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

2. Review Ceiling Frame Calc’s

3. 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.

4. Sketch picture below allow 100mm for wall frames (not shown)
Internal room sizes shown
4.100
3.300
5.800
8.350

5. 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

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

7. Hanging beam
= Order 4.5m

8. 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

9. 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

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

11. Variations to ceilings

14. 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

15. Two ceiling designs

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

17. Lean – to – Roof construction P 26
This type is constructed against an existing wall or other roof
Simple extensions
Carports
Verandahs
Awnings

18. Rafter / Ceiling variations
Common on carports, open exposed rafters

19. Other types

20. 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

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

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

23. Gable Roofs and Components

24. Name the components (handout)

25. 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

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

27. Definitions Continued
½ Span : The Plan length of the rafter

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

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

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

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

32. Continue on roofs
Ridge
Rafter
Strut
Purlin
Hanging beam
Ceiling joist

33. 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

34. 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

35. Set Out on Steel Square Calculate rise per / m. run
Transfer to steel square
Reduced rise
Reduced run

36. Set out pattern rafter
By either stepping out with square, or by calculation