The short answer
A sagging roof usually means something under the covering has moved or weakened: rafters pushing the walls out, timbers too light from the start, rot from years of damp, or a new covering that outweighs what the frame was designed for. The tiles show the problem but do not cause it. Have a structural engineer or surveyor inspect it before roofing work starts.
A dip along the ridge or a belly in the slope is a structural warning. Understand it first, then decide what to do to the covering.
Reading the shape of the sag
Where the dip appears tells you where to look. A drop along the crest of the roof points to the main supports. A shallow belly between eaves and crest points to the rafters, the slanting timbers that hold up the covering. A sag following a line across the slope often follows a purlin, a long beam running level along the slope that supports the rafters midway.
Is a sagging roof an emergency?
It can be serious, so never write it off as cosmetic. A roof that has moved is carrying load in ways it was not built for, and it may keep moving. From the garden, look with binoculars, photograph from the same spot every few weeks and watch for new cracks in walls and ceilings beneath. Stay out of the loft if the timbers look badly bowed, and keep the rooms beneath the dip free of anything heavy.
A taut string held against the crest line shows a dip far better than the eye can. It also gives a surveyor a record of how the shape has changed between visits, which helps separate a settled old sag from one that is still developing.
Roof spread and weak timbers
A pitched roof pushes outward as well as down, because the feet of the rafters want to slide towards the walls. Ceiling joists, the horizontal timbers across the house, or collar ties usually hold them together. If a joist has rotted, been cut for a hatch or pulled from the wall, the rafters spread and the crest sinks.
What else makes a roof dip?
Rafters that were too slim when the house went up will flex under the covering and under anything stored in the loft. Rot is the other cause to rule out. Years of damp from a leaking valley or slipped tiles soften timber, especially at the foot of each rafter, where it bears on the masonry.
Some of these faults are old and stable, while others are active. Telling them apart is the surveyor's work, not the tiler's. A new covering laid over a moving frame hides the evidence and fixes nothing. It can even add load to timbers that are already struggling, so the order of work matters as much as the work itself. Strengthening first and re-covering second is almost always cheaper than the other way round.
Heavier coverings and the weight check
A frame is built for one kind of covering. Change natural slate or clay to concrete, or add a second layer, and the load on the same timbers changes. A swap of one covering for another can also weigh more if the old one was thin or partly missing.
Is approval needed for a heavier covering?
It can be. Under Planning Portal guidance, a new covering that makes the roof 15 per cent or more heavier calls for Building Regulations approval, and an engineer's or surveyor's advice should come beforehand. Approved Document A words its test as a roof loading increase of more than 15%, so the safe course is to ask before you order materials.
Planning permission is a separate question, and replacing like with like on a house is normally permitted development under Class C. For the structure, ask Somerset Building Control on 0300 303 7790 or your own building control body. For a listed house, put the question to Somerset Council's planning team at the outset, so that nothing has to be returned or taken down.
Who to call, and in what order
Start with the engineer, not the roofer. A structural engineer or chartered surveyor can say whether the frame needs strengthening, whether ties are missing or timbers must be replaced, and whether a new covering is safe to put on it. The roofer then works to that advice, and the covering goes on a frame that is fit for it. If the report asks for new ties, bigger rafters or a steel beam, those jobs are done before a single tile is laid, and we quote for the roofing that follows.
Can you tell me over the phone if it is dangerous?
No, and nobody honest could. We look at the roof from both the garden and the loft, photograph what we see and tell you plainly when an engineer's opinion is needed. If water is coming in, we make it safe first, then repair or re-roof once the structural advice is in.
Our answer on heavier roof tiles and Building Regulations covers the weight rules, and whether roofers replace rafters explains timber repair. The pitched roofing page sets out the rest. You receive one price in writing once the survey and the engineer's advice are both done.
More on sagging roofs
Will new tiles cure a sagging ridge?
No. New tiles sit on the frame and follow its shape. If the ridge has dropped because the rafters spread or a timber has rotted, the new covering will dip in the same place. The frame is repaired first, on an engineer's advice, and the covering goes on afterwards.
Can I leave a sag that has been there for years?
Possibly, if it is stable, but an old dip can still be growing slowly. Photograph it against a fixed point and measure it twice a year. New cracking, a shifted line of tiles or a change after heavy rain or snow is a reason to ask an engineer to look.
The full picture: Pitched roofing in Taunton ยท More short answers