Lesson Review - 6.11.07 Roof Trusses M. S. Martin – Nov. 2007.
Post on 17-Dec-2015
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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
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
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
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