Bridging the space above a room, the house structure and providing support for a roof is achieved by timber roof truss as a structural framework of timbers. Trusses usually are installed at regular intervals, and are connected by longitudinal timbers such as purlins. Bay is the space between each truss.
Timber roof trusses are started developed and used in medieval. Before that the roofs had been supported by coupled rafters ? pairs of rafters linked by horizontal beams. But  they were structurally weak, and lacking any longitudinal support were prone to racking, a collapse resulting from horizontal movement. The gallery shows several types of construction on the real objects.
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