First, breathe: most cracks in walls are cosmetic — plaster shrinkage, seasonal movement, a house simply being a house. A small number signal something structural. Here's the 30-second triage surveyors actually use, the causes behind each type, and the point at which a professional should look.
The 30-second crack triage
- Hairline, vertical, under 3mm, in plaster only → almost always cosmetic. Fill, paint, move on.
- 3–5mm, reappearing after filling, or crossing corners → worth monitoring: photograph with a date and a coin for scale, re-check monthly.
- Diagonal cracks spreading from door and window corners → the classic movement signature — get it assessed.
- Stepped cracks following mortar joints in brickwork outside → assess promptly, especially with clay soil and nearby trees.
- Wider than 5mm, doors/windows sticking, sloping floors, or a crack you can see daylight through → professional inspection now.
What actually causes wall cracks
Shrinkage and settlement — new plaster, new builds bedding in, timber drying: harmless and common. Thermal and seasonal movement — materials expand and contract; clay soils swell and shrink with the seasons, opening and closing hairlines. Subsidence — downward foundation movement, classically from clay soil dehydrated by tree roots or leaking drains softening the ground; the diagonal-from-openings pattern is its signature (see how surveyors check for subsidence). Lintel failure — cracking above windows and doors where a corroded or overloaded lintel is giving way. Next door's building works — excavation and structural alterations can crack neighbouring homes; if works are under way next door, photograph everything now and read about party wall protections.
When to bring in a surveyor — and which survey
If a crack is in the amber or red categories above, a specific defect report answers the exact question — what caused this crack, is it active, what's the repair — for a fixed fee, in days. Buying a house with visible cracking? Step up to a Level 3 Building Survey: it examines the structure as a whole, not just the symptom. A surveyor can also tell historic movement (very common, often long-stabilised) from active movement — the distinction that decides whether anything needs doing at all.
If someone else caused it
Where cracking follows a neighbour's basement dig, extension or a developer's works, evidence beats argument: dated photographs, then an independent report attributing cause. If liability is disputed, our expert witness surveyors turn that evidence into a court-ready CPR Part 35 report.
Worried about a crack? A specific defect report gives you the answer in days — fixed fee, no drama → get a quote · 0204 579 8270.
