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of opposing rafters were thus initially tied together by a horizontal tie beam, to form coupled rafters. But such roofs were structurally weak, and lacking any longitudinal support, they were prone to racking, a collapse resulting from horizontal movement. Timber roof trusses were a later, medieval development. A roof truss is cross-braced into a stable, rigid unit. Ideally, it balances all of the lateral forces against one another, and thrusts only directly downwards on the supporting walls. In practice, lateral forces may develop; for instance,

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