Why is my roof ridge sagging?

Seen against the sky, the ridge where the two slopes join should read as a straight line. A dip, a hump or a lean is a symptom, not a fault in itself. It tells you that something under the tiles has deflected: the ridge board, the rafters, the purlins or the walls carrying them.

Old houses are rarely dead straight, and a gentle undulation that has looked the same for years usually belongs to a roof that settled long ago. What matters is whether the shape is changing. Photograph the ridge from one spot every few months. Watch for fresh slipped tiles, cracked pointing along the top, and new gaps beside a gable wall. Our guide to roof structure defects in older Stroud houses covers the other faults that can sit behind the same symptom.

In a cut roof, built from separate rafters meeting at a ridge board, the rafters push outwards as well as down. Ceiling joists and collar ties are meant to hold them together. When those ties are missing, cut through during a past alteration or decayed, or when the wall plate that carries the rafters shifts, the walls are pushed apart and the ridge drops to follow.

The evidence is often at eaves level: the tops of walls leaning outwards, cracks where internal partitions meet the ceiling, or tiles sliding along the lower courses. This is why a sagging ridge deserves a proper look rather than a quick cosmetic fix. Laying new ridge tiles over a roof that is still spreading treats nothing, and the cause has to be understood before the covering is touched.

Purlins are horizontal timbers set under the rafters partway up each slope, stopping the rafters from bending. If a purlin was too small for its span, has split along its grain, or has rotted where its ends sit in the masonry, the rafters sag above it and the whole slope dips. The ridge can follow.

These timbers are only visible from the loft, so we examine their joints, their bearing ends and any marks of old movement, such as packers or added props. Rot nearly always leads back to a leak that went unnoticed. If you can already see stained timber or daylight, our answer on torn felt in the loft explains what that says about the layer above the rafters. Repairs run from adding support beneath a purlin to replacing it, and an engineer or surveyor should specify the work where loads are uncertain.

A roof built for one covering can struggle with another. Cotswold District Council's 2000 guidance puts stone slate at about 1,000 kg for every 10 m² of roof, and a modern concrete tile is a different load again. Approved Document A treats a roof loading increase of more than 15% as significant, and the Planning Portal says a covering adding 15% or more needs Building Regulations approval. Where the existing structure cannot take more, strengthening follows.

That is a reason to keep like for like on older houses. A lighter roof replaced by a heavier one, with the timbers unchecked, can start a sag that takes years to appear. Ask building control, and follow the Planning Portal's advice to consult a structural engineer or surveyor first. We would sooner price that check into the job than find the problem with the tiles half laid.

Treat a ridge that is moving, or that arrives with new cracks, slipped tiles or doors that have started to stick, as something to have surveyed soon. A stable dip in a long-standing roof is less pressing, though it is still worth recording with dated photographs.

Our survey looks at the ridge from outside, then at the rafters, purlins, ties and wall plates from the loft. MyBuilder's 2026 guide puts a roof inspection at £150 to £500. Where tiles are creeping along the ridge as well, roof tile clips and nails sets out the fixing rules we check. If water is coming in, stopping it comes before anything else. Your written quote follows once the cause is known, and our 10-year workmanship guarantee comes in writing. More tiled-roof topics sit under pitched roofing.

It is tempting to treat a wavy ridge as a tile problem: lift the ridge tiles, re-bed them in fresh mortar and hope the line looks better. Tiles follow whatever is beneath them, so a ridge laid over a sagging ridge board will simply sag too, and the new mortar will crack where the roof keeps moving.

Cosmetic work has its place once the structure is sound. Under BS 5534:2014, ridge and hip tiles are expected to be fixed mechanically; a dry-fix ridge, or a bedded one that is properly clipped, can then follow the corrected line. Cost guides put dry ridge at £40 to £60 per metre in MyBuilder's guide, which is modest next to timber repairs.

So the order matters. Find the cause, correct the timbers, and only then finish the ridge. A good survey report will say which of those stages your roof needs, in plain words, with photographs of the loft timbers beside it.