An auxiliary truss is an extra truss placed between the load-bearing main trusses of a roof to provide intermediate support. Where the main trusses take up the heaviest forces and provide the large span, the auxiliary truss serves to support, for example, the purlins or the roof boarding over a shorter distance. An auxiliary truss is therefore often lighter in construction than a main truss.
Application
- Reducing the span — if the distance between the main trusses is too great for the purlins, an auxiliary truss picks up the intermediate bay
- Supporting roof boarding — in light roof structures where the boarding needs extra support
- Irregular roofs — with roof planes that have bends, dormers or recesses, where a full main truss does not fit
The auxiliary truss usually rests on the same wall plate or on a supporting structure as the main trusses. Because it does not always have to transfer the full load, it may be built as a half truss or a shortened truss.
When designing, it is important that the forces from the auxiliary truss are correctly conducted to the load-bearing parts; an auxiliary truss “in mid-air” without proper support has no structural purpose. The correct dimensioning and positioning follow from the structural calculation.
Related terms
- Truss
- Half truss
- Purlin
- Roof boarding
- Roof construction
- Wall plate
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