These roofs have two basic elements: i) rafters and ii) purlins The purlins give intermediate support to the rafters, and are supported on end walls. The intermediate supports so provided in the form of purlins, reduce the size of the rafters to the economical range. Such a roof is also known as rafter and purlin roof. The rafters are provided fairly close (40 to 60 cm c/c). Each rafter is thus supported at three points i) at the bottom, on the wall through wall plate ii) at the top, by the ridge beam, and iii) at the centre by a purlin. By supporting the rafter at its mid-point with a purlin, the span is halved, thus enabling the rafter to be made considerably lighter than it would need to be if it spanned the whole distance from eaves to the ridge. For larger roofs, two or more purlins may be provided to support each rafter.
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