What tree roots are really doing underground

There is a stubborn image of tree roots as powerful underground claws, punching through whatever lies in their path. The reality is far more gentle, and rather more interesting. Most roots spread outwards rather than downwards, travelling through the upper layers of soil where oxygen, moisture and nutrients are actually available. On many sites, the bulk of a tree's feeding roots sit within the top 600 millimetres of soil, and a great many of the finest roots never go deeper than 300 millimetres.

This matters because it tells us roots are opportunists, not battering rams. They follow water, they exploit existing cracks and gaps, and they thicken slowly over decades. A root will very rarely break sound, well-laid concrete through sheer force. What it will do, quite happily, is wander into a flaw that was already there.

How far do roots actually spread?

As a working rule, roots extend well beyond the branch spread. One and a half to three times the radius of the canopy is typical, and on light, sandy or free-draining ground they may travel further still, simply because they have to go looking for moisture. A mature oak with a 15-metre canopy can easily have roots reaching 30 metres or more from the trunk.

The practical consequence is that distance should be measured from the trunk, not from the edge of the branches. If a tree stands ten metres from your house, its roots are almost certainly already in your garden's soil, whether you can see them or not. That is not a reason to panic — it is a reason to understand the ground you are on.

Species more likely to cause trouble

Some trees are thirstier than others, and it is water demand, not root strength, that drives most problems near buildings. The species most often implicated in subsidence claims in the UK include:

• Willow, poplar and Lombardy poplar — famously thirsty, often planted near water, best kept well away from structures.
• Oak, ash and elm — large, long-lived and capable of drawing significant moisture from shrinkable clay.
• Horse chestnut and London plane — handsome street trees, but substantial water users at maturity.
• Leyland cypress — fast-growing, frequently planted as hedging, and a common culprit where it has been allowed to grow tall close to a property.
• Eucalyptus and birch — popular ornamentals, but surprisingly demanding on drier soils.

At the other end of the scale, hawthorn, hazel, holly, yew, elder and most small ornamental trees have modest water requirements and are far less likely to be a concern.

Why foundations suffer — it is usually the soil, not brute force

Where damage does occur, the mechanism is nearly always shrinkage rather than penetration. On shrinkable clay soils, roots draw moisture from the ground during the growing season. The clay contracts, the ground level drops slightly, and foundations that were never designed to move can settle unevenly. Cracks appear in walls, doors stick, and the pattern often follows seasonal wetting and drying.

This is why the same tree can be harmless on chalk or gravel and problematic on heavy clay. It also explains the counter-intuitive case of heave: remove a large, established tree from clay soil and the ground can swell as it re-wets, pushing foundations upwards. Felling a mature tree is a decision worth taking advice on, not a quick fix.

Drains, pipes and quiet root ingress

Drainage is where roots genuinely earn their reputation. Older clayware and pitch fibre pipes, laid in short sections with jointed connections, are vulnerable. A slight settlement crack or a failed seal is an open invitation, and once inside, fine roots branch out into the familiar mat that eventually blocks the pipe.

Modern plastic drainage with properly sealed joints is far more resilient, and roots do not generally penetrate an intact pipe. If you have mature trees and older drains, periodic CCTV surveys are money well spent, and a blocked drain is much cheaper to deal with early than after a collapse.

Planting sensibly and living happily alongside trees

Prevention beats remediation every time. When planting, allow generous space from buildings, boundaries and drainage runs. As an indicative guide on clay soils:

• Small trees (mature height under 8 metres): 5 to 6 metres from a building.
• Medium trees (8 to 15 metres): 10 to 12 metres.
• Large trees (over 15 metres): 15 to 20 metres, and more for willow, poplar and oak.

Where space is tight, a properly installed root barrier can help, though it must be deep enough and installed by someone who knows what they are doing. Reducing a crown by pruning lowers a tree's water demand over time, but simply hacking back roots can destabilise the tree and cause decay — it should never be a first resort.

Remember too that trees may be protected by a Tree Preservation Order or sit within a conservation area, and permission is usually needed before any work. If you are worried about a tree near your home, start with a quiet, informed conversation: a qualified arboriculturist, your insurer and your local authority can all help you weigh up the risks sensibly. Most British trees and British homes coexist perfectly well for decades. A little understanding of what is happening beneath the lawn is usually all that is needed to keep it that way.