When the span of the roof exceeds 5 meters and when there are no inside walls to support the purlins, framed structures, knows as trusses are provided at suitable intervals along the length of the room. Spacing is generally limited to 3 metres for wooden trusses. In this system, the roof consists of three elements; i) rafters to support the roofing material (i.e. tiles etc)) ii) purlins to provide intermediate support to rafters, and iii) trusses to provide support to the ends of purlins. These trusses span in the same directions in which the couple of rafters run. The trusses also support the ridge piece or ridge bean. The various types of trusses in use are:
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