Lesson Review - 6.11.07 Roof Trusses M. S. Martin – Nov. 2007.

Lesson Review - 6.11.07

Roof Trusses

M. S. Martin – Nov. 2007

Definition / Design

A truss is a self contained frame, designed to transfer roof loads, typically to external walls

Most modern roofs can be adapted to a trussed system

Members are typically joined by gangnail plates

Types – click mouse to reveal types

King Post Queen Post Hammer Beam Standard Truncated Girder Etc.

Standard truss

Terminology – Know your terms, click mouse to review each one

Bottom chord – the horizontal member forming the base of the truss, which will also support the ceiling below

Top chord – the angled member at the top of the truss, in a standard truss this is where a rafter would be

Web – these are the internal members of the truss that help distribute the load to the external walls

Camber – An upward curvature built into the bottom chord to compensate for deflection

Girder truss – A truss that supports other trusses or beams, differs in shape depending on where it is in the roof

Nail plates – the connectors made from a light gauge steel that join individual components of the truss

Gable end truss – the first truss at a gable end Raking truss – a gable end truss altered to suit outriggers for a verge overhang

Terminology / Stresses continued – Click mouse to reveal each point

Panel points – these are the connection points in a truss, (eg) where a web meets a top chord

Fish plate – an alternative bolted connection at a panel point Load bearing walls – in a trussed roof these are still the support walls, but

are typically the external walls

Tension & Compression

Review handout to help understand how this process takes place

Camber – P12 textbook – click mouse to go over main points

Trusses are built with a slight camber in the bottom chord

This is designed to provide maximum calculated deflection

Under no circumstances should trusses be supported along their span

With a trussed roof, internal walls are merely partitions / non-load bearing

Minimum clearance is 12mm

Support & Connections – click mouse to go through main points

The structure supporting the trusses must be level & square

External walls are load bearing with trussed roofs

Or intermediate panel points for cantilevered trusses

Either support directly over studs or change top plate

Where not supported by studs Use thicker top plate, or Double plate

Fixing to non load bearing walls

Lifting roof trusses

If lifting manually, you should always use support timbers to drag them up

If using a crane be careful to sling them correctly

The advantage of a crane, many trusses can be lifted at once

Some typical roofs

Roofs continued

Location of trusses