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Verulam

Hipped Roofs and Rafters This is a subject clearly of continuing interest to members. Mr M. Bowden, from Bromley in Kent, writes: Having read the two latest comments, I find that my analysis of the forces involved are quite different to their proposals: e.g. I cannot see how a purlin force applied perpendicular to a rafter can induce an axial force in it. Could I suggest that the use of purlins makes the triangular shape of the roof a redundant structure? My reason for saying this is as follows. With normal 4in x 2in (yes: imperial: do you remember it?!) rafters, the triangular structure is stable, the compression forces are small, but the span from the ridge to the eaves gives great bending overstress and deflections in the rafters. These are reduced by propping the centre of the rafter with a purlin. The carpenter has to position the purlin carefully, otherwise he will be forcing the rafter outwards and it will become disjointed at either the eaves or the ridge. Obviously, the purlin would be positioned before the load is applied, but the need for careful positioning is the same. (See Figs 1,2.)