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Signs Of Subsidence
Diagonal cracks, sticking doors and a floor that has dropped — the pattern that suggests the ground under the house has moved.
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What's happening?
Subsidence is downward movement of the ground supporting a building's foundations, so part of the house drops relative to the rest. The pattern is what identifies it: cracks that run diagonally, are wider at the top than the bottom, appear inside and outside in the same place, are usually wider than 3mm, and are worst near a corner or an extension. Doors and windows in the affected area start to bind because the openings are no longer square. In much of England, clay soils shrinking in a dry summer and swelling again in a wet winter are the usual mechanism, which is why subsidence damage often appears in late summer and seems to ease in winter — and then comes back worse.
What usually causes it?
- Clay subsoil shrinking as it dries out, most severe after a long dry summer
- A tree or large shrub close to the house drawing moisture out of a clay soil
- A leaking drain or water main washing fines out of the ground beneath a footing
- Foundations built on made-up ground, an old pond, a filled pit or soft spots that consolidate over time
- A shallow-founded extension or bay next to a deeper-founded main house, so the two settle differently
- Removal of a mature tree causing clay to swell back, which is heave rather than subsidence but cracks in similar places
- Historic mining or natural cavities beneath the site
Is it dangerous?
Structural movement is serious and needs diagnosing, but it is very rarely an immediate collapse risk in a domestic house — the damage usually develops over seasons. Treat it as urgent rather than an emergency, unless a wall has bowed, a floor has dropped noticeably, a lintel is sagging or a chimney has moved, in which case stop using the affected area and get someone out. The reason to act quickly is not that the house is about to fall down; it is that the cause keeps working and repairs done before the cause is fixed are wasted.
Can I fix it myself?
Nothing structural, but three things genuinely help. Monitor and photograph the cracks with a dated ruler over months, because a surveyor's first question is whether the movement is progressive. Check for the obvious cause: a tree within its own height of the wall, or a drain that gurgles, smells or has a damp patch nearby. And contact your insurer before commissioning any repair — subsidence is normally an insured peril and they will want to investigate. Do not remove a tree on the assumption it is the culprit, and do not fill the cracks.
When should you call a builder?
Call a builder and your insurer once cracks show the diagonal, tapering, inside-and-outside pattern, or once doors and windows in one part of the house have started to bind. Any tree you suspect may be protected — a tree preservation order or a conservation area means you must get consent before pruning or felling, and doing it without permission is an offence. Expect a structural engineer, a drain survey and often a period of monitoring before anyone proposes a repair.
What will the builder actually do?
- Record and monitor the crack pattern over time so progressive movement can be distinguished from historic
- Arrange the investigations that identify the cause — trial pits to see the foundations, a CCTV drain survey, soil sampling
- Fix the mechanism first: repair the leaking drain, manage or remove the offending vegetation with any consent needed
- Carry out the engineer's remedial design, which may be underpinning, a root barrier, or simply repair once movement has stopped
- Stitch and rebuild the damaged masonry with helical reinforcement and rebuild sections where needed
- Reinstate lintels, floors, plaster and decoration once the structure is stable
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