Question answered
What is roof spread and how is it fixed?
The short answer
Roof spread happens when the rafters of a pitched roof push the tops of the walls outward because nothing holds their feet together. The signs are a sagging ridge, walls that bow or crack at eaves level and tiles that keep slipping. The cure restores the missing tie, and a structural engineer should design it before any roofing work starts.
Roof spread is the slow outward push of a roof on the walls that carry it. It shows as bowing and sagging, and it is a structural fault, not a tiling job.
Why a pitched roof tries to shove its walls apart
Two rafters leaning together form a triangle, and the weight of the roof runs down them. At the bottom, part of that force turns sideways and tries to slide the rafter feet along the wall plate. In a sound roof, ties such as ceiling joists, collars and purlin struts resist the push, and the load travels down through the walls.
Spread begins when that resistance fails. A tie rots, is cut out, or was never fitted. The rafters then press outward and downward, the ridge drops and the wall tops drift out of line. It happens slowly, over years, often at a rate nobody notices from day to day.
A sound frame stops it, and the design of that frame varies with the roof type, as we explain in cut roof or trussed roof. Older cut roofs rely on a handful of members, so losing one matters.
Signs you can see for yourself
Sight along the ridge from the pavement. A ridge that dips between the gables, or a slope that looks hollow, is the classic sign. Then look at the walls. Bowing near the top, horizontal cracks in the brickwork at eaves level and a gap opening between the wall and the ceiling inside all deserve a note.
Indoors, hairline cracks that follow the line where ceiling meets wall (and that reopen after being filled), upstairs doors that begin to stick and bedroom walls that no longer feel plumb can all come from movement. None proves spread by itself, since other causes exist, such as settlement or cracked lintels.
Keep dated photographs. A crack that has grown over several months tells an engineer more than a single visit can. Do not go up on the roof for a closer look, and read why you should not walk on roof tiles before you think of it.
What sets it off
The usual causes are changes to the frame. A collar or tie taken out to gain headroom in the loft is one. Rot where the rafter feet bear on the wall plate is another, often after years of a slow leak or a failed gutter. Timber that has been eaten away or weakened, or a joint that has slipped, also allows movement.
A switch to heavier tiles can start it as well. A roof designed for light tiles can be pushed past its limits by heavy ones, which is why Approved Document A calls a load increase of more than 15% a significant change. Where the new covering is much heavier, the Planning Portal says to have a structural engineer or surveyor look first.
Roofs with too little triangulation from the outset spread more readily, particularly where a past owner removed a tie to gain headroom. Covering and timber both affect the load, and our page on interlocking or plain tiles covers the weight side of that choice.
Why a surveyor calls the engineer in first
Any visible movement in a wall, a ridge that has clearly dropped, or cracks that are widening call for a structural engineer before roof work begins. They assess the walls, the timbers and what restraint is left, and write a specification for the builder and roofer to follow. That protects you, because the first question, whether the walls can still carry the roof, is not one a roofer can answer.
Typical remedies include tying the rafter feet back together with new ties or steel straps, replacing rotten wall plates, strapping the roof to the walls, renewing purlin supports and, where the walls have travelled far, rebuilding their tops. A lighter covering is sometimes chosen to reduce the load.
Building control may need to be involved, as the Planning Portal lists structural alterations among the cases that call for approval. Where the work is notifiable, we handle that notification for you.
Where the roofer fits in
Once the structure is secure, the covering can be dealt with. Tiles may be lifted, stacked and relaid, with loose or broken ones replaced as part of that. A moving gable is also the time to look at the tile hanging on the walls below, since the same movement cracks it.
Our part is the survey and the roof side. We photograph the ridge, the eaves and the loft timbers, tell you plainly if we think the fault is structural, and point you to an engineer. We would not price a re-tile over a roof that may still be moving. The written quote for the roofing work, a single total, follows once the structure is settled.
For the broader question of which form suits a building, see pitched or flat roof. The full list of tiled-roof services is on the pitched roofing page.
Spreading roofs: further questions
Does a sagging ridge always mean spread?
No. Rotten rafters, a failed purlin, overloaded timbers or settled walls can also drop a ridge. That is why the loft inspection comes first. We trace where the sag begins and whether the wall tops are moving, then say whether an engineer is the next step.
Would re-tiling with a lighter covering stop it?
A lighter covering reduces the load, but it does not replace a missing tie. If the cause is a removed collar or a rotten plate, the structure needs repair first. An engineer may well specify a lighter covering as part of a wider fix, but not as a substitute for one.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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