A Sistered Rafter, Strengthening an Existing Roof

Sistering means fixing a new length of timber directly alongside an existing rafter, face to face, and bolting or screwing through both so they act together as a single, stronger member. The original rafter stays in place — it is reinforced, not removed — which is what makes the repair so much less disruptive than a full replacement.

Sistering suits a rafter that is undersized for its span or has begun to sag under long-term load, but is otherwise sound timber with no rot, splitting, or insect damage. Where the existing rafter is structurally compromised rather than simply underspecified, or where access genuinely does not allow a matching section to be fitted alongside it, full replacement becomes the more sensible route. A site visit tells us which category a given rafter falls into.

For anything beyond a straightforward like-for-like reinforcement, yes — we would expect a structural engineer to specify the new timber section, the fixing pattern, and confirm the approach is adequate for the loads involved, particularly where the roof has already been altered or where Building Control sign-off is required. We work directly from those specifications rather than guessing at sizing ourselves.

In most cases, no. Sistering is carried out from inside the roof space, working alongside the rafter from below rather than lifting tiles or slates from above. The roof covering is typically only disturbed if access genuinely requires it, or if the brief extends beyond the rafter repair itself.

Not always. Where the weakness is confined to a particular section — commonly the mid-span, where deflection is greatest — a partial sistered length positioned to span past the weak point and well onto sound timber either side is often sufficient. A full-length sister is used where the whole rafter is undersized for its span, rather than just locally affected.

Structural coach bolts or heavy-duty structural screws, sized and spaced to the specification for the job, drilled through both timbers together so every hole lines up precisely. The aim is for the original rafter and the new sistered section to behave as one solid structural member, not two pieces of timber loosely strapped together.

A sistered rafter sits within the roof structure itself, so once any loft lining, insulation, or boarding is reinstated, the repair is not something you would see in day-to-day use of the space. Where the loft is left open and accessible, the sistered section is visible but neat — two timbers bolted cleanly together rather than an untidy patch.

Yes — all structural carpentry we carry out, including sistered rafter repairs, is covered by our standard workmanship guarantee, confirmed in writing as part of your quote. Where the work has been specified by a structural engineer, we build strictly to that specification and keep you informed if anything on site differs from what the drawings assumed.