Will my roof timbers need replacing during a re-roof?

1What gets looked at once the roof is stripped

With the tiles and underlay gone, the whole frame is visible for the first time in decades. The main parts are the rafters, which run up the slope, the purlins, the heavier horizontal beams that support them midway, and the wall plate, the long timber resting on the masonry, which the rafters bear on. Where a ridge board runs along the top, the joints with it are checked, as are the hips and valleys on more complex roofs.

Battens are the first to go on most old roofs. They are thin and sit right under the covering, so they take the most water. We press and probe for softness, look at nail holes for rust and check for daylight along the lines of the frame. A sagging line in the roof slope from the street often points to a purlin or a rafter under strain, and we look closely wherever the profile dips.

Nobody expects you to go up a ladder to judge any of this. We photograph the condition as it is found, so whatever we recommend can be followed from the pictures at your kitchen table.

2Rot, insects and water: what a repair looks like

Timber fails for a few reasons. Water is the main one: a lifted flashing, a leaking valley or a failed gutter keeps the nearby wood damp, and damp wood rots. Insect attack such as woodworm may be old and inactive or live, and a specialist can say which. Rafter feet that sit on the wall plate close to a leaking gutter are a classic spot for decay.

Repairs are usually targeted. A rafter with a rotten foot can have a new length of timber bolted alongside it, a method called sistering, or be cut out and spliced. A decayed wall plate is replaced in sections. Sound timber is left alone, and where a repair is made the new timber is treated. Replacing the lot is rarely the answer, since the aim is the least work that leaves the structure sound.

The causes matter as much as the damage. If a gutter that was overflowing rotted the plate, the gutter is put right too, as replacing fascias and gutters with a new roof describes.

3When structural changes need approval

The Planning Portal lists when re-roofing needs building control approval: where the thresholds are exceeded, where structural alterations are carried out, where the new covering behaves differently in fire, or where it adds weight of 15 per cent or more. Swapping a few rotten rafters like for like is repair. Cutting through a roof member, removing support from a purlin or loading the roof in a way that changes how it carries weight is structural alteration.

Approved Document A calls the strengthening or replacement of a structure that cannot take extra load a material alteration. On weight, the Planning Portal suggests an engineer or surveyor should have a look before work starts when the new tile is a different weight from the old. Our guide to a heavier roof covering sets out the detail. We explain what is notifiable, and we handle the building control notification where one is needed, so the paperwork is never left to chance.

4Insulation, chimneys and other things the open frame invites

Part L treats a strip-down to the rafters followed by a rebuild as renovation of the roof's thermal element. Approved Document L expects the whole element to be brought up to standard once over 50 per cent of its surface has been renovated, or when over 25 per cent of the external building envelope is being renovated. In practice, insulation can become part of the job even if you only came for a re-cover.

An open frame also lets us study the chimney stack. A flue that has been leaking marks the timbers around it, and the page on what happens to the chimney when you re-roof covers the leadwork and repointing. If you plan to add solar panels later, say so at the survey, because the frame has to carry them: see fitting a new roof with solar. Roof windows and loft conversions change the structure further and have their own rules.

5Cost, reuse and what we tell you beforehand

Timber repair is where a re-roof price is least predictable, because the answer lies behind the covering. At survey we set out what we expect from the loft and from the signs outside, and what would alter the picture. Anything found on strip-out is photographed and talked through with you before extra work begins. The price reaches you in writing after the visit, as one figure that reflects what the survey showed.

It also helps to decide early whether old tiles will be kept. Battens and timbers under sound tiles may be good enough for a gentle re-cover, as discussed in reusing old roof tiles. A stripped roof is also the point at which repairs cost least in access, since the scaffold is already standing. For the wider process, see our roof replacement page, which covers coverings and the order of work.