Tree roots near your house: drains, foundations and subsidence

Published 2 November 2026

When a crack appears above a window or the drain backs up, a big tree in the garden is the obvious suspect. Sometimes it really is the tree. Often it’s only part of the story, and the fix, and who pays for it, depends on which problem you actually have.

This guide splits the three things people usually blame on roots (blocked drains, cracked walls, lifted paths) and explains how each works in Swansea, Neath Port Talbot and Carmarthenshire.

First, which problem do you have?

• Roots in a drain: slow or blocked drains, often found by a drainage firm’s camera survey.
• Cracks in the house: cracks that open in dry summers and close again in wet winters are the classic sign of soil movement under foundations.
• Lifted paving, low walls or drives: roots thickening just below the surface.

Each has a different cause and a different person to call.

Roots in drains: the root is usually the second problem

Roots don’t normally break a sound pipe. The RHS explains that roots grow towards water, so watertight drains are generally left alone, while leaking joints in older pipes give roots a way in. Swansea Council’s tree policy says the same thing plainly: “Trees themselves very rarely break or damage the drain in the first place.”

So cutting the tree down doesn’t fix a drain. The pipe needs repairing or lining, or the roots simply come back from whatever is nearby.

Who owns the pipe in Wales?

• Inside your property boundary, a pipe serving only your home is your private drain. Dŵr Cymru Welsh Water says these “are your responsibility”, and Citizens Advice Wales confirms you pay for repairs and can choose any contractor.
• Outside your boundary, or shared with neighbours, it’s usually Welsh Water’s. Since the 2011 private sewer transfer, a drain that crosses out of your property becomes a lateral drain looked after by Welsh Water (Neath Port Talbot Council’s drains guide sets this out).
• Not sure? Welsh Water says it will help work out who owns a pipe. Its sewer line is 0800 085 3968.

It’s worth checking your buildings insurance too; Citizens Advice Wales notes some policies cover, or require cover for, private drains.

Cracks and subsidence: it’s about clay and water

Roots very rarely push a house over. The RHS says roots “are unlikely to directly penetrate sound footings”. The real mechanism is indirect: on shrinkable clay soils, a thirsty tree dries the ground in summer, the clay shrinks and the foundations drop slightly. The RHS puts it at around 5–10mm of movement before cracks show.

The British Geological Survey notes that the clays most prone to shrink–swell are mainly in the south-east of Britain, and its GeoSure data can’t confirm the risk at any single address; only a site-specific assessment can. In practice that means two homes in the same street can behave differently, so don’t assume either way.

Distance and species matter. According to the RHS, high water demand trees such as oaks, willows, poplars and cypresses (including Leyland cypress) can dry the soil out to about their own height from the trunk; most other trees, about half their height.

If you suspect subsidence:

1. Tell your buildings insurer first. They normally arrange the engineer’s investigation and level monitoring.
2. Don’t fell the tree on a hunch. On some clays, removing a large, established tree can let the ground swell back up (“heave”), which the RHS warns can cause its own damage.
3. Ask for a tree report alongside the engineer’s report so the tree’s role is actually established.

Heavy pruning is sometimes offered as a middle way, but the RHS says it only helps where the crown is cut back by at least 70%, repeated every few years. For many trees, that isn’t a realistic option.

If the tree is protected

A Tree Preservation Order or conservation area doesn’t stop you dealing with real damage, but it does mean asking first. Planning Portal Wales says your application to the council must give reasons for the work, and a decision can take up to 8 weeks. In a subsidence case, the engineer’s and tree reports are your reasons, so include them.

If consent is refused, or granted with conditions, the Welsh Government’s Protected Trees guide explains you can claim compensation from the council for the resulting loss. For orders made after 2 August 1999 the claim must be £500 or more, and it must be made within 12 months of the decision.

Our protected trees page and the guide on whether you need permission to cut down a tree in Swansea explain how to check.

If it’s a council tree

Swansea Council’s tree policy sets out what it needs before it will act on a subsidence claim:

• an engineer’s or surveyor’s report and a tree professional’s report, including the soil type and the roots found;
• crack monitoring or a level survey over a suitable period;
• details of other vegetation nearby.

The policy says that where a council tree is shown to be a cause, permission to remove it “will not unreasonably be withheld”. It also says the council won’t prune or fell its tree to stop roots getting into a drain that is already broken, because the pipe is the problem. Where a council tree is physically touching a house, the council says it will act, usually by pruning.

In Neath Port Talbot and Carmarthenshire, contact the council direct about its trees; NPT’s customer line is 01639 686868. Our guide to signs a tree could be dangerous covers reporting other council-tree problems.

If it’s a neighbour’s tree

The owner of a tree is responsible for the damage it causes if they haven’t taken reasonable care of it; Swansea’s tree policy notes the owner “could be held liable”. Talk to them, and to your insurer, before anything else. Any cutting back is limited to the parts that cross onto your side of the boundary, and a protected tree still needs consent first. Our guide to a neighbour’s tree overhanging your garden covers your rights.

How our teams can help

Once an engineer or insurer has a clear picture, our teams can carry out the agreed work: a measured crown reduction, careful tree removal in sections close to a house, or stump grinding afterwards. Where Leyland cypress hedges sit close to a house, see our Leylandii and conifer removal page. We work across Swansea, Mumbles, Sketty, Neath and Llanelli. Get a free quote.