Lesson Review 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