Most cracks in walls are harmless. Some are the first visible sign of a structural problem that gets more expensive every month it is ignored. Here is how to tell the difference – the same first checks our surveyors make.
Cracks you usually don’t need to worry about
- Hairline cracks (under 1mm) in plaster, especially around new plasterwork or recently decorated rooms – normal drying and seasonal movement.
- Straight cracks along plasterboard joints in ceilings and stud walls.
- Fine cracks above doors and windows that do not grow year on year.
Cracks that deserve attention
- Diagonal cracks wider than 3mm (a pound coin’s thickness), especially stepping through brickwork mortar joints outside.
- Cracks wider at the top than the bottom, or ones you can see daylight through.
- Cracks that reopen after filling, or appear suddenly after a dry summer – the classic pattern of subsidence on clay soils.
- Cracks with sticking doors and windows or sloping floors in the same corner of the house.
- Horizontal cracks in walls – in older homes these can indicate corroding cavity wall ties, a common hidden defect in 1930s–1970s houses.
What surveyors actually do about cracks
A structural crack assessment categorises damage using the BRE scale (0–5), identifies the cause – movement, thermal, lintel failure, tree roots, leaking drains – and tells you in plain terms whether repair is cosmetic filling or structural work. Where movement may be ongoing, crack monitoring over a few months establishes whether it is live or historic; insurers usually insist on this before funding underpinning.
When to act
Get a professional opinion if a crack is wider than 3mm, growing, stepped through brickwork, or paired with sticking doors or sloping floors – and always before buying a property with visible cracking (a Level 3 Building Survey covers this in depth). One fixed-fee inspection settles the question and, in most cases, the answer is reassuring.
Worried about a crack? Call 020 4579 8270 for a fixed-fee crack assessment anywhere in London, or book online.
Cracks in plaster: when to worry
Plaster cracks follow the same rules as above with one addition: distinguish the plaster from the wall behind it. Hairline crazing, cracks along board joints and cracks around recently dried plaster are surface-level. But a plaster crack that mirrors a crack in the masonry behind, steps diagonally, or keeps returning after filling is reporting movement in the structure – treat it as structural until shown otherwise.
How are structural cracks in walls fixed?
Depending on cause: repointing and stitching bars (crack stitching) for local masonry cracking; wall tie replacement where corroded ties are the culprit; lintel replacement over openings; drainage or tree works where clay movement is the driver; and underpinning only in the minority of confirmed, progressive subsidence cases. The fix follows the diagnosis – never the other way round, which is why we assess before anyone quotes for repairs.