# Gable Roofs and Components

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

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

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

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

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

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

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

Ridge

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

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

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

Purlin

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

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

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

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

Fan Struts

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

Angles continued

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

Other supports

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

Half Scissor Struts

Half scissor strut

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

Parallel strutting beam

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

This one is being used as a hanging / strutting beam

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

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

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

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

Position of collar ties

Remember 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

Review

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

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

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

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

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