Are tree roots damaging my house?

The instinct on seeing a crack near a tree is to remove the tree. On clay soils that can be exactly the wrong first move, because clay that has been kept dry by a tree for thirty years rehydrates and swells once the tree goes, lifting the ground in what is called heave. Heave damage can exceed the original subsidence damage and is harder to repair, so the sequence matters more than the speed.

What you'll need

• A camera
• A ruler or crack gauge
• A notebook

Step by step

1. Look at the crack before you look at the tree.Movement cracks associated with subsidence are typically diagonal, run through brickwork rather than along mortar joints, are wider at one end than the other, often wider at the top, and frequently appear near windows and doors where the wall is weakest. Hairline vertical cracks in render, fine cracks at plasterboard joints and cracks that follow mortar lines usually mean something else.
2. Rule out the common alternatives.Thermal movement, shrinkage in new plaster, a failed lintel, sulphate attack, a leaking drain washing soil out, and simple settlement in a new building all crack walls. A leaking drain is by far the most under-diagnosed cause and is much cheaper to fix than anything else on the list.
3. Establish the soil type.Tree-related subsidence is overwhelmingly a shrinkable clay phenomenon. On sand, gravel or chalk it is very unlikely, and the diagnosis should move elsewhere. The British Geological Survey mapping and any nearby ground investigation will tell you.
4. Measure the distance and identify the species.Note the distance from trunk to the affected corner and the species. A poplar at 8m on clay is a plausible cause; a rowan at 8m on gravel is not.
5. Note the season.Clay shrinkage damage typically appears or worsens in late summer and autumn after a dry period and partially closes over winter. A crack that behaves that way through a year is strong supporting evidence. One that does not vary seasonally is probably something else.
6. Monitor before acting.Photograph the crack with a ruler in shot, monthly, and date every photograph. A crack gauge glued across it gives a proper record for a few pounds. Twelve months of monitoring turns speculation into evidence and is what an engineer or insurer will want.
7. Tell your insurer if the damage is significant.Subsidence is usually covered by buildings insurance and claims are handled with specialist investigation — trial pits, soil analysis, root identification and level monitoring. Do not commission major remedial work first; it can complicate a claim.
8. Get a structural engineer, not a tree surgeon, for the diagnosis.The question is what is moving and why. A chartered structural engineer with subsidence experience answers that. An arboriculturist then advises on the tree, once the cause is established.
9. Do not fell first on clay.If a tree has been drying clay soil for decades, removing it lets that clay rehydrate and swell, and heave can lift and crack a building severely. Where a tree is implicated on clay, staged crown reduction under professional advice is often preferred to removal.
10. Check whether the tree is protected before any work.A TPO or Conservation Area status applies even where damage is alleged. Councils do consider consent applications supported by engineering evidence, but the evidence has to come first.

Tips

• A leaking drain is the single most commonly missed cause of localised subsidence, and a drain survey costs a fraction of anything else in this process.
• If a neighbour's tree is implicated, the same evidence-first sequence applies. A demand to fell backed by nothing rarely succeeds; an engineer's report changes the conversation entirely.
• Cracks under 5mm are usually monitored rather than repaired straight away, because repairing while movement continues just cracks again.

Common mistakes

• Felling the tree as the first response — On clay it risks heave, which can be worse than the original problem, and it destroys the evidence needed for an insurance claim or a consent application.
• Filling and painting the crack — It hides the very thing you need to monitor, and if movement continues it simply reopens.
• Assuming any crack near a tree is the tree's fault — Thermal movement, drain leaks, lintel failure and new-build settlement are all more common. The soil type test alone rules trees out on most sites.
• Not telling the insurer — Subsidence claims have specific requirements and doing unapproved remedial work first can prejudice cover.

If it doesn't work

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