Trees and Subsidence: How Close Is Too Close to Your House?

Every summer we get the same call. A crack has appeared, it wasn’t there in spring, and there’s a large tree in the garden that suddenly looks a lot more suspicious than it did before.

Sometimes the tree is the problem. Often it isn’t. The difference comes down to three things: what the ground is made of, how far away the tree actually is, and what species it is.

Why trees cause subsidence at all

Trees don’t push buildings over. They dry the ground out.

A mature tree can draw a substantial volume of water out of the soil during the growing season. On sandy or gravel ground that barely matters, because those soils don’t change volume much when they lose moisture.

On clay, it matters a great deal. Shrinkable clay contracts as it dries. If that shrinkage happens unevenly beneath a foundation — more on the side nearest the tree, less on the far side — the building settles unevenly. That is subsidence.

It’s why cracking so often appears or widens in late summer after a dry spell, then seems to improve over winter when the ground rehydrates.

This is a Peterborough problem in particular

Much of Peterborough and the surrounding area sits on Oxford Clay — the same clay that supplied the city’s brickworks.

Clay of that type is firmly in the shrinkable category. Combine it with the mature trees found across older residential streets and the shallow foundations common in pre-war housing, and you have the classic conditions for tree-related subsidence.

The same applies across much of Hertfordshire and North London, where London Clay behaves in a similar way.

How close is too close?

The rough industry guidance is that a tree’s influence can extend to roughly its mature height, and for high water-demand species that distance is treated more cautiously.

But distance on its own doesn’t decide it. What matters is the combination:

• Soil type — shrinkable clay is the precondition. Without it, the tree is very unlikely to be causing structural movement.
• Species and water demand — oak, poplar, willow, elm and eucalyptus are high demand. Birch, holly and most fruit trees are considerably lower.
• Mature height, not current height — a sapling planted three metres from the house is a decision about the next thirty years.
• Foundation depth — a modern foundation founded below the zone of seasonal moisture change is far less vulnerable than a shallow Victorian footing.
• Crack pattern and timing — seasonal cracking that opens in summer and closes in winter points strongly towards clay shrinkage.

Don’t fell the tree first

This is the single most expensive mistake we see, and it’s an understandable one.

When a mature tree is removed from shrinkable clay, the ground it had been drying out slowly rehydrates and swells. That’s called heave, and it lifts the ground unevenly. Heave can do more damage than the original subsidence, and it is considerably harder and more expensive to remedy.

Removing a tree also removes the evidence. If an insurer later asks what caused the movement, the thing that would have answered the question is now a stump.

Get the assessment done first. If removal turns out to be the right answer, it can be done in a controlled way — sometimes in stages, sometimes alongside foundation work.

What an investigation actually involves

A tree-related subsidence assessment normally combines several things.

A structural appraisal establishes the crack pattern, where the movement is concentrated and whether it is active or historic. A trial pit exposes the foundation so its depth and bearing stratum can be measured directly, and a soil sample goes for laboratory testing to establish how shrinkable that particular clay is.

Root identification from the trial pit can confirm whether roots are actually present at foundation level, and which species they belong to. That last detail matters, because a nearby tree being visible is not the same as that tree being responsible.

Where movement may be ongoing, crack monitoring over a period of months gives a far more reliable answer than a single visit.

What if it isn’t the tree?

Quite often it isn’t, and that’s a genuinely useful outcome.

Leaking drains washing fines out of the ground, a nearby excavation, thermal movement, or simple historic settlement that stopped decades ago all produce cracking that looks alarming and needs no tree removed at all.

Being told in writing that your building is stable is worth having, particularly if you are selling or dealing with an insurer.

Frequently asked questions

Should I remove a tree that’s close to my house?

Not before it has been assessed. Removing a mature tree from shrinkable clay can trigger heave, which is often more damaging than the original problem.

Which trees cause the most problems?

High water demand species such as oak, poplar, willow and elm are the usual concern. Species matters less than soil type, though — on non-shrinkable ground even a large tree rarely causes structural movement.

Why do cracks get worse in summer?

Clay dries and shrinks during the growing season, so ground movement peaks in late summer. Cracks that open in summer and partially close in winter are a strong indicator of clay shrinkage.

Will my insurer cover tree-related subsidence?

Subsidence is covered by most buildings policies, but insurers will normally require a structural engineer’s report with supporting evidence before agreeing a course of action.

How long does an assessment take?

An initial appraisal is usually a single visit. If trial pits, soil testing or crack monitoring are needed, the full picture can take several weeks to a few months — monitoring in particular needs time to be meaningful.

Worried about a tree near your property?

If a crack has appeared and there’s a mature tree nearby, the useful first step is establishing whether the two are actually related — before anyone reaches for a chainsaw.

We’re a chartered structural engineering practice with offices in Peterborough and Borehamwood, covering the East of England, Hertfordshire and Greater London, and we’re fully insured to £5 million professional indemnity.

Send us a photo of the crack and the tree, and we’ll give you an honest view on whether it needs investigating.