What is roof spread, and what are the signs?

A pitched roof is a set of sloping timbers, called rafters, meeting at the ridge at the top and resting on the wall plate at the bottom. The slope wants to push those timbers outwards at the eaves, where the slope ends above the wall. In a sound roof something stops that. Ceiling joists, collars or trusses tie the two sides together so the load goes down the walls rather than sideways.

Roof spread starts when that restraint is missing, cut or has failed. The rafter feet slide a little, the ridge sinks to follow them, and the top of the wall is pushed out of line. It is usually slow. A roof can move for years before anyone sees it, and the movement often stops if the cause is put right. The useful first question is what made it move, not how to hide it.

Spread is not the same as a roof that sags between its supports. If the line of the slope bows in the middle, our answer on why a roof sags in the middle covers that. Often the two faults sit together.

From the pavement opposite, sight along the ridge, the line at the very top. It should run straight. A dip, or a ridge lower at one end, is the first thing to note. Then check the eaves. Gutters hanging away from the brickwork, fascia boards that no longer sit true, and a visible gap where the roof edge meets the wall all point the same way.

On the walls, look for horizontal cracks near the top, or brickwork that leans slightly outward under the eaves. At a gable end, the triangular wall at the side of the house, a gap can open between the wall and the verge, the tiled edge of the roof. Tiles may begin to slip or sit unevenly because the battens, the strips the tiles hang from, have shifted with the timbers below.

Photograph each sign in good light, from the same spot if you can. Pictures taken a few months apart are the simplest way for a surveyor to tell whether the movement is still happening.

Upstairs, check the corners of the top-floor rooms. Cracks where the ceiling meets the wall, or a ceiling line that looks slightly lifted at the edge, can come from walls moving outwards. Doors on the top floor that begin to stick for no clear reason may be catching on a frame that has shifted. These signs have other causes too, so none of them proves spread alone.

In the loft, with a torch and from the hatch or a firm boarded area, look at where the rafters meet the wall plate. A rafter foot that has slid, gaps at joints, or ties that have been cut through are all worth recording. Do not walk across the ceiling between joists. Plasterboard will not carry you, and it is easy to put a foot through it.

If the loft has been converted at some point, look at what was taken out. Timbers removed to make headroom are one of the more likely reasons a roof starts to push outwards.

The usual causes are removed or weakened restraint, a heavier covering, rot or movement in the timbers, and an older design that never had proper ties. A loft conversion that cut through collars or ceiling joists without replacing their job with steel or new timber is a common story. So is a roof re-covered with a far heavier tile than its frame was built to carry. Our answer on heavier roof tiles and Building Regulations sets out when that needs approval.

Rot at the wall plate or at rafter feet, usually from a long-running leak, lets the timbers sink. The style of the roof matters as well. A traditional cut roof, built piece by piece on site, relies on the carpenter having tied it properly, while a trussed roof carries its ties in the triangles. We compare the two in cut roof or trussed roof.

Whatever the cause, the cure is structural and has to be designed around the timbers actually in your loft.

Do not have tiles re-laid over a roof that is still moving. New tiles will slip again, and a patch only hides the evidence. Start with someone qualified to judge the frame: a chartered surveyor or a structural engineer. Planning Portal guidance points to both of them when a covering changes a roof's weight, and the same sense applies when the roof itself is moving.

Structural alterations are one of the triggers for Building Regulations approval, so ask the engineer to say what the repair will need and which body, the council or a registered building control approver, signs it off. Their design may add ties, steel or new timbers, and the covering is then made good around it.

Our part is the roof above the timbers. We look over the tiles, ridge and verges, take pictures for the surveyor's file and tell you if the trouble lies in the frame instead. Our pitched roofing page covers that work, and our price is set once the engineer has defined what the structure needs.