A roof truss is a load-bearing, usually triangular assembly of timber or steel that stands in the plane of the roof, takes up the roof loads and transfers them into the load-bearing walls.
Characteristics
The triangle is at the heart of the truss because it is a rigid, stable shape that resists deformation. Within that triangle several members each play a part:
- Rafters, or principal rafters, form the sloping sides.
- The tie beam runs horizontally across the base and stops the feet spreading.
- The collar beam ties the rafters together higher up.
- The king post links the ridge to the tie beam and steadies the frame.
Trusses are set at regular centres along the length of the roof. Between them run the purlins, which carry the common rafters or the roof covering, so the whole roof shares its load evenly.
Application
Roof trusses are used wherever a roof needs to span a space without intermediate support. By collecting the weight of the covering, along with snow and wind loads, and channelling it down to the walls at set points, they allow open interiors below. They are found in houses, barns, halls and industrial buildings alike, either cut on site in the traditional way or delivered as prefabricated units ready to lift into place.
Related terms
- Truss
- Roof structure
- Purlin
- Tie beam
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