Almost every building has cracks, and the overwhelming majority are cosmetic. But a minority signal genuine structural movement - and the anxiety comes from not knowing which kind you're looking at. Here's how engineers think about it.
Common harmless causes
- Shrinkage - new plaster, new construction and modern materials drying out; fine cracks, often straight, stable once formed
- Thermal movement - seasonal opening and closing, typically at junctions between materials
- Vibration and minor settlement - hairlines around openings and in corners, common after nearby works
- Old movement, long finished - historic cracks repeatedly filled and stable for decades
Patterns that earn an inspection
- Stepped or diagonal cracking through brick or stonework, especially widening as it rises
- Cracks wider than about 3mm, or any crack that is clearly growing
- Cracking mirrored inside and outside at the same location
- Cracks accompanied by sloping floors, sticking doors and windows, or leaning/bulging walls
- Cracking near trees on clay ground, or after drainage leaks - classic subsidence drivers
- New cracking after structural alterations - walls removed, chimney breasts taken out, lofts converted
What an engineer actually does about a crack
An inspection looks at width, pattern, direction, age and context - plus floors, openings and the outside of the building - to identify the mechanism causing the crack. The output is a report saying what it is, whether it's active, and what (if anything) to do: often simple repair, occasionally monitoring over time, rarely investigation or designed remedial work. That written answer is also exactly what lenders, buyers and insurers need when a crack is holding up a transaction or claim.
If a crack in your property fits the second list - or is simply worrying you - a structural survey settles it. This is a sensitive area to guess in, and we'd always rather tell someone their crack is harmless than have them not ask.