Repairs: roofs and rainwater goods

The original specification for these pre-trussed roofs (sometimes known as TRADA roofs) was subsequently amended to include lateral and chevron or diagonal bracing timbers to reinforce the roof across its width, mainly to take account of the loads imposed, probably onto one aspect of the roof only, by the prevailing wind.

Many pre-trussed roofs have now been improved by retro-fitting bracing timbers and steel strap anchors. But many have not, and large numbers are still encountered needing some works, if only as a precautionary measure.

Properties in sheltered locations may be at comparatively low risk from distortion of the frame due to wind loads, nevertheless it is sensible to recommend repairs in accordance with the BRE Good Building Guides 8 and 16.

Solution 1

Timber diagonal bracing boards should be fixed to the underside of the rafter frames, with each fixing secured beneath the rafter at two points. The diagonal spars are often made up of 2 or more sections overlapped, depending on the height of the roof. Overlaps need to cross over at least 2 rafter sets to achieve adequate load spread.

Note: The fixings could be nailed, but the author recommends screw fixings – certainly in any situation where a retro-fit is being handled – because it is amazing how much damage vibration can cause to plaster finishes (especially thin coats of plaster skimmed over nail heads or plasterboard joints).

Solution 2

Likewise in any retro-fit situation it is likely that the chevron braces will have to be built up in several sections unless you are particularly lucky in finding a roof access trap hatch and landing area large enough to manhandle the lengths of timber required!

Before fixing extra restraining timbers in place, check for any distortion in the pre-trussed frames.

The handbook Building Elements: Roofs and Roofing: Performance, diagnosis, maintenance, repair and the avoidance of defects advises that lateral distortions in the frame out of true by 10mm in 1m of rise are probably capable of being rectified. If the lateral distortion is greater than 40mm overall, have the frames checked by a chartered structural engineer to consider their structural integrity before proceeding further - they are probably candidates for replacement.

Rafter frame and wall collapse in Normandy