Why Is My Roof Sagging?

A pitched roof is a timber frame. Rafters descend from the ridge to the wall plate (the timber that sits on top of the outer wall), and other members stiffen the frame: collar ties, purlins and struts that carry weight down to internal walls. If one of these is overloaded, cut, rotted or pushed out of place, the roof drops. The covering follows, so the ridge dips or the slope ripples.

Several causes show up again and again. The covering may have been changed to a heavier tile than the frame was built for. Struts or ties may have been taken out in a loft conversion, or to clear storage space, leaving rafters to span further than intended. Timber may have decayed where a slow leak has run unnoticed, or been eaten by beetle. And the walls themselves may have moved: if the wall tops spread, the rafters lose their bearing and settle.

Each of these is a different fault with a different cure, which is why the cause has to be found before anyone gives you a price for the roofing.

Weight is the cause people overlook. A concrete tile is far heavier than a slate, and a frame built for one covering may not carry another. Under Planning Portal's re-roofing guidance, a covering adding 15 per cent or more to the roof's weight needs approval, and the same page tells owners to ask an engineer or a surveyor about the load first. Approved Document A sets a significant change at more than 15%. Our answer on changing roof tiles and weight covers the question in full.

Loft work is the other regular culprit. Taking out a strut to make a room, cutting a rafter for a roof window or removing a chimney breast changes how loads travel through the frame. Planning Portal is firm that Building Regulations approval always applies to taking out a chimney breast, since the stack and breast carry load as part of the building.

If a previous owner made alterations, you may not know what was done. A survey that examines the frame from inside the loft, with photos, is the way to find out, and your building control body can tell you what approvals exist for the work.

Sometimes the roof is not the cause at all. If outer walls lean or bow, or cracks have opened at the wall tops, the rafters lose support and the roof sinks. That is a problem with the building and needs a structural engineer. Look for cracks in brickwork near the eaves, doors and windows that have started to stick, and a gap opening where the roof meets a neighbouring wall.

Rot is easier to spot. Damp, crumbling timber in the loft, especially at rafter feet and around chimney stacks, tells you that moisture has found its way in over a long period. Cure the leak first, then judge which timber is still sound. Our answer on when roof battens need replacing explains how damp timber behaves.

From the ground, look along the ridge and eaves from a distance on each side of the house. A straight line that has become a curve is a warning. If you suspect the structure is weak, do not walk about in the loft: stay on the joists, photograph the timbers and ask for help.

Call one early if you can see a clear dip or bow in the ridge or slope, if cracks run through walls near the roof, if timbers are cut or split, or if the sag is getting worse. A structural engineer or chartered surveyor can assess the frame and specify the repair, whether that is new ties, supports or replacement rafters. RICS can point you to members who carry out surveys.

A roofer then does the covering work to suit. Re-tiling over a sagging frame hides the issue and loads the weak point further. Our survey looks at the frame and the covering together, and the pictures we send show you the timbers as we found them. The roofing side then comes as one written price, and we will say plainly when a structural opinion should come first. Our pitched roofing page covers re-covers and repairs.

Dangerous structures should be reported at once. If part of the roof looks ready to fall, keep people out of the space beneath and phone the emergency services or the council's building control team. Check with the council that deals with building control where you live, since the right office depends on the borough or district.