Repairs: roofs and rainwater goods

Traditional 'cut timber' roofs

These are simple frames, as sketched below, that are likely to include purlins and probably some props or struts to support the purlins and relieve some load onto internal walls (hopefully load-bearing, but not always).

In practice, the loads imposed by the roof covering (the dead loads) and the live loads, such as wind, rain, snow and occasional builders and their ladders, are transmitted in a pattern: the feet or lower ends of the rafters will tend to want to push outwards; the top of the rafters will want to push together as the feet push out.

So, in checking works on an old or new structure it is important to establish that the timbers on each side of the roof are suitably opposed and that the loads cancel each other out by pushing against each other more or less equally on each side of the ridge board.

The purlin should be fixed at about, or just below, the halfway point up the roof slope. That way, it not only picks up some of the weight and loading from the rafters, it also helps restrain the outward thrust tendencies of the rafters. If the purlin is above the halfway point, it will be in the area where the rafters are pushing together, and will not be in a position to offer any restraint.

Simple, or at least it should be. This straightforward and desirable state of affairs is not so often found in practice!

Pre-trussed roofs

The timber pre-trussed rafter frame has been the dominant roof construction since the late 1960s (see image below).

The frames are comparatively quick and simple to erect in most situations and can be pre-designed for specific load or span factors and then manufactured off-site, reducing reliance on skilled on-site labour and fair weather (although erecting the frames in high winds is not desirable for obvious reasons).

Single pitched roofs

Typically, this type of roof is found on single-storey additions for, say, a kitchen or outhouse in an old terraced house. They are often prone to particular problems caused by the difficulty of tying-in the frame. Over time, single pitch roofs can sag or collapse quite badly.