What happens when roof timbers need replacing?

A traditional pitched roof is a frame. Sloping rafters carry the load from the apex to the outer walls. The wall plate is the timber laid on the masonry that receives their lower ends. Mid-slope, a purlin runs sideways beneath the rafters to hold them up, while ceiling joists stop the feet from spreading outwards. On top of the rafters sit the battens that carry the slates or tiles, together with a layer of underlay.

Many roofs built since the 1960s use factory-made trusses instead, triangular frames in lighter timber, spaced closer together. They have no purlins and cannot be altered casually, because each member has a job. A roofer who cuts a truss to make room for something is asking for trouble. If you plan loft conversions or storage later, get that checked before a new covering is chosen.

Rot needs moisture, so it starts where roofs leak: below a broken valley, at a chimney, along a leaking gutter, or at the wall plate where water has run down the wall. Wet rot leaves timber dark, soft and crumbling across the grain. Dry rot is rarer and more serious, with a musty smell, cracked and cuboidal timber and sometimes a grey strandy growth, and it can spread into brickwork. It needs a specialist report before anyone starts cutting, because treating the cause matters more than swapping the timber, and a roofer should say plainly when the problem is beyond roofing.

Wood-boring insects are a separate matter. Furniture beetle leaves small round holes and gritty dust, and has often been dead for years. Fresh, pale dust below the holes suggests it is still active. Death watch beetle favours old, damp hardwood such as oak, so it appears in some period houses. An old flight hole is no cause for alarm on its own, but a specialist assessment is sensible, because the answer decides whether you treat, strengthen or replace. Our page on waste from a re-roof covers how rotten wood leaves the site.

Not every defect is decay. Some rafters were never big enough, particularly in Victorian terraces and in houses that were reroofed with heavier tiles than their original covering. The giveaway is a roof that dips between purlins, a ridge that sags, or rafters that bow. Past alterations also leave scars: a purlin notched for a chimney flue, a trimmed truss, joists cut through for a hatch.

Where a timber is undersized, it is usually fixed by sistering, which means bolting or screwing a new piece to the side of the old one so that the two share the load. Sometimes a steel flitch plate or a new purlin is the answer, and for larger changes a structural engineer is brought in to calculate the sizes. The calculation is then followed on site, and building control is told where the work is notifiable. Our page on a metal roof on a house notes how weight changes this check, as does the section on heavier coverings in our repair or replace advice.

The roofer cuts out only what has failed. A short length of rafter foot, for example, is taken out and a new piece spliced in, bolted to the sound timber beside it, and sitting on the wall plate on a damp-proof strip. A rotten plate is replaced section by section so the roof never loses its support. New timber is graded and treated softwood, sized to match, and cut square so loads go where they should.

Battens are almost always renewed on a re-roof, so most of the carpentry is simple. Fascias, soffits and verge boards get the same inspection. If the old covering is being reused, the order of work changes, and our note on reusing roof tiles explains how. Timber that is sound but damp is allowed to dry before it is covered, because sealing wet wood under a new roof is the quickest way to start the next rot. Where a leak caused the damage, the source is cured first, otherwise the new timber would only suffer the same fate in a few winters.

This is where a re-roof quote can move. The survey looks for the signs: stains on the ceiling, a sagging ridge, damp at the eaves, daylight at the gable, dust below the rafters. Photographs from inside the loft record everything that can be reached, but a hidden rafter foot at the back of the eaves is only visible once the covering is off. For that reason the written quote explains what it allows for and what would be an extra if a timber turned out worse than it looked.

As a guide to the wider job, MyBuilder's roof replacement guide, updated in August 2026, puts typical UK roofer day rates at £250 to £300. Treat it as a national yardstick, never a quote for your house. We agree any extra timber with you before it goes in, in writing, with a photograph. The full sequence is covered on our roof replacement page, and our written 10-year workmanship guarantee covers the carpentry as well as the covering. Send a few photographs of your ceilings and loft with the enquiry form, and the survey can start by looking at the likeliest trouble spots.