Single roofs Rafters of single roofs do not require any intermediate support. This type of roof has a number of limitations. It can only be used for small spans. If greater spans are required, larger roof sections would be needed.
A double roof is a roof whose rafters are of such a length that they require an intermediate support. This support is usually a beam which is secured under the rafters at a point half way between the ridge and the wallplate. This beam is known as a purlin.
A trussed roof is built (often away from the building site) as a series of interlocking rafters, joists and jacks. These make a series of A-frames with far better support than cut roofs. The added benefit? They could be installed as one piece when completed. Because these rooftops proved to be very sturdy and relatively easy to create, they soon became the popular choice for common wooden buildings.
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