Gable Roofs and Components

Created by Michael Martin April 2004 / revised April 2007 / Reviewed pictures added - March 2010

Continue on roofs - Components

Ridge

Rafter

Purlin

Hanging beamCeiling joist

Strut

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. runTan 22° = 404mmTrue length per m. run√ .404² + 1.0² = 1.079

Centre line length of Rafter½ span x true length per m.

4.250 x 1.079= 4.586Order length, if say 450 eave

(4.250 + .450) x 1.079= 5.071Order 5.1m length’s

Calculate lengths of rafter belowFor 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 1.113 = 3.450

Order length

(3.1 + .450) x 1.113 = 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

Cable truss system

Barap strut / brace

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

ReviewPlumb & level bevel

Ridge……

Highest horizontal member, runs across roofs length at the apex of the rafters

Set out of ridge……To suit rafter positionsFunction 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

strutsTypes 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