London Clay, trees and foundations: what homeowners need to know
Most of London sits on a clay that swells when wet and shrinks when dry. Add a thirsty tree and shallow Victorian footings, and you have the most common cause of foundation movement in the city.
How do trees affect foundations on London Clay?
Trees affect foundations on London Clay by drawing water out of the ground. London Clay is a highly shrinkable soil, so as roots dry it out it reduces in volume and the foundations above it drop. When the tree is removed or the weather turns wet, the clay takes the water back and swells, which can push foundations upwards. Both movements crack walls, and both are worse when footings are shallow.
The risk depends on three things: how shrinkable the clay is, how much water the tree takes, and how close and how deep the roots reach compared with your foundations. A house can sit happily next to a mature tree for decades and then crack after an unusually dry summer, or after the tree is felled.
Why London Clay behaves the way it does
London Clay is a stiff clay, brown and weathered near the surface and blue-grey deeper down, that underlies much of the capital north and south of the Thames. Its very fine particles hold a lot of water and its plasticity is high, which is the technical way of saying it changes volume a great deal as its moisture content changes.
Near the surface, the clay goes through a seasonal cycle. It dries and shrinks through late summer and early autumn, then rewets and swells over winter. Without trees, this zone of seasonal movement is fairly shallow, typically within the top metre or so. Tree roots can extend the drying much deeper and further out, and the effect can build up over a run of dry years.
In some parts of London the clay is capped by sands and gravels left by the Thames and its tributaries. These granular soils do not shrink and swell in the same way, which is one reason two streets in the same borough can behave very differently. A desk study or a trial pit tells you what is actually under your house.
Victorian and Edwardian houses: shallow footings on moving ground
Much of London's housing was built between the 1850s and the First World War on stepped brick footings, often less than a metre below ground. Those footings sit right in the zone that dries and swells each year. That is why minor seasonal cracking around bays, doorways and rear additions is so familiar in terraces and semis across boroughs such as Haringey, Barnet and Ealing.
Bay windows and later rear additions are often the weak points. They are usually built on even shallower footings than the main house, they tend to be closer to garden trees, and they are joined to the main walls with little or no allowance for movement. When the clay under them moves, the junction with the main house is where the crack shows.
Older houses are also more tolerant than they look. Lime mortar lets brickwork absorb small movements, and fine cracks that open and close with the seasons are not always a structural problem. The real question is whether the movement is getting progressively worse.
Which trees cause the most problems?
Foundation guidance used across the UK groups trees by their water demand. High demand species include oak, willow, poplar, elm, eucalyptus and the larger cypresses. Plane, the classic London street tree, sits in the moderate group with ash, lime, sycamore and most fruit trees. Birch, holly and hazel are among the lower demand species, though any large tree on clay deserves attention.
Distance matters as much as species. For the thirstiest trees, the zone of influence on highly shrinkable clay can reach further from the trunk than the tree is tall. Mature height is what counts, so a young tree planted close to the house today can become tomorrow's problem.
Hedges count too. A dense line of conifers along a boundary can dry the clay as effectively as a single large tree. So can a neighbour's tree or a street tree, which is why the tree that matters is not always the one in your own garden.
How deep should foundations be near trees?
For new foundations on shrinkable clay, recognised guidance sets a minimum depth based on the soil's shrinkage potential, then increases it according to the tree species, its mature height and its distance from the foundation. On highly shrinkable London Clay, even with no trees nearby, foundations usually need to go down to at least around 1 m. Within the influence of a large, high demand tree the required depth can be considerably more.
Once the depth goes much beyond about 2.5 m, deep trench fill becomes awkward and unsafe to excavate. An engineered solution is then normally used, such as mini piles with a ground beam and a suspended floor. Building Control will expect any departure from the standard tables to be justified with calculations.
Our tree influence and foundation depth assessment, from £450 + VAT, identifies the relevant trees, checks the soil and sets the depth each part of the foundation needs. The main inputs are:
- Soil shrinkage potential (high, medium or low)
- Tree species and its water demand
- Mature height of the tree, not its current height
- Distance from the tree to each part of the foundation
- Whether the tree will be kept, removed or newly planted
Heave: the problem with cutting trees down
Removing a large tree near a house is not always the fix it seems. Clay that has been dried by roots for decades slowly rehydrates once the tree has gone, and it swells as it does. This heave can lift floors and foundations over several years, and the cracking it causes is often harder to stabilise than the original subsidence.
For that reason engineers rarely recommend felling a mature tree close to an older house without first understanding how dry the clay is. Staged crown reduction, carried out by an arboriculturist, is sometimes the better option. If the tree has a Tree Preservation Order or stands in a conservation area, you need the council's consent before doing any work to it.
Where trees have been or will be removed near new foundations, the design includes heave protection. That typically means a compressible layer against the sides of trench fill or under ground beams, and a void or compressible material beneath suspended floor slabs, so swelling clay cannot lift the structure.
Protecting an existing house or a new extension
If you are extending, the new foundations will often be deeper than the original footings. The two parts will then respond differently to the clay, so we usually detail a movement joint or flexible connection at the junction rather than tying new to old rigidly. Our foundation design covers depth, heave protection and these junction details.
Where a tree has to stay close to an existing house, a root barrier can limit how far roots reach towards the foundations and drains. It only works if it is deep enough and in the right place, so it should be designed rather than improvised. Repairing leaking drains helps as well, because roots are drawn towards the moisture they provide.
If cracks have already appeared, read our guide to subsidence signs and what to do before taking any action on the tree.
Frequently asked questions
Can a neighbour's tree cause subsidence to my house?
Yes. Roots do not respect boundaries, and on London Clay a large tree in a neighbour's garden or on the street can dry out the soil under your foundations. If an investigation links the movement to a tree you do not own, your insurer will usually deal with the tree's owner or the council. Avoid making demands before you have evidence, as a structured investigation makes the case far more clearly.
Should I cut down a tree that is close to my house?
Not without advice. Removing a mature tree from dried-out clay can cause heave, where the soil swells back over several years and lifts the foundations. An engineer can assess the risk, and an arboriculturist can advise on pruning or crown reduction as an alternative. Check too whether the tree has a Tree Preservation Order or sits in a conservation area, as the council's consent may be needed.
How do I know if my house is built on London Clay?
Geological maps show London Clay across much of north, west and south London, but local sands and gravels can sit on top of it. The only reliable way to know what is under your foundations is a trial pit or borehole, with the soil examined and ideally tested. For extensions and basements, a desk study is a sensible first step and shows where to investigate.
Do I need deeper foundations for an extension near a tree?
Usually, yes, if the ground is shrinkable clay. The required depth depends on the soil, the tree species, its mature height and its distance from the new foundation. Building Control will check that the depth follows recognised guidance or is justified by an engineer. Where the depth becomes very large, piles with a ground beam are often more practical than deep trench fill.
Will planting a new tree damage my foundations?
It can in future, depending on the species and how close it is to the house. Choose lower water demand species near buildings and give larger trees plenty of room, judging by their mature height rather than their size when planted. Fast-growing conifer hedges close to the house are a common cause of problems on clay and are best avoided.