1. Gable Roofs and Components
Created by Michael Martin April 2004 / revised April 2007 / Reviewed pictures added - March 2010

2. Continue on roofs - Components
Ridge
Rafter
Strut
Purlin
Hanging beam
Ceiling joist

3. Remember set out for common rafter – Text P.30
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

4. 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

5. Review calc’s for rafter length
Calculate lengths of a rafter, with pitch of 22° and half span of 4.250m (timber frame)
Rise per m. run
Tan 22° = 404mm
True length per m. run
√ .404² + 1.0² = 1.079
Centre line length of
Rafter
½ span x true length per m.
4.250 x 1.079
= 4.586
Order length, if say 450 eave
( ) x 1.079
= 5.071
Order 5.1m length’s

6. Calculate lengths of rafter below
For roof pitch of 26° and span of 6.200m, 450mm eave (timber frame)
Rise per m. run
Tan 26° = 488mm
True length per m
√ .488² + 1.0² = 1.113
Centre line length
3.1 x = 3.450
Order length
( ) x = 3.951, order 4.2m lengths

7. Ridge, Wind braces,Purlins & Struts
These areas covered in your text (carp 11 Basic Roof & Ceilings) from page 30
RIDGE: is the highest member of the roof, traditionally narrow in sectional size and runs horizontally for the roofs length

8. Ridge

9. Ridge – P. 30
If there is a need to join it, it can be a scarf joint, or a butt joint.
Splices usually have a fish plate on each side

10. Ridge continued – P. 31
Set out of ridge is done before it is erected, often only on one side
To suit the rafter positions
It can be marked manually or lay it over ceiling frame at plate position and transfer marks

11. Purlins or Underpurlins – P. 32
These members are fixed to the underside of the rafters, parallel to ridge
They support the rafters continuously mid-span
Typically, maximum 2100mm span

12. Purlin

13. Positioning purlins – on underside of rafter – P. 32
This is done by measuring up from the wall plate at required spacings
Mark top side of purlin
Run a string line across length, put temp. nail on every 3rd rafter
Lift purlin into position & clamp
Double skew nail to rafters

14. Joining purlins Purlins are supported by struts
Spaced at 2100mm centres, depending on size & grade
With additional strut under any join
Most common method of joining, is by half lap and nailing

15. Struts – P.33 These members are placed under the purlins
They transfer the roof load to one of two places, typically
(1) can be transferred to internal load bearing walls, or
(2) can be transferred to a strutting beam
Chocks support base of strut and at top of fan struts

16. Size and type of struts
Typically struts are 75 x 75mm, but will vary depending on load, stress grade and type of timber used
Most common struts are inclined and fan struts, sometimes called flying struts

17. Fan Struts

18. Fitting at angles - AS1684 – P. 34
It is not always possible to fit struts at 90° you should become familiar with tolerances on page 34

19. Angles continued

20. Supporting struts over internal walls – P. 35
It is preferred to position struts directly over studs, but not always possible
Alternate distribution of load is shown below
Your reference

21. Other supports

22. Scissor struts – P. 36
These are deep sectioned timbers, supported over external walls and bolted
Can be full scissor or half

23. Half Scissor Struts

24. Half scissor strut

25. Support to scissor struts
The foot of the scissor strut must be bolted to a rafter
Preferably a ceiling joist as well
As well as in the centre where they cross
Sizes should be taken from AS 1684

26. Parallel strutting beam

27. Strutting beams
Over large spans, it may be necessary to use a strutting beam
Usually placed parallel with the hanging beams in the roof space
But, must not rest on any part of the ceiling frame
To achieve this the ends are packed up at least 25mm above the ceiling joists
If sizing meets the code, you may also combine the member to become a
hanging / strutting beam

28. This one is being used as a hanging / strutting beam

29. Patent type struts Used where conventional methods can’t be
Two types, Barap using a steel rod and
Cable truss systems
Barap strut / brace
Cable truss system

30. New products – “Wesbeam” e-strut, e-purlin, etc.

31. Wind bracing - Wind bracing is designed to prevent any
movement of the roof, or raking out of
Plumb. These forces can cause cracking

32. Collar ties – P. 39
Light sectional timbers used for additional support
Like spreaders, to prevent rafters from sagging at the purlin position
Fixed to alternative pairs of rafters, either at 900 or 1200 centres
Placed on top of purlin, parallel to the joists
Can be half scalfed or bolted on face

33. Position of collar ties

34. Remember set out on steel square
Calculate rise per m. run
Transfer to steel square
Reduced rise
Reduced run

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

36. Review

37. Review
Plumb & level bevel
Ridge……
Highest horizontal member, runs across roofs length at the apex of the rafters

38. Set out of ridge……
To suit rafter positions
Function of purlin…..
These are the members fixed to the underside of the rafter, to give them support mid-span
Purlins joined….
Typically with half scarf joint, with strut under joint

39. Where is strut positioned……
Placed under purlins to transfer the load either to load bearing walls or a strutting beam
Three types of struts….
Inclined struts, fan struts, scissor struts
Types of scissor struts….
Full and half

40. Wind bracing…..
Positioned at each end of a gable roof, at 45° angle to prevent any movement or raking out of plumb
Collar ties….
Placed above purlin like a spreader between rafters, to help prevent sagging of rafters. Provide additional support / tie

41. Ensure when installing strutting beam…
That it has clearance above ceiling frame,minimum 25mm, pack up at ends