Tree roots and house foundations: when to worry

Most trees growing near a house do no structural damage at all. Roots do not push foundations over; the problem, where there is one, is water. On shrinkable clay soil a large, thirsty tree can dry the ground under a shallow foundation in a hot summer until the clay shrinks and the building moves with it. That needs three things at once: clay soil, a thirsty tree close enough to reach under the building, and foundations shallow enough to be affected. Without all three, the tree next to your drive is almost certainly not the reason for the crack in your wall. With all three, the first call is your insurer, not a tree surgeon.

This guide is general information, not structural advice.

How subsidence actually happens

Clay soils change volume with moisture. In a dry summer a mature tree draws large quantities of water out of the soil; the clay shrinks, and anything sitting on it settles. The RHS calls this a seasonal soil moisture deficit that usually peaks in late summer, and notes that structural damage is generally only a problem on shrinkable clay soils, with buildings up to four storeys built before the 1950s most at risk because they frequently have comparatively shallow foundations (RHS: trees near buildings).

Movement has to be substantial before you see anything: the RHS puts it at roughly 5mm to 10mm before cracks develop. The Association of British Insurers makes the same point from the other side, that many houses crack from time to time and only rarely is it subsidence (ABI: subsidence). Roots lifting a path are a nuisance, not subsidence: a path has no foundation.

Is Birmingham on clay?

Partly, and it varies street by street, so do not assume either way.

Birmingham's bedrock is Triassic: higher ground to the north-west on the Sherwood Sandstone Group, lower ground to the south-east of the Birmingham Fault on the Mercia Mudstone Group, both largely masked by glacial till typically less than five metres thick (BGS memoir, Birmingham sheet 168). That matters because the Mercia Mudstone is a mudstone, not a sand. The British Geological Survey says some parts of it may shrink and swell with moisture enough to cause structural damage to buildings, while noting that it appears to cause few serious geotechnical problems compared with higher plasticity clays such as London Clay (BGS: Mercia Mudstone Group). Glacial till can also be clay-rich.

So there are areas of shrinkable ground across the West Midlands and areas where it is not an issue, and the difference can be within a few hundred metres. Your home search pack, your insurer or a structural engineer can tell you what is under your address, drawing on the BGS shrink–swell dataset (GeoSure). A tree surgeon cannot.

Distance, height and species

Two rough rules are widely used, and both are planting guidance rather than a verdict on an existing tree. For species with high water demand, the RHS says the soil drying influence extends outwards to about the height of the tree. For intermediate and low water demand species, it is unlikely to extend more than half the tree's height. The thirsty species the RHS names are oak (responsible for the most UK subsidence cases), willow, poplar, eucalyptus and the cypresses, including Leyland cypress; among lower water demand species it names magnolia, liquidambar, catalpa, hazel, spruce, larch and fir.

Put together: a 15m oak or willow 8m from a pre-1950s house on clay is worth a professional opinion. A 6m magnolia at the end of a modern garden is not. A mature Leylandii hedge tight against an older house is a more common culprit in the West Midlands than people expect, and one reason Leylandii hedge removal comes up so often. Foundation depth matters as much as the tree: modern houses built near existing trees are designed for it, a 1930s semi on shallow footings is not.

The signs of subsidence

Look for a pattern, not a single crack. Typical signs, drawing on ABI guidance:

• Diagonal cracks that taper, wider at the top, through the brickwork rather than just the plaster
• Cracks at weak points: the corners of door and window openings, and where an extension or bay meets the main wall
• Cracks thicker than a 10 pence coin, which is the Association of British Insurers' rule of thumb
• Cracks that appear suddenly after a dry spell, or open in late summer and close in winter
• Doors and windows sticking for no obvious reason, rucked wallpaper, or gaps at skirtings
• Cracks visible on both the inside and outside of the same wall

Fine hairline cracks in plaster, cracks along a plasterboard joint and the settlement cracks in any new build are usually something else, as are cracks that have not changed in years.

What to do first, in order

1. Do not fell the tree. On clay with a long-standing moisture deficit, removing a large tree lets the soil take up water again and swell, lifting the foundations. That is heave, which the RHS says can follow tree removal on such soils, and repairs are often more serious than for subsidence. Felling may also destroy the evidence your insurer needs.
2. Photograph and date the cracks, with a ruler or a coin against them, and repeat monthly. A crack that changes with the seasons tells an engineer far more than one photograph.
3. Tell your buildings insurer. Subsidence is a standard peril on UK buildings policies, the excess is usually much higher than for other claims, and doing your own works first can prejudice a claim.
4. Let a structural engineer investigate. The RHS advice is the same: notify insurers, get an engineer, and arrange crack and level monitoring, usually over at least one seasonal cycle.
5. Bring in an arboriculturist if trees are implicated, who can advise on staged crown reduction, a root barrier or removal with heave precautions, and put it in writing for the insurer and the council.

Drains and roots

Roots rarely break a sound pipe, but they will find and exploit a joint or crack that is already leaking, then block it. A leaking drain can also wash fine soil away and cause movement of its own, which the ABI lists as a separate cause of subsidence. If blockages keep recurring near a tree, get the drain surveyed by camera first. Severn Trent's position is that pipes within your boundary are yours, while the larger sewers under the road are theirs (Severn Trent: blocked drains).

Protected trees, and the work itself

A structural problem does not override tree protection. If the tree has a Tree Preservation Order or stands in a conservation area you still need the council's consent or notice before felling or pruning, and councils routinely ask for an engineer's report linking the tree to the damage. Insurers are used to this and will usually support the application. See how to check if a tree has a TPO and do I need permission to cut down a tree.

Where a tree is genuinely implicated, the outcome is more often managed reduction than removal, because less crown means less leaf area and less water drawn. Our price card gives crown reduction at £220–£300 for a small tree, £320–£480 medium and £480–£750 large, and tree removal at £280–£380, £480–£720 and £850–£1,400 by size, with a free survey over £1,200. See the crown reduction cost guide and tree removal cost guide.

We are an introducer: we pass your enquiry to one vetted local tree surgeon. For a suspected subsidence claim, speak to your insurer first. The tree work follows the engineer's report, not the other way round.