![]() ![]() ![]() A roof truss is cross-braced into a stable, rigid unit. Timber roof trusses were a later, medieval development. But such roofs were structurally weak, and lacking any longitudinal support, they were prone to racking, a collapse resulting from horizontal movement. Pairs of opposing rafters were thus initially tied together by a horizontal tie beam, to form coupled rafters. For larger spans and thinner walls, this can topple the walls. Rafters have a tendency to flatten under gravity, thrusting outwards on the walls. The space between each truss is known as a bay. Trusses usually occur at regular intervals, linked by longitudinal timbers such as purlins. This is an example of a "double roof" with principal rafters and common rafters.Ī timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof. Key: 1: ridge beam, 2: purlins, 3: common rafters. Two king post trusses linked to support a roof. ![]()
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