along this path by describing factors that affectthe functional performance of details.

In architecture, function certainly includes the technical performance of the details that con-tribute to making a building safe and secure for its occupants. But function also includes featuresthat affect the qualities of the forms, surfaces, and spaces that compose the building. A space thatis firm and dry but that has an aggravating echo or glaring light does not function as well as itcould.

The detailer is challenged to address the function needs of the building when it is new butalso long into the future and sometimes beyond the lifetime of those who designed or construct-ed it. Buildings constantly change in response to natural forces, such as the daily cycles of tem-perature and light, as well as in response to seasonal changes. A basic grasp of physics and of bio-logical and chemical processes are part of the detailing process. Other functions concern the peo-ple who engage with the building every day, altering it internally and externally through count-less actions.

The detail patterns that relate to function address the breadth of these topics.They are organ-ized into thematic groups to focus the detailer’s attention on each topic individually. Each pat-tern builds awareness of the issue and includes directions toward possible solutions. The patternsdescribe the natural processes involved, as well as the codes, standards, and conventional prac-tices that are relevant to discovering appropriate detailing solutions.

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INTRODUCTION

For water to penetrate through a building assembly threeconditions must all occur at the same time:

1. There must be an opening through the assembly.2. There must be water present at the opening.3. There must be a force to move the water through the

opening.

If any one of these three conditions is not met, water will notpenetrate the assembly. In designing any exterior detail,therefore, we can pursue one or more of three strategies:

1. We can try to eliminate openings in building assemblies.2. We can try to keep water away from openings in build-

ing assemblies.3. We can try to neutralize forces that move water through

openings in building assemblies.

Complete success in any one of these three strategies willresult in the complete elimination of water leaks. But some-times in detailing we pursue two of these strategies or evenall three of them at the same time, because this gives addedsecurity in case one of them fails due to poor workmanship orbuilding deterioration. Let us consider each of these strate-gies briefly and list the detail patterns that relate to each.

1. ELIMINATING OPENINGS IN BUILDING ASSEMBLIES

Every building is full of openings. A shingled roof has anopening under each shingle. A wall has cracks around win-dows and doors, and around joints between the units ofmaterial from which the wall is made. Additional cracks andholes may form as the building ages and deteriorates. Wecan attempt to eliminate all these openings by using pre-formed gaskets and sealants. As a sole strategy this is unre-liable. Gaskets may not seal securely if they are the wrongsize or resiliency, or if the surfaces they touch are rough orunclean. Sealants may fail to adhere properly if the materi-als to which they are applied are not scrupulously clean andproperly primed or if the installer does not compress thesealant fully into the seam. Both sealants and gaskets candeteriorate from weathering and from the flexing andstretching they may undergo as the building ages. A build-

ing skin that relies on sealants and gaskets alone for water-tightness will leak sooner or later. Furthermore, even a smalldefect in a sealant or gasket that is exposed to the weathercan leak very large amounts of water, just as a small hole ina bathtub can create a very large puddle.

Sealants and preformed gaskets are extremely useful,however, as components of an overall strategy for making abuilding skin watertight. Therefore, it is important to knowhow to detail sealant joints and gasket joints correctly andhow to incorporate them into more complex schemes forcontrolling water penetration. The detail pattern that relatesto eliminating openings in building assemblies is

Sealant Joints and Gaskets (p. 35)

2. KEEPING WATER AWAY FROM OPENINGS INBUILDING ASSEMBLIES

There are a number of effective ways to keep water away fromopenings. Often it is useful to keep most water away from anopening simply to reduce the volume of water that must bedealt with at the opening itself. In many cases we can easilyand securely keep all water away from an opening.

The detail patterns that relate to keeping water awayfrom openings in building assemblies are:

Wash (p. 7)

Overlap (p. 12)

Overhang and Drip (p. 14)

Drain and Weep (p. 18)

Cold Roof (p. 21)

Foundation Drainage (p. 23)

3. NEUTRALIZING FORCES THAT CAN MOVE WATERTHROUGH OPENINGS IN BUILDING ASSEMBLIES

There are five forces that can move water through an open-ing in a wall or a roof: (1) gravity, (2) surface tension, (3)capillary action, (4) momentum, and (5) air pressure differ-entials. In most cases, it is surprisingly easy to detail a build-ing assembly so that all five of these forces are neutralized,and the most secure strategies for keeping water out of abuilding are based on this approach.


C H A P T E RControllingWater Leakage1

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6 PAR T I � DETAIL PAT TERNS

We have already encountered the detail patterns for neu-tralizing two of these forces, because these same patternsare useful in keeping water away from openings in build-ings. The force of gravity is neutralized by

Wash (p. 7)

Overlap (p. 12)

Surface tension, a force that causes water to cling to theunderside of a surface where it can run into an opening, isneutralized by

Overhang and Drip (p. 14)

The patterns for neutralizing the other three forces are:

Capillary Break (p. 25)

Labyrinth (p. 27)

Rainscreen Assembly (p. 28)

Upstand (p. 33)

The capillary break neutralizes capillary action. Thelabyrinth neutralizes momentum, and the rainscreen assem-bly and the upstand neutralize air pressure differentials. Bycombining these seven patterns in each exterior joint of abuilding, we can make a building entirely waterproof.

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A WASH is a slope given to a horizontalsurface to drain water away from vul-nerable areas of a building. In general,every external horizontal surface of abuilding should have a wash. More per-meable materials should have a steeperslope to shed water more quickly.

1. A window or door sill, whethermade of stone, concrete, wood, ormetal, always has a wash to keep waterfrom accumulating next to the door orsash. A minimum slope for this type ofwash is about 1 in. per foot (1:10 or1:12). A steeper slope drains waterfaster and is more secure, because themore quickly water is removed from asurface, the less time it has to leakthrough. It is also more difficult forwind to drive water up a steeper slope.

2. The wash on this concrete chimneycap keeps water away from the vulnera-ble crack between the clay flue tile andthe concrete. The slope should be at least1:12. The outer edge of the cap shouldhave a thickness of at least 3 in. (75 mm)to discourage cracking of the concrete,not the feather edge that is commonlyused (see Clean Edge, p. 179). The crick-et on the upslope side of the chimneyconsists of two washes that divert wateraround the shaft of the chimney. �

Wash


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3. The coping on a building parapethas a wash to keep standing wateraway from the seams in the parapet.Usually the wash drains toward theroof, to minimize water staining of thebuilding faces.

4. The bottom surface in a horizontaljoint between wall panels should havea wash to drain water to the outside.Even if the joint will be closed at theoutside face with sealant, the washshould be provided to discourage leak-ing if the sealant should fail.

5. The sloping roof is a special case ofthe wash. A shingled roof will not shedwater unless it has a considerableslope. If the slope were too shallow,water would linger on the roof andflow around and under the shinglesand penetrate the gaps beneath. Eachtype of shingle material has its own rec-ommended minimum slope. A slopesteeper than the minimum is advisableon exposed sites where rain is oftendriven against the building by wind. Agood rule of thumb is to avoid roofslopes less than 4:12. Wood shingles,asphalt shingles, and unsoldered metalroofing can go as flat as 3:12 with aspecial underlayment (consult theappropriate literature from trade asso-ciations or manufacturers for moreinformation). Steeper slopes shedwater faster, thus are less prone toproblems, but may be more costlybecause the roof area is increased, andworkers will have greater difficultymoving about the steeper surface.Many roofing materials can be installedat a very steep slope, even on verticalsurfaces.

6. So-called flat roofs are seldom flatbut are given a positive slope towardpoints where water is removed by roofdrains or scuppers, because standingwater on a roof can cause deteriorationof the roof membrane and even struc-tural collapse. The correct name for“flat” roofs, in fact, is “low-slope” roofs.Drains in a low-slope roof should belocated either at points of maximumstructural deflection (usually themidspan of a beam or joist) or at lowpoints purposely created by sloping thestructure that supports the roof.

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Tapered insulation or roof fill shouldbe used if necessary to create an addi-tional slope that will cause water todrain properly from a roof. If a drain islocated at a point of maximum struc-tural deflection, the minimum recom-mended slope is 1⁄8 in. per foot (1:100),and more slope than this is desirable. Ifa drain is located at a low point createdby sloping a beam, the overall risealong the length of the beam should beat least twice the expected maximumdeflection in the beam, plus another 1⁄8in. per foot (1:100) of the length of thebeam, to be sure water cannot betrapped by the curvature of the beam.The detailer should work closely withthe structural engineer to design a sys-tem of roof drainage that complies withthese guidelines. This is especiallyimportant if the roof is composed ofcambered elements such as precast con-crete planks or beams.

It is desirable (and mandatory undersome building codes) to provide a com-plete, independent set of auxiliary roofdrains or scuppers to take over in casethe primary drains become cloggedwith debris. The auxiliary drains orscuppers are usually located 2 in. (51mm) higher in elevation than the pri-mary drains and must be served bytheir own network of piping.

7. A rooftop plaza is usually drainedthrough open joints between its dead-level paving stones or tiles. The waterdrops through the joints and is fun-neled to a system of roof drains by thelow-slope roof membrane below. Thesame recommended slopes apply to thismembrane as to any low-slope roof.The plaza paving is held level by small,adjustable-height pedestals that standon the roof membrane and support thepaving units at each intersection. Thesepedestals are marketed in several pro-prietary designs and are usually madeof plastic. �


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8. Another special case of the wash isindicated on architectural drawings bythe note “pitch to drain.” The rain gut-ter at the eave of a roof is usuallypitched (sloped) to drain water towardthe nearest downspout. Commonslopes used for gutters are 1⁄8 in. or 1⁄4 in.per foot (1:100 or 1:50). A steeperslope gives a greater capacity to handlewater in a heavy rainstorm.

9. An industrial or basement floor slabis often pitched toward floor drains toeliminate puddles of standing water. Arule-of-thumb pitch for slab drainage is1⁄4 in. per foot (1:50), but to preventpuddles, this should be increased forsurfaces that are not very flat and canbe decreased for very smooth surfaces.In the case of a floor or paving, howev-er, pitches should not become toosteep, or they will be awkward forpedestrians and vehicles to navigate.

10. If there is no interior floor drain, aresidential garage floor is usuallypitched so water dripping off a car willrun under the garage door and out.Minimum pitch recommendations arethe same as for industrial and base-ment slabs.


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11. Roads, driveways, and walks areusually crowned to shed water in bothdirections and to avoid puddling. Theslope on each side of the crown shouldbe at least 1:200. Parking lots shouldslope at least 1:100 to shed water, butnot more than 5:100.

12. The ground surrounding a build-ing should slope away from the build-ing at a rate of at least 2:100 for at least6 ft (1.83 m). This helps keep waterfrom puddling against the foundationand leaking into basements and crawlspaces.

A wash assures that gravity will actto keep water away from an opening,but its action can be overcome by strongwind currents. Thus a wash that is con-tained within a joint is often combinedwith an air barrier and a pressure equal-ization chamber to form a rainscreenjoint (see Rainscreen Assembly, p. 28). �


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In an overlap, a higher surface isextended over a lower surface so watermoved by the force of gravity cannotrun behind or beneath them. For anoverlap to work, the surfaces must besloping or vertical. Porous materialsneed a greater overlap and steeperslope to be effective.

1. Roof shingles and tiles keep waterout by overlapping in such a way thatthere is no direct path through orbetween them. Each unit covers a jointbetween units in the course below. Theoverlap only works, however, if the roofsurface slopes steeply enough so thatwater runs off before it can find its wayaround the backs of the shingles or tilesto the open cracks beneath.

2. Wood bevel siding sheds water byoverlapping each board over the onebelow. The weak spots in wood sidingare the end joints, which should becaulked and flashed to prevent waterpenetration.

3. Flashings keep water out by over-lapping. Flashing is used to create over-lap wherever the overlap or slope ofbase materials is insufficient to preventwater intrusion. This simple Z-flashingof sheet metal or thin plastic keepswater from coming through the crackabove a window or door frame.

4. This lintel flashing in a masonrycavity wall is another example of over-lapping. Any water that penetrates theouter brick facing is caught by themetal or plastic flashing sheet and isconducted through weep holes to theoutdoors. Notice the overhang and dripon the outside edge of the flashing.These keep water out of the crackbetween the flashing and the steel lin-tel (see Overhang and Drip, p. 14).


Overlap

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5. A reglet (also called a raggle) is anupward-sloping slot in a vertical sur-face into which a flashing or the edgeof a roof membrane may be inserted.The slope (wash) acts to prevent waterfrom being forced into the vulnerablejoint by gravity, and the overlap of theupper lip of the reglet over the flashingkeeps water from reaching the jointbetween the two components. Thereglet shown in this drawing is a tradi-tional type that is largely obsolete; it ismolded into glazed terra-cotta tiles thatare built into a parapet wall by masons.Shims and/or a sealant bead must beinserted into the reglet to hold theflashing or membrane in place.

6. This contemporary type of reglet iscreated in a concrete wall or spandrelbeam by using a preformed strip ofmetal or plastic that is nailed lightly tothe formwork before the concrete ispoured. The opening in the reglet isusually closed temporarily with anadhesive tape or a strip of plastic foamto prevent its being accidentallyclogged with concrete. There are manypatented profiles for this type of regletthat are intended to interlock securelywith a folded edge on the top of theflashing. Diligent inspection is neededjust prior to concrete pouring to be surethat the reglet is installed right side up.

If a reglet is wetted, water may findits way through by capillary action. Acontinuous bead of sealant between theflashing and the reglet can be helpful inpreventing this.

7. There are also a number of patenteddesigns of surface-mounted regletsmade of plastic or metal. A bead ofsealant is intended to keep water frombehind the reglet. This is somewhatrisky, because the success of the detailis entirely dependent on perfect work-manship in installing the sealant andperfect adhesion of the sealant to thewall.

An overlap is generally very effectivein preventing entry of water driven bythe force of gravity. If wind is allowedto blow through an overlap, however, itmay carry water with it. An overlap isuseless against standing water, so itcannot be used on a level surface. �


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Overhang and Drip

Adhering drops or streams of waterrunning down the wall of a buildingcan be kept away from an opening inthe wall by a twofold strategy: (1) cre-ating a projecting profile (an overhang)just above the opening and (2) creatinga continuous groove or ridge in theunderside of the projection (a drip) sothat gravity will pull the adheringwater free of the overhang.

1, 2. The size of an overhang is deter-mined by its function. The width of anoverhang that protects a seam or jointneed not extend far from the face of thesurface it is protecting. An overhangthat is meant to protect a tall exteriorwall must be much wider to be effective.The wider the overhang, the greater thewall area below that will be protected,because wind-driven rain falls at anangle, not straight down. The angle offalling rain during a storm is difficult topredict accurately, but a good rule ofthumb is to add 20 to the wind speed (inmph) at the time of the rain. The sum isthe approximate angle from the verticalof the falling rain. Rain falling with a 20mph (32 kph) wind would fall at anangle of about 40 degrees off of vertical;at 40 mph (64 kph) it would fall atapproximately 60 degrees off of vertical.Greater overhang width also moves thesplash of the water on the ground belowfarther from the wall face, decreasingsecondary wetting and soiling of thesurface.

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1, 2. These are two versions of a doorsill detail: one executed entirely inwood and the other in a combination ofwood and aluminum components.There are two openings that must beprotected in either case: the crackbetween the door and the sill, and thejoint between the sill and the wall ofthe building. The door cannot fit tight-ly to the sill, because a generous clear-ance is required to allow free operationof the door. We would certainly weath-erstrip this crack, but the weatherstripis intended only as a barrier to the pas-sage of air and cannot be relied upon toprevent water from passing. We wouldwant the installer to bed the sill insealant, but the sealant work might beimperfect, and it would deteriorateover time. The overhang and drip is asimple, economical, and highly effec-tive detailing element that shows up inmany kinds of details. In these twodrawings we see it used to protect thetwo openings beneath a door. In thelower part of both these details, the silloverhangs the wall below. In the woodsill detail (1), the drip is simply agroove milled into the bottom of thewooden sill. The groove must be wideenough and deep enough so that a dropof water cannot bridge it: Usually awidth of 1⁄4 in. (6 mm) and a depth of 1⁄8in. (3 mm) are about right. In the alu-minum sill detail (2), the drip is formedby the downturned outer edge of theextrusion. In either case, adheringdrops of water cannot move across thedrip, because, to do so, they wouldhave to move uphill, against the forceof gravity. Therefore, they collect at theouter edge of the drip and fall free.Notice in both cases that the sill has awash to drain water away from thedoor.

On the bottom of the door in bothdetails is a second type of overhang anddrip that protects the crack between the

door and the sill. The overhang is pro-vided by a wooden or aluminum dripstrip that is screwed tightly to the door.The underside of the drip strip is con-figured so that water must drip free atthe outer edge, well clear of the crackbetween the door and the sill. The topof the drip strip, of course, has a steepwash in each case. �


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3. Standard exterior details of woodframe houses contain several examplesof the overhang and drip principle. Theroof shingles overhang the fascia boardand slope upward so that water willdrip clear of the joint between the fas-cia and the shingles. The lower edge ofthe fascia projects below the horizontalsoffit so that water running down thefascia will drip free of the crackbetween the fascia and the soffit. Thewhole eave, of course, is a large over-hang and drip that keeps water off thevulnerable upper edge of the wall andalso gives some protection to windowand door openings. At the base of thewall, a traditional water table detailconsists of an overhang and dripdesigned to keep water out of the crackbetween the wood wall and the foun-dation. Whether or not a water table isused, the bottom edge of the sidingshould be spaced away from the foun-dation wall to create another overhangand drip.

4. The stone or concrete coping atop amasonry parapet wall is sloped towardthe inside of the building to help pre-vent staining and leaking of the outersurface of the wall. A generous over-hang and drip are provided to keepwater out of the mortar joint immedi-ately beneath the coping. Additionally,the metal flashing in this mortar jointprojects outward and downward to pro-vide another overhang and drip.

The seam between the metal coun-terflashing and roof membrane wherethe roof joins the parapet wall is poten-tially troublesome. The counterflashingand roof membrane often fit closelyenough that water entering the seamwould be pulled into it by capillaryaction. The overhang and drip in thecounterflashing profile keep the seamdry. As a backup precaution, the coun-terflashing is also folded out to create aCapillary Break (p. 25). For ease ofinstallation, the counterflashing isoften made in two pieces as shown.The first piece is embedded in the wallby the masons, and the second piece isinserted into the first and screwed to itby the roofing installers.

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5. A drip should always be providedunder the outer edge of an overhangingstory of a building. In a wooden build-ing, the bottom edge of the siding canusually be projected below the soffit toprovide a drip. In a concrete or stonebuilding, a drip groove around theouter edge of the soffit will preventleakage and staining of the soffit area.

6. Internal flashings in masonryveneers sometimes catch and divert rel-atively large volumes of water as themortar joints in the veneer above ageand deteriorate. Each flashing shouldproject completely through the outerface of the masonry by roughly 3⁄4 in.(20 mm) and turn down at 45 degreesto keep the draining water from wet-ting the mortarless horizontal jointbeneath the flashing. The detailershould resist the urge to recess theouter edge of the flashing into the mor-tar joint. This might look better than aprojecting flashing, but it can lead toserious leakage and deterioration prob-lems beneath the flashing.

7. A larger scale overhang and drip inthe form of a porch roof or marqueeoffers the building user the opportunityto leave a door or a window open forventilation or access even during mod-erately severe rainstorms.

The problems in making the cracksaround exterior doors waterproof aresuch that it is not a bad idea to providea small protective roof above everyexterior door in a building. �

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Drain and Weep

It is often wise to include provisions forcollecting and conducting away anywater that may leak through the outerlayer of a building cladding system.This internal drainage system is a frankand useful acknowledgment that thingscan go wrong in sealants, glazing com-pounds, gaskets, mortar joints, andmetal connections, whether caused byfaulty materials, inadequate workman-ship, building movement, or deteriora-tion of materials over time. Such adrainage system also releases anywater that condenses inside the assem-bly or enters it from interior sources. Itis inexpensive insurance against thedamage that can be caused by uncon-trolled leakage and the expense ofrebuilding a wall of flawed design. Aninternal drainage system is comprisedof spaces or channels that conductwater by gravity to weep holes or otheropenings that direct the water backoutdoors.

1. The rafter detail of a traditionalRedwood-framed greenhouse is ex-tremely simple. The sheets of glass thatbear on the rafter are bedded in glazingcompound and secured with a strip ofRedwood held on with screws. This isnot a rain screen detail; any defect in

the glazing compound will result inwater leakage between the glass andthe rafter. Because of surface tension,water that has leaked through will clingto the rafter and run down its sides.This detail furnishes a small drainagegutter milled into the rafter on eitherside to catch this water and conduct itto the bottom of the rafter, where it iswept to the outdoors. Contemporarymanufactured skylights and green-house assemblies have similar integraldrainage features.

2. The outer wythe of a masonry cavi-ty wall is expected to leak water, espe-cially as the mortar joints age and dete-riorate. The leakage drains down thecavity until it encounters an interrup-tion of the cavity such as a window ordoor lintel or the base of the wall. Ateach of these points, a continuousflashing collects the water and drains itthrough weep holes that are providedat horizontal intervals of from 2 to 4 ft(0.6 to 1.2 m).

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3. The horizontal mullion of an alu-minum curtain wall acts as a gutter toaccumulate leakage if the seal betweenthe glass and the glazing gasket isimperfect. Weep holes discharge thisleakage back to the outdoors. A win-dow of average width might have threeweep holes distributed across its sill.

Wind can drive water back througha weep hole if there is not an adequateair barrier between the weep hole andthe interior of the building. This possi-bility can be minimized by locating theweep hole in a sheltered location thatis not likely to become wet and byinserting a baffle behind the weephole. The baffle is made of a nondecay-ing, noncorroding open-celled materialthat allows water to filter out by gravi-ty but slows entering air currentsenough so that they are unlikely to beable to move water through the open-ing. A typical baffle material is a non-woven mat composed of stiff plastic fil-aments.

4. In detailing a rainscreen panel sys-tem, it is important to design a three-dimensional system for draining theopen joints. Especially crucial is thedesign of the intersections of the hori-zontal and vertical joints, which need tobe detailed carefully for ease of assem-bly and for rain-tightness. Any cavitybetween the rainscreen panels and theair barrier wall must also be drained,using much the same detail as for amasonry cavity wall (see detail 2). �

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Unobstructed Drainage

Water sometimes leaks into an assemblybecause of an inadequate or poorlymaintained drainage mechanism.Designers should trace the path of waterfrom where it first contacts the buildingto its point of discharge to be sure thatthe path will be effective. The bestdrainage mechanisms remove the waterswiftly and directly. Special attention iscalled for when the drainage lines areconcealed inside of roof or wall assem-blies, because they are difficult to detectand expensive to repair.

1. Once water is collected in adrainage mechanism, keep it movingsmoothly to the discharge point byavoiding flat slopes or circuitous paths.Even short distances where water is sta-tionary can allow water-borne sedi-ment to collect, possibly slowing theflow of water. Transitions, such as aroof drain junction in a low-slope roof,intersecting sloped roofs, and bends inthe leader that carries water downthrough the building are all pointswhere turbulence is expected and care-ful detailing and installation is neededto avoid obstructions and leaks.

2. Anticipate where an obstruction mayoccur and include features that mini-mize its threat. Use rainfall intensitydata for the building location to calcu-late the volume of water to be carried bythe drainage system; include a safetyfactor in case unusual weather, icedamming, or poor maintenance occurs.The safety factor should double if multi-ple adverse factors are expected. Scalegutters, leaders. and scuppers generous-ly and avoid extreme reductions in thesize of the channel that carries thewater. Provide accessible cleanouts atthe locations where obstruction is mostlikely. Where possible include detailsthat separate waterborne debris fromthe moving water. Filters or strainers atthe point where water enters a roofdrain, gutter, or leader are a commonsolution but require periodic mainte-nance to remove debris.

3. The joints in water drainage sys-tems for precipitation, snowmelt, andcondensation typically are not sealed astightly as plumbing pipes that containwater under pressure. When drainagechannels are obstructed, water may col-lect to sufficient depth to cause thesejoints to leak. Drainage lines concealedwithin a wall cavity or roof assemblyshould be watertight to avoid leaks thatare difficult to find and correct. �

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The outer surface of a roof in a snowyclimate should be kept cold in winter toprevent snow from melting. Roofdrainage systems often become cloggedwith snow and ice during cold winterweather. When melt water that runsdown the roof reaches ice-clogged gut-ters, drains, or eaves, pools can formthat are deep enough to back uparound shingles and flashings andthereby leak into the building. By ven-tilating the underside of the roof deckwith outdoor air, the roof can be keptcold enough so that the snow will notmelt except in above-freezing outdoortemperatures that will also melt thesnow and ice in the drainage system.

1. Most building codes require a pre-scribed minimum amount of net freeventilation area at eave and ridge in asloping roof—commonly 1⁄300 of the areaof the space to be ventilated. A build-ing with a ceiling area of 3,000 squarefeet, for example, would require a totalof at least 10 square feet of net freeventilation opening. Half this amountshould be distributed along the ridge orhigh in the gable ends, and the otherhalf should be distributed along theeaves. These high and low ventilationopenings allow convection to work effi-ciently to remove heat from the roofspace or attic. Appropriate ventilationlouvers for this purpose are availablefrom a number of manufacturers, wholist the net free ventilation area for eachproduct.

2. It is important to detail the roof sothat all ventilation takes place abovethe thermal insulation in the roof. Mostbuilding codes require a minimum of 1in. (25 mm) of air space between theinsulation and the roof sheathing. Inany situation in which there is a chancethat the insulation might accidentallyblock the ventilating cavities beneaththe roof sheathing, vent spacer chan-nels made of foam plastic or paper-board should be used to maintain open

air passages. These channels are espe-cially appropriate when loose fill, battinsulation, or icynene spray-in-placefoam insulation is used.

Roof ventilation also serves to carryaway any water vapor that may escapethrough defects in the vapor retarder orthrough ceiling penetrations, such aslight fixtures and attic hatches. �


Cold Roof

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3. In cold climates, the requiredamount of insulation may exceed theavailable depth provided by ceilingjoists or roof rafters. In this case, araised-heel truss may be the best meansto provide plenty of space for attic insu-lation and ventilation at the eave.

4. Some buildings with low-slope roofsin very snowy environments are fur-nished with a strong horizontal latticeconstruction several feet above the roofthat catches and holds snow, keepingthe roof membrane below free of snowand ice. When the weather is warmenough to melt the snow, the waterdrips through the lattice and is carriedaway by the membrane and roofdrains. The space between the roofmembrane and the lattice is open to theair and is tall enough for inspection andmaintenance as well as free air flow. �


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Basements tend to leak. Water is almostalways present in the surrounding soil.There are always openings in basementwalls: Concrete and masonry founda-tion walls are full of cracks, pores andutility line penetrations, and the jointbetween a basement floor slab and afoundation wall is difficult to makewaterproof. Also present are strongforces to move any water through theopenings, especially hydrostatic pres-sure. Removing water from the soilaround a basement by means of foun-dation drainage is the surest way tokeep the basement from leaking.Foundation drainage has the addedbenefit of reducing or eliminating thewater pressure that tends to collapsethe basement walls. These principlesalso apply to buildings without base-ments because groundwater can alsoharm slabs on grade and crawl spacefoundation systems.

1. Slopes and swales are a first line ofdefense against water around a base-ment. They provide a simple system ofsloping surfaces (washes) of earth orpaving that encourage surface water todrain away from the basement ratherthan toward it. Gradients of 2 to 10 per-cent are recommended for a distance ofat least 6 ft (1.83 m) from the house.

Also part of the first line of defenseare roof drainage systems, eitherperimeter gutters or internal roofdrains, which keep roof water awayfrom the foundation and basement.

2. The second line of defense againstwater around a basement consists ofopen drainage pipes that are laid inporous material at the base of the base-ment wall. Sometimes on very wet sitesdrainage pipes are laid under the floorslab as well. The porous materialagainst the wall may be either crushedstone (of uniform particle size, for max-imum porosity) and/or a thick panel ormat of synthetic material that containslarge internal passages for water. Whenwater moves through the ground

toward the basement wall, it first reach-es the porous layer, where gravity pullsit rapidly downward. As the water accu-mulates at the base of the wall, it entersthe open drainage pipe and flows bygravity either to an outlet down theslope from the building or to a sump inthe basement floor, from which it isejected by an automatic pump.

The drainage pipe has a line of holes orslots in it to allow water to enter. Thefunction of the pipe is to provide anunobstructed lateral passage for waterthrough the crushed stone. Providedthe pipe is placed lower than the slab of

the basement it is protecting, it makesno difference whether the holes faceup, down, or sideways, except thatdownward-facing holes allow water toenter the pipe at a lower elevation thanthe other orientations.

Fine soil particles can be carried intothe drainage layer by water percolatingthrough the soil. Eventually, these par-ticles may clog the pores of thedrainage material. To prevent this, it isgood practice to provide a synthetic fil-ter fabric between the drainage materi-al and the soil. The fabric allows waterto pass freely while straining out thesoil particles. �


Foundation Drainage

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Many exterior building materials arenot completely waterproof. When watercomes in contact with permeable exteri-or materials, such as wood, concrete,stucco, and masonry, some moisturemay migrate into the material and maycontinue to move through the assembly,using porous materials as its path.Moisture can be prevented from movingthrough assemblies by using either acavity or an impermeable barrier.

1. A cavity is simply a void that inter-rupts a path through one or moreporous materials. Moisture that reachesa cavity may evaporate or may draindown into the cavity and be directed bya flashing through weeps to the exteri-or. Cavities are often used in verticalassemblies, such as behind stone ormasonry veneers. Only metal tiesbridge the cavity, and they do not com-promise the moisture break, becausethey are not water permeable.

An impermeable barrier may bemade using a durable flashing material,such as a compatible sheet metal or flex-ible synthetic flashing. Groundwater isprevented from migrating through theconcrete and masonry foundation mate-rials, a process called “rising damp”, bya continuous piece of through-wallflashing. This flashing is installed nearthe elevation of finish grade, where the

foundation meets the superstructure ofthe building.

Some of the precipitation that fallson the coping of this building may enterthe assembly, either through the copingmaterial or through its joints. Through-wall flashing is installed below the cop-ing to isolate its moisture from the wallbelow. Parapet materials below the cop-ing may also get wet, and condensationmay occur in the upper portion of thewall cavity when temperatures fallbelow the dew point. Through-wall

flashing is also installed near the base ofthe parapet to prevent moisture fromthese sources from entering the lowerportions of the wall or the roof assembly.Through-wall flashing at the base of acavity will also need weeps, but theseare not required when through-wallflashing is not used below a cavity.Through-wall flashing enhances resist-ance to moisture intrusion, but it maycompromise the structural integrity ofthe wall so consultation with the struc-tural engineer is advisable. �


Moisture Break

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Water can pull itself by capillary actionacross and even upward through a nar-row crack, but not a wide one. To pre-vent capillary entry of water, we createa capillary break by enlarging a crackinternally to a dimension large enoughso that a drop of water cannot bridgeacross it, at least 1⁄4 in. (6 mm).

1. This drawing shows a vertical edgebetween two exterior cladding panelsthat we want to place only 1⁄8 in. (3mm) apart. If this edge is wetted, waterwill be drawn into the narrow openingby capillary action. When the waterreaches the capillary break, however, itwill be unable to bridge it, and it willnot pass farther toward the interior ofthe building unless pushed by windforces.

2. In this horizontal joint betweenwall panels, a capillary break is createdby enlarging the clear dimension ofthe labyrinth joint in the center of the panels.

3. Traditional detailing of the sill of awood window shows a capillary breakcreated by milling a groove in theunder edge of the sash. �


Capillary Break

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4. There are two capillary breaks inthis detail of an aluminum window:one between the sash and the frame,and another between the aluminumframe and the stone sill.

5. A parapet counterflashing can pullwater by capillary action through thenarrow crack between itself and the

upturned edge of the roofing mem-brane underneath. This possibility canbe avoided by bending the flashing sothat it creates a capillary break.

A capillary break serves only to neu-tralize capillary action as a force thatcan move water through a building

assembly. It is a reliable and usefulcomponent of an overall strategy formaking an assembly watertight, but itis not capable of resisting water pene-tration caused by gravity, momentum,or wind. �


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If a joint is designed so that no straightline may be constructed through itwithout passing through solid material,a raindrop or a snowflake cannot passthrough the joint by its own momen-tum.

1. A windblown raindrop or snowflakepossesses momentum that can move itthrough an opening in a building wall.A raindrop striking this open horizontaljoint between two stone or precast con-crete wall panels, for example, willsplatter water through the joint towardthe interior of the building unless thejoint is configured as a simple labyrinth.

2. This is a labyrinth design for thevertical joints in metal-clad foam com-posite panels.

3. A labyrinth can also be executed inextruded aluminum or other metal.

4. This rigid metal or plastic baffle isanother approach to designing a verti-cal labyrinth joint. It is intended only toblock water driven by momentum, so itfits loosely in the grooves. In this typeof joint, the panel edges are not as frag-ile as in some of the other kinds oflabyrinth joints, and there are no left-hand and right-hand panel edges tokeep track of—both vertical edges ofevery panel are the same.

5. The astragal is a traditional laby-rinth design that is used to keep waterdrops from being blown through thevertical crack between a pair of swing-ing doors.

A labyrinth is a very useful part of anoverall strategy for preventing waterpenetration into a building, but it is notsufficient in itself to prevent the pas-sage of windblown water or snow; itmust be combined with an air barrierand a pressure equalization chamber(see pages 28 and 44). �


Labyrinth

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A detail that blocks air currents frompassing through a joint will preventwater from being pushed through thejoint by air pressure differentials. Forthe same reason that you cannot blowmuch air into an empty soft drink bot-tle, wind and water cannot readilyenter a joint made in this way.

1. By using a combination of Wash,Labyrinth, Capillary Break, and Overhangand Drip, we can design a low-mainte-nance wall or window joint that willresist the entry of water driven by theforces of gravity, momentum, capillaryaction, and surface tension. If this jointis wetted, however, and if a current ofair is passing through from outside toinside, the air current can blow orpump water and vapor through thejoint. To look at it another way, the pas-sage of the air current indicates that theair pressure outside the joint is higherthan the air pressure inside. This differ-ence in pressure represents potentialenergy that can move water from out-side to inside. Such differences in pres-sure exist on every building exposed to wind, which is why most water leaksin building skins occur in windy, rainyweather.


Rainscreen Assembly

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2. This is the same pair of details as inthe previous drawing, with the additionof a bead of sealant along the interioredge of the joint. We will assume for themoment that the sealant is perfectly air-tight. Now air can pass in and out of thejoint, but it can no longer pass throughit. If a sudden gust of wind raises thepressure on the outside of the wall, airwill be forced into the open interior ofthe joint by the increased external pres-sure. After only a very small amountof air has moved into the joint, however,the air pressure inside the joint willequal the air pressure outside, andair movement will cease. Because thetwo air pressures are equal, there is noenergy available to pump water, sowater will not penetrate past the joint.Damp-tolerant materials (see RobustAssemblies, p. 141)mustbeused tomakethe joint. The sealant joint in this detailnever becomes wet; it serves only as anair barrier. The large capillary breakinside the joint has now taken on a sec-ond function: It works also as a pressureequalization chamber (abbreviatedPEC), a container of air that is main-tained at the same pressure as the air out-side the wall by tiny movements of air inand out of the joint.

The wall panels themselves arereferred to as a rainscreen, meaningthat they act to screen out rainwaterexcept at the joints. The entire assem-bly of rainscreen, air barrier, and PEC isknown as a rainscreen assembly, andthe principle by which it works isknown as the rainscreen principle.

3. Let us look one more time at thesame joint, but this time let us assumethat there is a defect in the sealant.Perhaps the sealant never adheredproperly to one of the panels, or per-haps it has grown old and cracked, cre-ating a small opening through whichair or water can pass. Unless thesealant falls completely out of the joint,however, it will prevent most air frompassing, and the small amount of airthat does pass will not be sufficient to

disrupt seriously the automatic pres-sure-equalizing action that prevents thepumping of water through the joint. Asa rule of thumb, if the total area ofleaks in the air barrier is no larger thana tenth of the total area of the openingsthat the air barrier protects, leakage isunlikely.

Contrast this with a defectivesealant installation on the outside of thesame joint instead of on the inside. Thesealant will be bathed with water dur-

ing a rainstorm, and water will beforced through the defect by even smalldifferences in air pressure betweeninside and outside. This demonstratesthat the outside of a joint is not theplace to install an air barrier, becausein this position the air barrier onlyworks if it is perfect. The proper loca-tion for an air barrier is on the inside ofthe joint, where it is always dry andwhere small holes, cracks, or otherdefects will not impair its action. �


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4. This is the sill of an ordinary woodwindow. The detail to the left incorpo-rates all the principles we have identi-fied so far for keeping water from pen-etrating: There is a Wash on the sill toprevent water entry by gravity and anOverhang and Drip beneath it to pre-vent water entry by surface tension.The L-shaped crack between the sashand the sill is a Labyrinth that elimi-nates momentum as a force that canmove water through the window unit.The groove in the bottom of the sash isa Capillary Break. With the addition ofa reasonably airtight weatherstrip atthe inside end of the crack, the groovebecomes also a pressure equalizationchamber (PEC), and wind forces areneutralized. This detail represents acomplete strategy for keeping waterout—a true rainscreen detail.

The detail to the right differs fromthe one to the left only in the locationof the weatherstrip. If the weatherstripin this example has even a small leak,water can be forced through it duringwind-driven rainstorms and can easilybe pumped up onto the window stoolinside.

5. To the left is a door sill that repre-sents a complete rainscreen strategy forpreventing water penetration. The PECis the space under the aluminum dripstrip. The air barrier is a weatherstripon the inside face of the door (it couldalso be inside the crack). The rain-screen is the door itself.

The sill detail to the right shows anavailable type of drip strip that incorpo-rates a synthetic rubber weatherstrip.The weatherstrip is placed just to theinside of the PEC and will remain dryand effective in this location. If it wereplaced, instead, at the outside edge ofthe drip strip, the entire detail wouldbe unreliable.

6. The left-hand detail represents a hor-izontal joint between two metal panelsof a curtain wall system. It includes aWash, a Labyrinth, and an internal Drip.An air barrier is provided by two syn-thetic rubber gaskets that are insertedinto a narrow aluminum channel justbehind the metal panel. This is a rain-screen detail. Even if the gasket does notseal perfectly, this detail will not leak.


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(Rainscreen wall designs are oftenreferred to in manufacturers’ literatureas pressure-equalized wall designs.)

The right-hand detail is not a rain-screen detail. It relies completely on theintegrity of the sealant joint. It is muchsimpler and less expensive to manufac-ture and install, but it is unreliable,because any defect in the sealant willcause a water leak.

7. This is a sill detail from an alu-minum-and-glass curtain wall system.It has a synthetic rubber gasket that islocated on the outside of the glass. Ifthe gasket is slightly defective, waterwill move past it and into the interioraluminum channel beneath the glass.The manufacturer of the wall systemhas anticipated this possibility, howev-er, and has provided weep holes thatwill allow the leakage to drain back tothe outdoors. Furthermore, there is alsoa gasket on the inside face of the glassthat acts as an air barrier, preventingthe water from being pumped farthertoward the inside of the building. Inother words, if the external gasketleaks, this detail functions as a rain-screen detail. In a detail like this theexternal gasket is called a deterrentseal, because its role is only to deter thepassage of as much water as possibleand not to act as a perfect seal againstall water penetration. The internal gas-ket is called an “air seal” to indicatethat it functions as an air barrier in arainscreen detail.

8. The traditional masonry cavity wallis a rainscreen design. The outer wytheof masonry is the rainscreen. The cavi-ty is the pressure equalization chamberif it is compartmented. The inner wytheof masonry is the air barrier, and theweep holes provide not only fordrainage but also for the passage of airto equalize air pressure between thecavity and the outdoors.

9. This drawing represents an adapta-tion of the cavity wall rainscreen designto a building faced with story-high pan-els of cut stone or precast concrete. Theair barrier wall is composed of steelstuds and gypsum sheathing, coveredwith a rubberized asphalt mastic coatingto make it airtight and water resistant.

In looking at these last two rain-screen designs with their large PECs, wecan make three observations that are

important for the detailer to keep inmind.

First, the air barrier, whether it is abackup wall, a gasket, or a bead ofsealant, supports all the wind load onits proportion of the face of the build-ing. Every air barrier must be engineeredto support full wind load. In a masonrycavity wall, the backup wall, not thefacing, supports the wind load. In thestone or precast concrete wall shownhere, regardless of the stiffness of thepanels, the metal studs must be engi-neered to withstand the full wind load.At a door sill, the weatherstrip must besufficiently stiff to resist the force ofwind upon it. Fortunately, the area ofthe weatherstrip is small so the totalwind force on it is similarly small, butthe backup wall is large in area andmust absorb a large load. �


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Second, wind pressure varies consid-erably across the face of a building. In afreestanding tall building, pressures aremuch higher at higher stories of thebuilding than at lower stories, and pres-sures near the edges of a facade aremuch different than those in the middle;often some areas of a wall are subject tosuctions rather than positive pressuresbecause of the aerodynamics of a build-ing. Buildings in urban settings often

have highly variable pressure gradientsnear adjoining buildings. Because ofthis, it is important to divide buildingfacades into compartments.Compartments may be larger in the cen-tral portion of a facade, but they shouldbe relatively small at building edges andparapets. Air pressure in a smaller com-partment more quickly equalizes withthe outside air pressure, reducing thevolume of air and water entering. The

largest compartments may be up to twostories high and one structural bay inwidth. If the PEC is not compartmented,air can rush from one part of the build-ing facade to another within the PECand cause localized pressure differen-tials that may result in water leakage.The divisions between the compart-ments need not be absolutely airtight,but they should be designed to choke offmost airflow. The dividers can be madeof masonry, sheet metal, compressiblefoam, or any other material appropriateto the wall construction system.

Third, every pressure equalizationchamber, whether small or large, mustbe drained and wept to the outdoors todispose harmlessly of any water thatmay enter (see Drain and Weep, p. 18).

The rainscreen approach cannot beapplied to solid walls because a solidwall, by definition, cannot contain apressure equalization chamber. Solidmasonry or concrete exterior walls arethought of as face-sealed “barrierwalls,” meaning that they are so thickand so well constructed that they areunlikely to leak. The barrier wallapproach is far from foolproof, howev-er, because a single crack can allowwater to enter the building. �


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An upstand is simply a dam. The prin-ciple of the upstand is that wind pres-sure can drive water uphill only to aheight at which the hydrostatic pres-sure of the standing water retained bythe dam is equal to the pressure exert-ed by the wind. We use an upstand indetailing when it is impractical to pro-vide a reliable air barrier to preventwater from being driven through a hor-izontal crack by air pressure differen-tials. This can happen in situationswhere installation access to the properlocation for an air barrier is blocked bya spandrel beam or a column. It canhappen at the sill of a door or a windowas a gasket or weatherstrip ages, wears,and begins to leak large volumes of air.Sometimes we just want to be doublesure that a detail will not leak. In any ofthese cases, a simple upstand can serveto prevent wind pressure from pushingwater through a horizontal joint, evenif the joint is totally unsealed againstair leakage. The required height of anupstand is determined by the maxi-mum expected wind pressure. To findthe wind pressure, find the design windspeed for the building location in theappropriate building code. Then deter-mine the necessary height of theupstand according to the accompany-ing table, interpolating as necessary.

1. A manufacturer of sliding glassdoors recognizes that if the interiorweatherstrip becomes sufficiently wornwith years of use, the rainscreen actionof the sill detail may become inopera-tive. A 2 in. (51 mm) upstand at theinterior side of the door offers a degreeof backup protection by preventingleakage up to a maximum wind pres-sure of 10 psf (480 Pa), equivalent to a60 mph wind (100 km/h). A tallerupstand would offer even more protec-tion against leakage, but this advantagemust be weighed against the increasedtripping hazard of a taller sill. �


TABLE 1-1: Required Height of Upstands

Approximate Wind Speed Wind Pressure Upstand Height

45 mph (70 km/h) 5 psf (240 Pa) 1" (25 mm)

60 mph (100 km/h) 10 psf (480 Pa) 2" (51 mm)

90 mph (145 km/h) 20 psf (960 Pa) 4" (102 mm)

110 mph (175 km/h) 30 psf (1,440 Pa) 6" (152 mm)

125 mph (200 km/h) 40 psf (1,920 Pa) 8" (203 mm)

140 mph (225 km/h) 50 psf (2,400 Pa) 10" (254 mm)

Upstand

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2. This horizontal joint between metalcurtain wall panels has a 3 in. (75 mm)upstand, giving protection againstwater penetration at wind pressures ashigh as 15 psf (720 Pa), even if the gas-ket should be inadvertently omittedduring installation.

3. To be absolutely safe against waterbeing pumped back through a weephole by wind pressure, the hole can bedrained through a vertical weep tubethat exits the wall a distance below thepoint that is being drained. If there is avertical distance of 10 in. (254 mm)between the inlet and outlet of a weeptube, for example, it would take a windof approximately 140 mph (225 km/h)to pump water up and into the buildingthrough the tube. This is the principleof the upstand applied in a slightly dif-ferent manner, using the same table toequate heights of water to pressures ofair. Tube diameter must be at least 3⁄8 in.(9.5 mm) for good drainage; this isalso large enough to prevent capillaryentry of water.

When detailing an upstand, remem-ber that its ends must be dammed care-fully at vertical joints or the water willsimply drain out of the ends to becomeunwanted leakage. In aluminumcladding details, end dams are oftenplugs molded of synthetic rubber. �


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Sealants and gaskets are elastic materi-als that can be placed in a joint to blockthe passage of air and/or water whileallowing for relative movementbetween the two sides of the joint. Agasket is a strip of synthetic rubber thatis compressed into the joint. Mostsealants are mastic materials that areinjected into the joint and then cure toa rubberlike state. A gasket sealsagainst a surface by compressing tight-ly against it. A sealant seals by adher-ing tightly to the surface.

1. The width and depth of a sealantjoint must never be left to chance; theyshould be determined in accordancewith the procedure shown in ExpansionJoint, (p. 94). The plastic-foam backerrod is a very important part of everysealant joint: It limits the depth of thesealant to the predetermined dimen-sion, provides a firm surface againstwhich to tool the sealant, and impartsto the sealant bead the 1:2 hourglassshape that optimizes the strength andelasticity of the sealant. The backer rodshould be at least 20 percent largerthan the maximum joint width.

2. If the sealant joint is too narrow,normal amounts of movement betweenthe adjoining components can over-stretch the sealant and tear it.

3. If the sealant bead is too deep,stresses in the bead will be excessiveand tearing is likely.

4. Tooling forces the sealant materialto fill the joint, assume the desired pro-file, and adhere to the adjoining com-ponents. �


Sealant Joints and Gaskets

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5, 6. In a three-sided sealant joint,bond-breaker tape should be appliedagainst the back of the joint to allow forfull extension of the sealant bead whenthe joint opens.

7. If a sealant joint is too narrow, thesealant may become overcompressed,squeezing it out of the joint and tear-ing it.

8. Sealant should be applied at an airtemperature that is neither too hot nortoo cold. If application at very hot orvery cold temperatures is anticipated,the initial joint width should be adjust-ed to compensate for the seasonal over-stressing that might otherwise occur.


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9. A sealant lap joint may be dimen-sioned using the same procedures as fora butt joint.

10. Even if perfectly installed, sealantjoints may fail as the material ages andbecomes less elastic. Added protectionand durability can be provided by atwo-stage drained sealant joint. Thisconsists of the careful installation ofbacker rod and sealant within the jointand recessed behind the outer sealantjoint to produce a small cavity. Two-stage joints require minimum jointwidth of 3⁄4 in. (19 mm) for sufficientaccess by the installer. Vertical two-stage joints must be detailed to drainany water at the bottom of the joints.The cavity between sealants can also bepressure-equalized if the inner seal isairtight.

11. There are many types of glazingdetails that include wet (gunnable)sealants. In general, these incorporatesynthetic rubber spacers that regulatethe depth and thickness of the sealant,according to the principles laid out ear-lier. In the detail to the left in the draw-ing, the glass is set on synthetic rubberblocks and centered in the metal framewith the aid of compressible spacerstrips that also serve as backer rods.This detail minimizes the number ofdifferent components needed to installthe glass by eliminating any gaskets.The detail to the right uses a preformedsynthetic rubber gasket on the interiorside for easy installation and a neatappearance. The outside is sealed witha gunnable sealant for maximum secu-rity against leakage. �


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12. This is an example of a preformedsynthetic rubber gasket used to close amovement joint in a high-traffic hori-zontal surface, such as a roadway or aparking garage. The gasket is slightlywider than the joint and must be com-pressed during installation.

13. Preformed gaskets are widely usedto seal between window glass andmetal framing. In this example, aclosed-cell sponge gasket is insertedfirst; then the glass is inserted. Finally,a dense gasket in a roll-in wedge profileis forced between the inside face of theglass and the frame, compressing thesponge gasket and holding the entireassembly together. For additional secu-rity against water penetration, a beadof gunnable sealant is sometimesplaced over the outside gasket. This iscalled a cap sealant.

14. There are many types of syntheticrubber lockstrip gaskets that are usefulin glazing. This example incorporates apine tree spline that is inserted into aslot in a concrete sill or jamb. The glassis installed in the gasket, and then thesynthetic rubber lockstrip is insertedwith a special tool to make the gasketrigid and lock the glass in place.


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15. Preshimmed tape sealants aremade to be compressed between com-ponents of nonworking joints. The tapeis thick and very sticky. The semirigidshim rod in the center of the tape con-trols the thickness of the joint and lim-its the tendency of the surroundingmastic to squeeze out.

16. The waterstop is a preformed syn-thetic rubber gasket used to seal pourjoints and movement joints in concretefoundation walls. The example shownhere features a center tube that allowsthe waterstop to stretch or compressconsiderably in response to movementin the concrete walls. Many othershapes of synthetic rubber waterstopsare also manufactured, along withalternative designs made of rigid plas-tic, metal, mastic, even bentonite clay,which expands and seals when wetted.

Glazing and cladding details are usu-ally developed by manufacturers ofglazing and cladding systems ratherthan by detailers in architectural offices.However, it is important for designersand detailers to have a good grasp ofdetailing principles so that they are ableto assess manufacturer’s systems andinstalled work in the field. �


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PROPORTIONING SEALANTJOINTS

Sealant joints should be provided atfrequent enough intervals in a surfaceso that the expected overall movementin the surface is divided into an accept-ably small amount of movement ineach joint. Usually sealant joint spacingis determined by the desired sizes ofthe panels or sheet materials that makeup a wall.

Generally, a sealant joint should notbe narrower than 1⁄4 in. (6 mm). A joint

narrower than this is difficult to makeand has little ability to absorb move-ment. Joints can be as wide as 1 to 2 in.(25–51 mm), depending on the abilityof the sealant not to sag out of the jointbefore it has cured. The depth ofsealant in a joint should be equal tohalf the width of the joint but not lessthan 1⁄4 in. (6 mm) or more than 1⁄2 in.(13 mm). Thus, a 1⁄4 in. wide jointshould be 1⁄4 in. deep (6 � 6 mm), a 3⁄4in. wide joint should be 3⁄8 in. deep (19� 9 mm), and a 11⁄4 in. wide jointshould be 1⁄2 in. deep (32 � 13 mm).

To determine the required width fora sealant joint in a particular locationin a building, many factors must beconsidered. The spacing betweenmovement joints, the particular mate-rials used, and the climate at the build-ing location are some of the factors. A complete discussion of this topic,including example calculations ofsealant joints, follows in DeterminingWidths of Sealant Joints.


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Calculations of expansion joint intervalsand sealant joint widths are interde-pendent. The width of a sealant jointshould be determined by the designer ofthe building, the detailer, the specifica-tions writer, the suppliers of the compo-nents or materials on either side of thejoint, and the structural engineer. Thesecollaborators work together using allavailable information on temperatureextremes at the building site, the time ofyear when the sealant will be installed,the properties of the materials on eitherside of the joint, the properties of thesealant itself, and the structural charac-teristics of the frame and skin of thebuilding. For preliminary purposes, thefollowing equation may be used todetermine the width of any sealantjoint:

100W = (�L�T + Mo) + tX

where

W = required width of sealant joint

X = percent plus or minus movementcapability of sealant, expressed as awhole number

� = coefficient of expansion of skinmaterial

L = length of building skin betweenjoints

�T = annual range between extremehigh and low temperatures. If specifictemperature data are lacking, assumethat �T is 130°F (54°C)

Mo = anticipated movement due to suchnonthermal factors as structural deflec-tions, creep, or moisture expansion andcontraction

t = construction tolerance

This formula may be used with eitherconventional or SI units. Following arethree examples of its use.

SEALANT JOINT WIDTHCMALCULATIONS

Example 1 Calculate the required widthof a horizontal sealant joint for an all-aluminum curtain wall panel, dark gray

in color, that is 6 ft 8 in. or 80 in. (2032mm) high. The temperature rangesannually between –40° and + 100°F(–40° and +38°C). The building isframed with steel. A sealant with amovement capability of 125 percent isrecommended by the wall panel manu-facturer.

The annual range of air temperatureis 100° to –40°F = 140°F (78°C), butthe sun will heat the dark-colored panelto well above the air temperature. Asan estimate, we will add 40°F (22°C) tothe temperature to account for thisphenomenon, making a total tempera-ture range of up to 180°F (100°C). �

Determining Widths of Sealant Joints

in. /in./°F mm/mm/°C

Wood (seasoned)

Douglas fir parallel to grain

perpendicular to grain

Pine parallel to grain


Oak parallel to grain


Maple parallel to grain


Masonry and Concrete

Limestone

Granite

Marble

Brick

Concrete masonry units, normal aggregate

Concrete masonry units, lightweight aggregate

Concrete

Metals

Steel

Stainless steel, 18-8

Aluminum

Copper

Tin

Titanium

Zinc

Finish Materials

Gypsum board

Gypsum plaster, sand

Glass

Acrylic glazing sheet

Polycarbonate glazing sheet

Polyethylene

Polyvinyl chloride

TABLE 6-2: Coefficients of Linear Thermal Expansion of CommonBuilding Materials

0.0000038

0.0000580

0.0000054

0.0000340

0.0000049

0.0000540

0.0000065

0.0000486

0.0000079

0.0000085

0.0000131

0.0000065

0.0000094

0.0000077

0.0000099

0.0000117

0.0000173

0.0000231

0.0000168

0.0000290

0.0000090

0.0000310

0.0000162

0.0000126

0.0000090

0.0000742

0.0000796

0.0001530

0.0000720

0.0000021

0.0000320

0.0000030

0.0000190

0.0000027

0.0000300

0.0000036

0.0000270

0.0000044

0.0000047

0.0000073

0.0000036

0.0000052

0.0000043

0.0000055

0.0000065

0.0000099

0.0000128

0.0000093

0.0000161

0.0000050

0.0000172

0.0000090

0.0000070

0.0000050

0.0000410

0.0000440

0.0000850

0.0000400

04_488178_ch01.qxp 8/30/06 1:24 PM


The structural engineer estimatesthat deflections of the spandrel beamsand columns under live and wind load-ings can total as much as 0.04 in. (1mm) per panel.

The construction tolerance, theaccuracy of the aluminum panels asinstalled on the building, is estimated bythe curtain wall contractor to be ± 1⁄8 in.(3.2 mm).

From the accompanying table, wedetermine that the coefficient of ther-mal expansion of aluminum is0.0000128 in./in./°F.

Starting with the given equation

100W = (�L�T + Mo) + tX

and substituting,

100W = [(0.0000128 in./in./°F)(80 in.)25(180°F)+0.04 in.] + 0.125 in.

we have W = 1.02 in.; use a 1 in. widesealant joint or 11⁄8 in., if we wish to beconservative. The depth should be 1⁄2 in.

This example may be worked in SI(metric) units using the same formulaand procedure, so long as all the units oflength are consistent and the tempera-ture is converted from Fahrenheit toCelsius.

100W = [(0.0000231 mm/mm/°C)25(2032 mm)(100°C)+1 mm]

+ 3.2 mm

we have W = 25.98 mm; use a 26 mmwide sealant joint. The depth should be13 mm.

Example 2 Calculate the required widthof a sealant joint between white granitewall panels that are 4 ft 7 in. or 55 in.(1397 mm) in maximum dimension.The annual range of air temperature isfrom –10° to 110°F (–23° to 43°C). Thebuilding structure will be of reinforcedconcrete, and the structural engineerestimates that creep in the frame willeventually reach about 0.03 in. (0.76mm) per panel, but that structuraldeflections will be insignificant. Thesealant will have a movement capabilityof ±25 percent. The supplier andinstaller of the granite panels expectto work to an accuracy of ±3⁄16 inch (4.76mm).

From the table on p. 41, we find acoefficient of thermal expansion forgranite of 0.0000047 in./in./°F.Starting with the given equation

100W = (�L�T + Mo) + tX

and substituting,

100W = [(0.0000047 in./in./°F)(55 in.)25(120°F)+0.03 in.] + 3⁄16 in.

we have W = 0.43 in.; use a 1⁄2 in. joint.A depth of 1⁄4 in. is suitable.

Working in SI (metric) units:

100W = [(0.0000085 mm/mm/°C)(1397 mm)(66°C)+0.76 mm]

25

+ 4.76 mm

we have W = 10.93 mm; use a 11 mmjoint. A depth of 6 mm is suitable.

Example 3: Calculate the required widthof a vertical sealant joint in a brick wallwith a joint spacing of 21 ft 4 in. or256 in. (6.5 m or 6500 mm). The airtemperature range is up to 108°F(60°C). The contractor would like touse a sealant that has a movementcapability of ±12.5 percent. Accordingto Technical Note No. 18 of the BrickIndustry Association, brickwork willexpand over time by about 2⁄100 of 1 per-cent due to moisture absorption. A con-struction tolerance of ± 1⁄4 in. (6 mm) isexpected.

According to the table, the coeffi-cient of thermal expansion of brickmasonry is about 0.0000036 in./in./°F(0.0000065 mm/mm/°C). Startingwith the given equation

100W = (�L�T + Mo) + tX

and substituting,

100W = [(0.0000036 in./in./°F)(256 in.)(108°F)+(256 in.)

12.5

(0.0002)]+ 1⁄4 in.

we have W= 1.45 in.

Working in SI (metric) units:

100W = [(0.0000065 mm/mm/°C)(6500 mm)(60°C) +(6500 mm)

12.5

(0.0002)]+ 6 mm

we have W=36.7 mm

This is very wide, nearly 11⁄2 in.—which would make sealant installationdifficult. If a sealant with a ±25 per-cent movement capability were usedinstead, the joint would only need to be7⁄8 in. wide (22 mm) wide, which couldbe rounded up to 1 in. (25 mm). If anarrower joint is desired, then anothersealant with even greater movementcapability could be selected. �

04_488178_ch01.qxp 8/30/06 1:24 PM

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