Bulging and bowing walls: causes and assessment

A pitched roof pushes outwards at its base unless something ties the feet of the rafters together. Where ceiling joists act as that tie, and they have been cut, weakened or never properly connected, the roof spreads and pushes the tops of the walls out with it. The bulge is worst near eaves level and is often accompanied by cracking at the top corners of the end walls.

Loft conversions are a frequent trigger, because the ceiling joists that were quietly doing the tying get cut for a staircase or a floor. The roof then has nothing holding its feet, and the walls take the thrust.

Floors provide restraint too. A wall relies on the floors and roof to hold it laterally at intervals up its height. Where a floor has been removed, or where joists run parallel to a wall and were never strapped to it, the wall becomes tall and unrestrained and will bow under wind or under its own eccentricity.

Measuring the bow matters, because the amount of out of plane movement relative to the wall thickness governs how serious it is. A bulge of a few millimetres in a solid wall is a very different matter from a bulge approaching a third of the thickness, where the load path through the wall is genuinely compromised.

Establishing whether it is progressing is again the pivotal question, and again it is answered by monitoring rather than opinion. A wall that bowed decades ago and stopped can often be restrained and left. A wall still moving needs the cause removed first.

Remedies run from lateral restraint straps tying walls to floors and roof, through remedial wall ties, to taking down and rebuilding the affected section. The cheapest effective option depends entirely on the mechanism, which is why identifying the cause comes before pricing the repair.