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Flat Roof & Introduction to Gable Roof

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1 Flat Roof & Introduction to Gable Roof
Created by Michael March 2004 / reviewed march 2008

2 Flat roof construction
P23 & 24 in Carp 11 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

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

4 Variations to ceilings



7 Skillion roofs 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

8 Two ceiling designs

9 Lean – to – Roof construction
This type is constructed against an existing wall or other roof Simple extensions Carports Verandahs Awnings

10 Rafter / Ceiling variations
Common on carports, open exposed rafters

11 Other types

12 Simplest method of marking
With skillion roofs the simplest thing is to scribe the rafter over the supporting plates

13 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

14 Answer – length of rafter
√ 2.7² ² = √ = √ 8.1 = lin/m Order 3.0m lengths 900 2.700 4.200

15 Number of rafters (4.200) + 1 .600 = 7 + 1 = 8 Order 3.0m rafters

16 Roof Components – Gable roof

17 Component names Common rafter Ridge Collar tie Strut Purlin Tail piece
Ceiling joist or overhang ½ Span Plumb cut Span Eave width or eave overhang

18 Gable roof definitions
Complete answers on your work sheets Roof span: Is the horizontal distance measured overall the wall plates Span

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


21 Other Reference Other reference for gable roof definitions and sketch – carp 11, pages 27 to 30

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