Signs Of Subsidence

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

Who fixes this

• Structural repairs

Questions people ask

Sources & References

1. GOV.UK: Approved Document A (Structure)
2. GOV.UK: Tree preservation orders and trees in conservation areas
3. BRE Group: Building research and guidance