Roof Spread: the Wall Bulge That Is Not Subsidence

Roof spread is what happens when a pitched roof stops behaving like a triangle and starts behaving like a wedge. The rafters slide down and outwards, the tops of the walls they sit on get pushed out with them, and the damage shows up inside as cracked plaster and doors that will not shut. Homeowners almost always read that pattern as subsidence and call the wrong trade.

It is not subsidence, and in most cases it is not a roofing job either. This page covers how to recognise it, how to tell it apart from a sagging roof, what causes it, and why the first call should be a structural engineer rather than a roofer.

The mechanism, in one paragraph

A traditional British cut roof is a triangle. The rafters lean against each other at the ridge, and their feet sit on a timber wall plate along the top of each eaves wall. Because the rafters are at an angle, the weight of the roof does not just press straight down. It also pushes outwards at the feet. What stops the walls being shoved apart is the horizontal member closing the bottom of the triangle: usually the ceiling joists, nailed to the rafter feet, sometimes collars or ties higher up.

Cut that tie, weaken it, or load the roof more heavily than it was built for, and the triangle opens. The rafter feet travel outwards, the ridge drops, and the walls go with it.

The signs, roughly in the order you notice them

• Doors and windows sticking upstairs, particularly on the first floor and particularly at the top corner. This is usually the first thing anyone notices.
• A gap opening between the ceiling and the top of the upstairs wall, often running the length of the room. The ceiling stays put while the wall head moves away from it.
• A horizontal crack in the external brickwork near eaves level, sometimes with a stepped crack running down from it.
• An outward lean or bulge in the wall when you sight along it from the corner of the house. A spirit level or a plumb line held from an upstairs window confirms it faster than any amount of squinting.
• Rafter feet visibly sitting proud of the wall plate in the loft, sometimes with the wall plate itself split or rotten.
• A dipped or wavy ridge line, and tiles or slates opening up at the eaves.

One correction worth making, because it is repeated everywhere: the walls that get pushed are the eaves walls, the ones the rafter feet bear on. On a typical semi or terrace that means the front and rear elevations. A gable runs parallel to the rafters and does not take the thrust directly, though it can crack or lean if the ridge and purlins drop into it.

Telling it apart from the two things it gets confused with

Subsidence moves the building downwards, so the cracks are usually widest at the top and taper towards the bottom, they run diagonally, and they often appear at ground floor level near a corner, a drain or a large tree. Roof spread moves the building outwards at the top, so the crack pattern is highest on the wall and horizontal at eaves level. If the worst damage is at the top of the house and the ground floor is largely intact, spread is the likelier answer.

A sagging roof is a deflection problem, not a spread problem. The rafters or, more often, the purlins carrying them are undersized, overloaded or rotten, so the roof surface dishes and the ridge dips. Crucially, the walls stay plumb. If the roof line is wavy but a plumb line down the front wall is straight, you are looking at deflection and our page on sagging roof causes and repair is the relevant one. When the ridge drops and the walls lean out, the two are happening together, and the sag is a symptom of the spread.

What causes it

Ceiling ties cut for a loft conversion

This is the big one. Ceiling joists in a loft look like floor joists that happen to be a bit thin, and they are routinely cut, notched or removed to make headroom, to run a staircase, or to build a room in the roof. They are not floor joists. They are the tie that stops the roof splaying, and removing them without replacing that restraint with something else, a steel ridge beam or steel ties, is the single most common route to roof spread. If you are planning work up there, read our notes on loft conversion roofing first.

Purlins or collars removed

Same principle one level up. Collars, binders and purlin struts get taken out to open up a space or to fit rooflights. Each one is doing a job.

Re-covering a light roof in a heavier one

This is the cause people underestimate, so here are the actual manufacturer figures rather than a vague warning.

Natural slate in the most common British size, 500 x 250mm at around 5mm thick, weighs roughly 35 kg per square metre once laid, according to Cupa’s own coverage data. A standard concrete interlocking tile, taking Marley Modern as the example, weighs 50 kg per square metre at 75mm headlap and 54 kg/m² at 100mm headlap on Marley’s technical data.

Swap one for the other and you have increased the covering weight by about 43%, or 54% at the larger headlap, on a structure that was sized for slate. That is not a marginal change, and it is exactly the kind of alteration that pushes an already marginal roof over the line.

Building control takes the same view. Approved Document A treats a loading increase of more than 15% as significant, and where a re-covering exceeds it, strengthening works or replacement of roofing members are required rather than optional. The Planning Portal’s re-roofing guidance puts it plainly: if the new covering is significantly heavier or lighter than the existing one, the roof structure may need modifying or strengthening, and you should check with a structural engineer or surveyor before starting. Our page on planning permission and building regs for a new roof covers the notification side.

Note the “or lighter” in that sentence. Going markedly lighter is its own problem, because a roof that was holding itself down by sheer mass now needs proper anchorage against wind uplift.

A rotten or split wall plate

The wall plate is the timber the rafter feet sit on and are nailed to. It is buried at the top of the wall, so nobody looks at it, and it is exactly where water gets in when the eaves detail fails. Once it has gone soft, the nailed connection holding the rafter feet has nothing to grip and the feet start to travel.

Bay window and porch roofs

Small lean-to and hipped bay roofs spread for the same reasons on a smaller scale, and the symptom is a bay that appears to be pulling away from the house.

Why a roofer is the wrong first call

Roof spread is a structural movement problem that happens to be in the roof. A roofer can re-bed the ridge, re-fix slipped tiles and make the covering watertight, and none of that stops the walls moving. Worse, replacing the covering without addressing the structure can add load and accelerate the problem.

The correct sequence is:

1. Structural engineer first. They will establish whether the movement is live or historic, what caused it, and what the roof needs. A monitoring period is sometimes recommended before any repair, so that the fix addresses actual ongoing movement rather than damage that stopped decades ago.
2. Building control involvement where the remedy is a material alteration, which it usually is.
3. The trades afterwards, working to the engineer’s design: often a carpenter and a steel fabricator, with a roofer reinstating the covering at the end.

Insurance is worth checking at step one. Gradual structural movement is often excluded, but the picture changes if the trigger was a specific event such as storm damage. Our page on home insurance and roof cover explains what tends to be covered.

What the fix usually looks like

Engineers reach for a fairly standard set of options, chosen to suit the roof:

• Reinstating the tie, by putting proper ceiling ties or steel tie rods back across the roof at rafter foot level. Cheapest and most effective, but only possible if you can live with members crossing the space.
• A structural ridge beam, typically steel, so the rafters hang from the ridge rather than leaning on each other. This makes the roof non-thrusting and is the standard answer for a loft conversion or a vaulted ceiling where ties are not acceptable.
• Steel purlins or a steel frame to pick up the rafters mid-span and take load off the walls.
• Collars, but with a caveat: a collar placed high in the roof does less than a tie at the feet, and the higher it sits the more thrust it leaves behind.
• Lateral restraint straps tying the wall plate and the walls back into the floor and roof structure, often used alongside one of the above.
• Rebuilding the top courses of the wall where the brickwork itself has been displaced too far to be pulled back or left.

Whether the wall gets pulled back or simply stabilised where it is depends on the engineer’s judgement. Forcing a bulged wall back to plumb is a much bigger job than restraining it, and is not always the right call.

Catching it early

Spread is slow, so the useful thing is a baseline. When you next have a roof inspection or survey, ask specifically whether the ceiling ties are intact and continuous, whether the wall plate is sound, and whether any purlin struts or collars have been removed. Photograph the loft. If you buy a house with a loft conversion, ask to see the building control completion certificate and the structural calculations, because the conversion that was done without them is the one that pushes the wall out ten years later.

Frequently asked questions

What is roof spread? Roof spread is the outward movement of the tops of the walls carrying a pitched roof, caused by the rafters pushing outwards when the horizontal tie closing the roof triangle is missing, cut or ineffective. The rafter feet slide out, the ridge drops, and the eaves walls lean.

How do I know if I have roof spread or subsidence? Look at where the damage is worst. Roof spread concentrates at the top of the house: horizontal cracking near eaves level, gaps between the ceiling and the wall head upstairs, and an outward lean when you sight along the wall. Subsidence usually shows diagonal cracks that are wider at the top, often starting at ground floor level near a corner. A structural engineer can tell them apart quickly.

Can I put concrete tiles on a roof built for slate? Not without a structural check. A common natural slate covering is around 35 kg/m², while a standard concrete interlocking tile such as Marley Modern is 50 to 54 kg/m² depending on headlap, an increase of roughly 43% to 54%. Approved Document A treats an increase in roof loading of more than 15% as significant and requires strengthening or replacement of roofing members.

Does roof spread need a roofer or a structural engineer? A structural engineer, first and always. A roofer can make the covering watertight but cannot diagnose or design a fix for structural movement, and re-covering the roof without addressing the cause can make it worse.

Is roof spread covered by home insurance? Often not, because gradual structural movement is a common exclusion. Claims are more likely to succeed where a specific insured event, such as a storm, caused or worsened the damage. Check your policy wording before committing to any work.

How much movement is too much? There is no single figure a homeowner can apply, because it depends on the wall’s construction, its height, and whether the movement is still going. What matters more is whether it is progressing. That is why engineers frequently recommend a period of monitoring before deciding on a repair.

Can removing a chimney cause roof spread? It can contribute. A chimney breast often provides restraint and sometimes carries purlin ends, so taking it out without adequate support transfers load and can leave a roof less braced than before. It is a structural alteration and should be designed as one, as our page on chimney removal sets out.