Roof Sagging In The Middle

What's happening?

A pitched roof is held up by rafters, purlins, trusses or a mixture of timbers, and it should be straight. A dip between two points means a timber is bending or something that held it up has moved. The usual reasons are timbers that are too small for the span or were weakened, rot from a long-term leak, a heavy covering such as concrete tiles laid on a roof built for lighter ones, a support wall or chimney breast that has been taken out, or a roof whose rafters have spread because the ceiling ties are missing. Many roofs have a slight historic bow and have been stable for decades. What matters is whether it is new, worse, or has cracking in the ceiling or walls under it. That tells you whether it is moving.

What usually causes it?

• Rafters or purlins that are undersized or over a long span
• Rot in the timbers from a long-term roof leak or condensation
• A heavier roof covering than the roof was designed for
• A wall, strut or chimney breast removed in an alteration or loft conversion
• Rafters spreading because ceiling ties or collars are missing or have failed
• Roof truss members that have been cut or altered
• Ground movement or wall movement under the roof

Is it dangerous?

Yes, because the cause is structural. A roof that is still moving can fail, and the ceiling below can too. Do not store anything in the loft, and ask a professional to look before the next bad weather. Treat sudden dips, cracking ceilings and doors jamming as urgent.

Can I fix it myself?

From the ground, photograph the roof line from the same spot in the same light each month. From the loft hatch, photograph splits, rot, sagging beams or gaps at joints. Look at ceilings and wall tops for new cracks. Do not jack, pack or prop the roof yourself. Do not add weight.

When should you call a roofer?

Call a structural engineer first to say what is wrong and what repair is needed, then a roofer or carpenter to carry it out. Call a roofer first only if the sag is clearly from a known leak. If the roof is moving, cracking ceilings or doors are jamming, call the same day. Ask for a written report.

What will the roofer actually do?

• Inspect the roof from outside and inside, measuring the sag and noting any movement
• Check the timbers, joints, ties and bearings, and any alterations that took out support
• Identify wet or rotten timbers and the source of the moisture
• Strengthen or replace the failed members to the engineer's design
• Re-cover and weatherproof the area and check the ceiling below

Who fixes this

• Roof inspection

Questions people ask

Sources & References

1. GOV.UK: Approved Document A, structure
2. HSE: roof work
3. NFRC: choosing a roofing contractor