# Flat Roofs & Introduction to Gable Roofs

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Flat Roofs & Introduction to Gable Roofs
M. S. Martin / March 2004 / Reviewed & Combined March 2007, Reviewed March 2010

Review Ceiling Frame Calc’s

Spec for example following copy on blank paper
Joists 150 x 38 sawn 600mm centres, max. span 3.6m Trimmers 100 x 38 sawn 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 below allow 100mm for wall frames (not shown)
Internal room sizes shown 4.100 3.300 5.800 8.350

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

Joists 8.350 – = – 1 = say 14 – 1 = 13 Order 6.0m

Hanging beam = Order 4.5m

Trimmers 5.800 – 1 = – Say 10 – 1 = 9 Trimmers at two ends plus internal wall 9 x 3 = 27, 27 x .600 = 16.2 lin/m Order 6.0m, 4.2m

Flat roof construction
Basic Roof & Ceiling Frame text – from P23 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 – P 25 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 construction P 26
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

√ 2.7² ² = √ = √ 8.1 = lin/m Order 3.0m lengths 900 2.700 4.200

Number of rafters (4.200) = = 8 Order 3.0m rafters

Gable Roofs and Components

Name the components (handout)

Answers Ridge Roof Tiles Rafter Tile Battens Collar Tie Hanging Beam
Purlin Anti-ponding Board Strut Purlin Strut Top Plate Strutting Beam Ceiling Joist Top Plate Soffit Bearer Stud Block Cornice Ceiling Lining Internal Wall Fascia Soffit Lining

Gable roof definitions
Roof span: Is the horizontal distance measured overall the wall plates Span Plate Stud Section A-A

Definitions Continued
½ Span : The Plan length of the rafter

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

Centre Line Length : The hypotenuse in the right angled triangle formed by the ½ span and the rise

Common Rafter : Is the member fixed to the ridge with a plumb cut and birdsmouthed over plate, extending to an eave.

Pitch : The amount of slope or gradient in a roof, typically expressed in degrees
30°θ

Continue on roofs Ridge Rafter Strut Purlin Hanging beam Ceiling joist

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

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