What should I do about subsidence cracks in a dry summer?

Much of southern and eastern England sits on shrinkable clay, which loses volume as it dries. In a hot dry summer, clay under and beside a house shrinks — especially where a thirsty tree is drawing water out of it — and the foundation follows it down. That is subsidence. It is usually seasonal and partly reversible, which is why cracks in clay areas open in late summer and close again over winter, and why a single photograph in August proves nothing on its own.

Safety

What you'll need

• A camera or phone
• A ruler or a coin for scale
• A notebook or a phone note for dates
• A spirit level, for checking whether a wall is plumb (optional)
• Your buildings insurance policy number

Step by step

1. Photograph everything before you touch it.Each crack, with a ruler or a coin against it for scale, from the same position each time, with the date. This record is the single most valuable thing you will produce, for the engineer and for the insurer.
2. Work out whether it is in the plaster or in the structure.A crack that stops at the plaster and does not appear on the outside of the same wall is almost always shrinkage or settlement of the finish. One that lines up inside and outside, and follows the mortar joints in a staircase pattern, is structural movement.
3. Measure the width and note the direction.Hairline to 1mm is cosmetic. Up to about 5mm is usually repairable decoration. Above that, and especially above 15mm, it needs assessment. Diagonal cracks that are wider at one end are the ones that matter; straight vertical cracks at a wall junction usually are not.
4. Check the doors and windows.Doors and windows that have started binding in the last few months, in the same part of the house as the cracks, are a strong sign of real movement. Ones that have always stuck are not.
5. Look for the obvious cause outside.A large tree or hedge close to the wall, a leaking gully or downpipe, a drain that has been running into the ground, or a recent extension. Trees and leaking drains between them account for most domestic subsidence claims.
6. Monitor across seasons before acting.Photograph the same cracks monthly for six to twelve months. Clay shrinkage cracks that open in September and close by March are seasonal movement, and that pattern changes the diagnosis and the repair completely.
7. Ring the insurer before commissioning any survey.Subsidence is covered by standard buildings policies with a higher excess, typically £1,000. The insurer will normally appoint their own engineer, and a survey you commission first may not be accepted or reimbursed.
8. Do not fill the cracks while they are still moving.Filler in a live crack cracks again within months and destroys the evidence of how much it has moved. Repair is the last step, once monitoring shows movement has stopped.
9. Be very careful about removing a tree.Removing a mature tree from clay can cause heave, which lifts the foundation and can do more damage than the shrinkage did. This is an arboriculturist and engineer decision, and the tree may also be protected or in a conservation area.
10. Deal with leaking drains, whatever else is happening.A leaking drain washing fines out of the ground is both a common cause and a cheap fix by comparison. A CCTV drain survey is the standard first investigation and is worth doing regardless.

Tips

• Cracks that are wider at the top usually indicate downward movement below; wider at the bottom often indicates heave. It is one of the more useful things a homeowner can observe.
• The classic subsidence pattern is a diagonal crack running from a window or door corner outwards and upwards, visible on both faces of the wall, following the mortar joints.
• Not all cracking is subsidence. Thermal movement, lintel failure, wall tie corrosion, plaster shrinkage in a new build and settlement of a recent extension all produce cracks and are all treated differently.
• Cracks at the junction between an extension and the original house are extremely common and usually not structural — the two structures move differently and the joint takes the strain.
• Declaring a subsidence claim affects the insurability and saleability of the house for years, so it is worth being confident before you make one — but concealing genuine structural movement is far worse.

Common mistakes

• Filling and redecorating immediately — It hides the evidence needed to diagnose the movement, and the crack reappears within a year. Monitor first, repair last.
• Cutting down a tree to solve it — Removing a mature tree from clay soil releases the moisture it was taking out, the ground swells, and heave can crack the house more severely than the drying did. Get advice before touching a tree.
• Commissioning a private structural survey before ringing the insurer — Insurers normally appoint their own engineer and may not accept or pay for someone else's report. Report first, then follow their process.
• Panicking at any crack in a hot summer — The great majority of cracks in a dry summer are cosmetic and seasonal, and they close again over winter. Documentation and patience diagnose it more cheaply than anything else.

If it doesn't work

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