Flat Roofs & Introduction to Gable Roofs

Ref:

M. S. Martin / March 2004 / Reviewed & Combined March 2007

Review Ceiling Frame Calc’s

Spec for example followingcopy on blank paper

• Joists 150 x 38 sawn Oregon @ 600mm centres, max. span 3.6m

• Trimmers 100 x 38 sawn Oregon @ 600mm centres

• Hanging beam 240 x 45 hardwood, max. span of 5.4m

• On sketch following, draw in joists and hanging beam to scale and indicate trimmer positions also.

Sketch picture belowallow 100mm for wall frames (not shown)

Internal room sizes shown4.100

3.300

8.350

5.800

Formula's

No. of joists = width of room - 1 spacing

Length of hanging beam = length of room + 2 x wall thickness

Trimmers = length of wall – 1 spacing

Joists

8.350 – 1

.600

= 13.916 – 1

= say 14 – 1

= 13

Order 13 @ 6.0m

Hanging beam

4.150 + .200 = 4.350

Order 1 @ 4.5m

Trimmers

5.800 – 1 = 9.666 – 1

.600

Say 10 – 1 = 9

Trimmers at two ends plus internal wall

9 x 3 = 27, 27 x .600 = 16.2 lin/m

Order 2 @ 6.0m, 1 @ 4.2m

Flat roof construction

• Section 8 – from P306

• Flat roofs combine the roof and ceiling frame in one

• Generally described as one with a pitch less than 10 deg.

• To allow for extra loads the rafters / ceiling jiosts are increased in sectional size

Ceiling types

• Ceilings can be either lined on the rake or rafters laid flat and use graded battens to give fall to roof

Variations to ceilings

Skillion roofs - P308

• Single sloping roofs• Walls at either end of roof at different

heights• Internal walls parralell to end walls will

be at different heights• Walls at 90 deg. to end walls will taper

in height under the sloping rafters / joists

Two ceiling designs

Simplest method of marking

• With skillion roofs the simplest thing is to scribe the rafter over the supporting plates

Lean – to – Roof constructionP309

• This type is constructed against an existing wall or other roof

• Simple extensions

• Carports

• Verandahs

• Awnings

Rafter / Ceiling variations

• Common on carports, open exposed rafters

Other types

Calculation of skillion rafters

(eg) calculate the quantity of 125 x 50 rafters required for the roof 4.2m long and 2.7m wide, rafter spacings 600mm centres, the difference in height of plates is 900mm

Length of rafter = √ (run of rafter)² + (rise)²

No. of rafters = (length of building) + 1 spacings

Answer – length of rafter

√ 2.7² + .900²

= √ 7.29 + .81

= √ 8.1

= 2.846 lin/m

Order 3.0m lengths

2.700

4.200

900

Number of rafters

(4.200) + 1

.600

= 7 + 1

= 8

Order 8 @ 3.0m rafters

Gable Roofs and Components

Name the components (handout)

Answers

Roof Tiles

Tile Battens

Top Plate

Fascia

CorniceCeiling Joist

Ceiling Lining

Strutting Beam

Internal Wall

BlockTop Plate

Soffit Bearer

StudSoffit Lining

Anti-ponding Board

Collar Tie

RafterRidge

Purlin

Strut

Hanging Beam

Purlin

Strut

Gable roof definitions

• Roof span: Is the horizontal distance measured overall the wall plates

SpanPlate

Stud Section A-A

Rise: Is the vertical height measured from the x – y line to where the upper edge of the rafters would intersect

Rise

Continue on roofs

Ridge

Rafter

Purlin

Hanging beamCeiling joist

Strut

Set Out for Common Rafter

• Plumb cut is at the top of the triangle

• We reduce the centre line length of rafter by ½ the thickness of the ridge to get our true length

• Birdsmouth has a maximum cut out of one third the rafter depth

• Level bevel is shown here at base of triangle

• Back of birdsmouth, is outside of frame and if you take a vertical line up, this is where it meets the x – y line

X-Y

LINE

Set Out on Steel Square

• Calculate rise per m. run

• Transfer to steel square

Reduced rise

Reduced run

Set out pattern rafter

• By either stepping out with square, or by calculation