A sagging roof rarely happens overnight. The ridge line starts to droop, a slope develops a shallow hollow, or the eaves bow outward, and from the pavement it looks like a tired, uneven roof. On a street of older houses it can be hard to tell whether the dip is new or has been there for decades, which is why the first question is always whether it is still moving.
The covering is almost never the cause. Tiles and slates follow whatever the timbers beneath them do. The rafters, purlins and wall plates that carry the roof may be too slight for the span, softened by years of damp, or carrying a weight they were never built for. Sometimes a support further down the house has been taken out.
This guide sets out the usual causes, the checks made in the loft and on the outside of the house, and how we decide between strengthening the existing frame and starting again with a new roof. The right answer depends on what we find, so we explain the choice before any work is priced.
Reading a dip: new, old or still moving
Begin with the view from the pavement. Sight along the ridge and along the line of the eaves, ideally from a few houses away so the whole roof is in view. A ridge that steps down at one point suggests a failed support. A gentle, even curve usually points to a long-standing problem. A hollow in a single slope, with the ridge still straight, often means the rafters on that side have weakened.
Inside the loft, a torch tells more. Rafters are checked for bowing, splits near the joints and sideways lean. Purlins, the long horizontal beams halfway up each slope that hold the rafters, must sit on solid supports. Where each rafter lands on the wall plate, a timber laid along the wall head, we watch for signs that the walls are being pushed outward. Fresh pale splits in old timber, or gaps between joints that were once tight, suggest recent movement. Dark, dusty cracks suggest the roof settled long ago and has stayed put since.
A simple way to check movement
You can help by measuring. Photograph the roof from the same spot every few weeks, or hold a taut string along the ridge and note the gap. Stick a dated pencil mark on a crack in the loft plaster or against a rafter.
A roof that is stable and has simply sagged slightly over a lifetime might not need structural work, only careful repair of the covering. One that is still moving needs the cause found before anything else is done, and our guide to terraced house roof repairs shows how that works when the roof is shared.
Timbers that were never big enough
Many older roofs were built by eye, from whatever timber was to hand. Rafters can be slim for the distance they span, and a long slope may have had no mid-height support at all. The roof coped while the covering was light and the timber was fresh, then started to bow as the wood dried, twisted and crept under load year after year.
Victorian and Edwardian houses show this often. Many have a single purlin per slope, resting on struts that land on an internal wall. If a strut was cut short, bears on a weak partition, or was never fixed at its foot, the purlin dips and the rafters follow. A roof with a long hip or a wide span, such as a bungalow with a hipped roof and a deep plan, can sag where the hip rafter is too light to carry the jack rafters that lean on it.
Spreading walls
Rafters push outward at the bottom as well as downward. Properly built roofs resist that thrust with ceiling joists or collars tied across the slope, and when those ties were left out, cut through for a loft hatch, or have rotted at their ends, the eaves can spread and the ridge falls. You may see this as gaps at the top of the party walls, bowing gable ends or cracks in the ceilings below.
The fix for undersized timber is almost always to add rather than replace. New rafters are fixed alongside the old ones, a new purlin or a strongback goes in with proper bearings, and ties are restored across the slope. A structural engineer calculates the sizes where the loading is unclear, and we work from that.
Rot, damp and slow water damage
Timber that stays wet loses strength. A leak that has run for years into the same rafter or wall plate can leave it soft, crumbly or full of fungal growth, and the roof settles where the wood gives way. The cause is often an old flashing, a blocked valley or a gutter that overflows onto the wall plate, so the roof sags in a line below the fault rather than at it.
We probe the timbers with a blade. Sound wood resists, and rotten wood lets the point sink in. The rafter ends sit in the dampest spot, so they get a close inspection, along with any strut feet and the timber under them. Dark staining, a musty smell, lifted paint on the ceilings and sagging plasterboard all point the same way. Where the wet came from matters as much as the damage: replacing a rafter without stopping the water just gives a new piece of timber the same fate. Our guide to bay window roof repairs shows how a small leak at a wall line can go undiscovered for a long time.
Cutting out and splicing in
If only the ends of the rafters are bad, the usual repair is to cut back to sound wood and fix a new section alongside, a method known as sistering or scarfing, and then support the whole on a new wall plate or bearing. Badly decayed members are replaced outright, one at a time, with the roof held up on temporary props.
Wood-boring insects are a separate question. We note any sign of them and, where the damage looks active, recommend a specialist timber survey before any new timber is fixed alongside it.
A heavier covering than the roof was built for
Re-covering is the most common reason a sound roof starts to sag. Frames built for plain clay tiles, thin slate or an older stone slate can struggle if the new covering is heavier, such as large concrete interlocking tiles or thick natural slate. The extra load shows as a slow dip that appears months or years after the work.
The rules treat this as a structural question. Approved Document A counts a rise in roof loading of more than 15% as significant. If the frame cannot carry it, strengthening or replacement follows, and that is a material alteration. The Planning Portal words it a little differently: approval is needed when a new material makes the covering 15 per cent heavier or more, and it recommends an engineer's or surveyor's opinion first. The two wordings differ, so the sensible reading is that a covering roughly 15 per cent heavier needs building control and a structural check. Ask your roofer to confirm that before any covering is ordered.
Why it matters before you buy tiles
A heavier covering also loads the walls and any purlin supports below, and it changes how the roof behaves in wind. Re-covering falls under Building Regulations once it passes the thresholds, and the building control body is then notified. We talk you through consent during the survey and send in the notification ourselves when the job requires one.
A modest weight increase can often be carried if the frame is sound and the timbers are in good order. Where the numbers are tight, adding rafters or a purlin before the new tiles go on is cheaper and neater than cutting into a finished roof afterwards.
When a support has been taken out
Alterations inside the house are a surprisingly frequent cause of a sagging roof. A purlin strut that landed on an internal wall loses its bearing when the wall is removed. A chimney breast in the loft, which acts as a solid support and sometimes carries timbers, is taken out to free up a room. A loft conversion cuts through rafters or collars to make headroom, and the frame that remains is not strong enough to span the gap.
Building Regulations approval always applies to a chimney breast coming out, according to the Planning Portal. Where your neighbour shares that breast, a party wall notice follows too, as GOV.UK counts taking chimneys out of a party wall as notifiable work. Our pages on chimney breasts that have been removed explain what the stack above needs once the breast below has gone.
Signs it is the alteration
The timing is the clue. If the dip appeared within a year or two of building work indoors, that is where we look first. We trace each strut and purlin down to its bearing, check whether the load now lands on a beam, a steel or nothing at all, and ask for any calculations from the original work.
The remedy is to put a proper support back: a new steel or timber beam in the right place, a padstone or a new bearing wall, sized by a structural engineer. The roof can then be jacked gently back to line where that is safe, though a frame that has taken a long set seldom returns fully straight, and you hear that from us at the outset. Cosmetic levelling is a job for the covering once the frame is sound.
Strengthen the frame or re-roof: how we decide
Strengthening is the right answer when the frame is basically sound and the fault is local: one weak purlin, a few rotten rafter feet, a missing tie. Re-roofing makes sense when the damage is widespread, the covering is also at the end of its life, or the frame needs so much repair that opening up the roof is the most economical route. Doing both together is often sensible, since the old covering must be stripped to reach the timbers anyway.
Cost depends on how much timber is affected, whether engineer's drawings are needed, how the roof is reached and what covering goes back on. A structural fix on a roof of normal size can take longer than people expect, because props and temporary covers go in first. Scaffolding is a large part of that: MyBuilder gives around £850 to £1,200 for a standard two-storey house. For comparison, its guide to a full re-roof runs from £5,000 to £12,000, so a repair that avoids one is worth pursuing when the frame allows. Once we have surveyed the timbers, our written quote puts one total price on the route we recommend, with photographs of the wood in question. Our roof repair cost guide covers how repair prices are set in general.
What to do in the meantime
Keep heavy items out of the loft, clear snow loads and blocked gutters where you can reach them safely, and avoid leaving damp patches to run on. If a ceiling sags, a door sticks or cracks appear in the walls, arrange the survey sooner rather than later.
The site is cleared when the job is done, and any propping or strengthening is explained in the written quote. See roof repairs for the full range.
