Trees and subsidence

Trees cause subsidence when their roots draw water from shrinkable clay beneath a building, making the clay shrink and the foundations drop, usually worst in late summer. Felling a tree is not a simple fix, because the clay can swell back and cause heave, so get advice before cutting anything.

How do trees cause subsidence?

Trees cause subsidence by drying out the soil, not by pushing on foundations. On shrinkable clay, roots draw moisture from the ground, and the clay shrinks as it dries, taking shallow foundations down with it. The effect is strongest in late summer and after long dry spells, when trees are using most water and rain is scarce. The clay usually swells again in winter, which is why tree-related cracks often open and close with the seasons.

Which trees and soils carry the most risk?

The risk is greatest where a large, thirsty tree stands close to a house built on shrinkable clay. Shallow foundations, common in older houses, bays and extensions, are more affected than deep ones. The risk is much lower on sand, gravel, chalk or rock, which do not shrink in the same way. Every site is different, so an engineer or arboriculturist judges each case.

• High water-demand species such as oak, willow and poplar
• Trees whose height is close to their distance from the house
• Fast-growing trees and hedges that are getting bigger
• Shrinkable clays such as London, Oxford and Gault Clay
• Shallow foundations under bays and extensions

Should I cut down a tree that might be causing subsidence?

No, do not fell a tree near your house without advice first. On clay, a mature tree may have kept the ground dry for many years. Remove it, and the clay can take up water and swell over a long period, lifting and cracking the building. This is called heave, and it can be as damaging as subsidence. Sometimes reducing or managing the tree is enough. If the tree is part of an insurance claim, the insurer and engineer will usually decide what should happen.

What if the tree is protected or belongs to someone else?

If the tree is protected or belongs to someone else, do not cut it yourself. Many trees are protected by a tree preservation order or a conservation area, and working on them without consent can be an offence, so check with your council before any felling or pruning. They can tell you whether a tree is protected and what consent or notice is needed. If the tree belongs to a neighbour or the council, talk to your insurer rather than acting yourself. Where there is a claim, the insurer usually approaches the tree owner.

Do root barriers stop subsidence?

Root barriers can help in some cases, but they are not a universal fix. A root barrier is a vertical membrane or wall set in a trench between a tree and a building, to keep roots out of the soil beneath the foundations. It can be useful where a tree must stay, for example because it is protected. It needs to be deep enough, positioned correctly and designed for the site, and it may not stop roots already under the house. An engineer or arboriculturist should advise.

How is tree-related subsidence investigated?

Tree-related subsidence is usually investigated with trial pits to expose the foundations and soil, root samples sent for identification, and soil tests that show how dry and shrinkable the clay is. A structural engineer’s investigation and report is a guide £500 to £2,000 inc VAT, varying by region and scope. Monitoring often follows, sometimes for up to 12 months, to show whether movement follows the seasons and whether action on the tree has worked. Drains are usually checked as well.

How are the cracks repaired?

Once the tree has been dealt with and monitoring shows the building is stable, most tree-related damage is repaired with crack stitching and making good rather than underpinning. Crack stitching is a guide £80 to £180 per metre of crack inc VAT. Where the tree must stay and movement continues, deeper foundations such as mini-piled underpinning may be needed, at a guide £2,000 to £3,500 per metre of wall. Prices vary by region and access and are confirmed in writing after a survey.

Questions about this guide