A Sagging Rafter, Properly Diagnosed and Repaired

A handful of causes, usually working alone or together: the rafter was undersized for its span in the first place, water ingress has weakened the timber over years, a supporting strut, collar tie or purlin bearing has failed, or added loads — loft insulation, water tanks, a later loft conversion — have exceeded what the original structure was ever designed to carry. Diagnosing which of these applies is what determines the right repair.

It depends on how far it has moved and why. A very slight, long-standing dip in an old roof is sometimes just cosmetic ageing. But active or worsening sag is a structural symptom, not a decorative fault, and should always be assessed properly rather than assumed to be harmless. If you can see daylight movement, hear new creaking, or notice the sag getting visibly worse, treat it as worth checking without delay.

In most cases, yes. Rafter repair and reinforcement — sistering a new length of timber alongside the existing rafter, or splicing in a new section — is normally carried out entirely from inside the loft, with the roof covering left undisturbed above. Full replacement of a severely damaged rafter occasionally needs a small amount of tile lifting for access, which we would always flag clearly before work starts.

By fixing the actual cause, not just the symptom. If the original sizing was marginal, a sistered rafter brings the section up to what the span and load genuinely need. If a strut or collar tie had failed, it is repaired or replaced as part of the same visit. If an added load — extra insulation, stored items, a water tank — was the trigger, we flag that so it is not simply reloaded straight back onto a freshly repaired structure.

For a straightforward, localised repair — one or two rafters, a clear and simple cause — we can usually assess and specify the repair directly, using standard like-for-like sizing. Where the sag is widespread, the cause is not straightforward, or the repair would change how load moves through the rest of the roof, we recommend a structural engineer assesses it and provides a specification first, which we then build to.

Structural rafter repairs fall under Building Regulations Part A in the same way new roof construction does, so in most cases, yes — particularly where a rafter is being sistered, spliced or replaced. Minor like-for-like repairs may not always need inspection; we advise on this as part of the assessment and leave the work accessible for inspection where it is required.

A single rafter, sistered or spliced with straightforward access from the loft, is often a one-day job including temporary propping and reinstatement. Where several rafters are affected, or a strut and collar tie need attention as well, it typically runs to two or three days. We confirm a time estimate as part of the written quote, after seeing the extent of the problem on site.

Yes. All our structural repair work is covered by our standard workmanship guarantee, with the details confirmed in writing alongside your quote. Where a repair has been carried out to an engineer’s specification, that specification and the completed work are both kept on file.