The short answer
Roof spread is the outward movement of the top of a wall because the rafters above it are pushing sideways as well as downwards. It happens when nothing holds the rafter feet from sliding apart, often after ties are cut or a heavier covering goes on. A structural engineer should look at it before any roofer prices the work.
Rafters that push as well as press
A pitched roof is made of pairs of rafters, the angled beams that rise from the wall to the ridge. Each pair leans on the other at the top. Weight acts straight down, but because the rafters are sloped, part of that weight also tries to shove the foot of each rafter outwards, along the line of the ceiling.
In a sound roof that sideways push is held. Horizontal timbers, called ceiling joists or ties, tie the rafter feet together across the width of the house, and sometimes collars or purlins brace the slope higher up. The forces cancel out inside the roof, and the walls carry only the downward load.
Roof spread starts when that balance fails. The rafter feet slide apart, the pair flattens slightly and the wall plate, the timber bedded on top of the wall, is pushed outwards. The wall itself can lean out at the top or crack, and the ridge may drop. It is a structural movement, a different thing from slates sliding or a tile slipping, which is why it needs different people.
Common causes: heavier coverings and removed ties
Two causes come up again and again. The first is a covering that is heavier than the one the roof was built for. A roof designed for light clay or slate that is later covered in something much heavier has to carry more load, and the rafters push harder. The Planning Portal puts the line at a new covering adding 15 per cent or more to the roof's weight: beyond that, Building Regulations approval is needed. It advises taking a structural engineer's view first, ahead of any work.
The second is alteration inside the roof. A joist or tie cut to make room for a loft ladder, a loft conversion or a new opening can remove the very timber that held the rafter feet together. A chimney breast taken out below, or a purlin trimmed, can have the same effect on the load path.
Timber decay, old movement in the walls and undersized original timbers also appear on the list. In every case we can describe the symptoms, but the cause is for an engineer to decide, and our page on heavier roof tiles explains why weight matters.
Signs you can see from the ground and the loft
From outside, look at the walls just below the eaves, where the roof meets the wall. Roof spread can show as a wall leaning outwards at the top, a crack along the line where wall and roof meet, or guttering that no longer sits tight against its board. The ridge line may dip, and the slopes can look flatter than they used to.
From inside the loft, check whether ceiling joists have been notched, cut through or removed near the eaves, and whether any rafter foot has moved on its wall plate. Look for a gap opening between a rafter and a joist, or at a joint that has been nailed rather than bolted.
A roof in the early stages shows very little. Hairline cracks in the ceiling plaster at the corners can be the first sign, and they appear alongside settlement and shrinkage, which is why photographs and a careful look matter more than a guess. Keep records over a few weeks, because a crack that grows is a different matter from one that stays the same.
Structural engineer first, then the repair
If you suspect spread, ask a structural engineer to inspect before anyone quotes for roof work. The engineer works out whether the movement is old and finished or still going, and specifies the cure, which might be new ties, steel or timber strapping, or reinforcement at the wall plate. It is not a job for adding more tiles.
Structural work can need Building Regulations approval, and your building control body is the place to ask. You can use the council's team or a registered building control approver. Either will want the engineer's drawings and calculations, so ask the engineer to prepare them in a form that can be handed over, and keep copies of everything for a future sale.
Our part, as pitched roofing specialists, comes after the diagnosis. We can survey the covering, battens and underlay, record the condition with photographs, and carry out the roofing side of the repair to the engineer's specification, for one written total price. Where weight is the cause, the covering may have to change as well, and the engineer will say so. Any tiles stripped to reach the timbers are stacked, counted and reused where they are sound, and the site is left clear when each day ends.
