Why is my roof sagging in the middle?

A healthy roof slope looks flat from the street, like a stretched sheet. When it dips, the covering is only following what the timbers underneath are doing. Tiles do not sag by themselves, so the cause sits in the frame: the rafters that run up the slope, the purlins that support them midway, or the walls that carry both.

Where the dip sits gives a clue. A hollow halfway up a slope often means rafters bending under load or a purlin that has deflected. A sag along the ridge points to the ridge board, the support walls or roof spread. Sudden unevenness near a chimney can mean the stack has settled, or that timbers around it have been cut.

Look at the roof from a distance and at different times of day. Low sun along the slope shows ripples that midday light hides. Take photographs from the same spot each time so any change can be seen. A dip that has stayed the same for decades is one thing; a deepening one is another.

Rot is a leading cause. A slow leak, a blocked valley or damp from a failed gutter can soften rafter feet and the wall plate, and weak timber gives way under the weight of the tiles. A related fault is rotten battens, the thin timbers the tiles are hooked over. We explain how to spot them in how to tell if roof battens are rotten, though battens on their own seldom make a whole slope dip.

Weight is the next cause. A concrete tile laid over a frame built for clay or slate adds load the original carpenter never planned for. Planning Portal guidance treats a covering that adds 15 per cent or more as a reason to involve building control.

Older roofs may simply have been built light. Timber sizes and spans in some past construction were modest, and a roof can settle slowly over generations. Finally, alterations matter: a purlin notched for a loft room, a removed strut or a cut brace takes strength out of the roof without any outward sign.

Not every wavy roof is a problem. Hand-made clay tiles vary in thickness, so an old plain-tile roof can look rippled yet be sound. Slate roofs sometimes show gentle undulation as the battens settle. What matters is movement and cause.

Treat it as structural when you see any of these: a dip that is getting deeper, cracked or displaced ceilings below, doors upstairs that now catch, daylight showing through at the ridge, or a visible bend in a rafter from inside the loft. Water staining on timbers near the dip adds to the case.

From the loft hatch, a torch will show a bowed rafter, a split purlin or a timber that has slipped off its bearing. Stay on boards or the hatch edge. The ceiling between the joists will not carry your weight, and nobody should step onto it.

If the signs are doubtful, a survey with photographs settles the question without committing anyone to a job. Our answer on roof spread covers the related fault where the walls move outwards.

The fix follows the cause. Where rot is limited, carpenters cut out the bad timber and fix new sections alongside sound wood, a technique called sistering. Where a purlin has deflected, a new support or a larger section can take the load. Where the roof was built light, extra bracing or new rafters may be added.

The covering usually has to come off first, at least over the affected area, so the work can be done with the frame open. That is also the time to renew rotten battens and underlay, the sheeting stretched under the tiles. Tiles that are sound are stacked and re-laid, and broken ones are replaced. Our page on matching old roof tiles explains how replacements are chosen.

Because structural alterations can bring Building Regulations into play, an engineer's design and the building control body's sign-off belong in the plan. Where the work is notifiable, we deal with the notification.

Waste leaves with us when the work is done.

Photograph the dip from the road and, only if it is safe, again from the loft hatch. Note any new cracks inside, and check for damp patches on the ceiling below. Water getting in comes first: stop it, because every wet week softens the timbers further.

Then arrange a survey from someone who will look at the frame, not just the covering. We inspect the covering and the loft timbers together and say whether the trouble sits in the tiles or in the timbers. Where the frame is at fault, we suggest bringing in a structural engineer. Our figure is written down after the visit as a single sum, matched to the fault and no bigger.

Do not let anyone lay new tiles over a dipping roof as a quick answer. The covering will follow the sag, and the problem will show again within a year or two. Our pitched roofing page explains the full range of work above the rafters.