Trees and buildings can be uneasy neighbours, and few topics cause more worry than roots creeping under a house. The good news is that the reality is far less dramatic than the folklore suggests. Understanding how roots actually behave — and what genuine warning signs look like — will help you enjoy your trees with confidence rather than fretting about the foundations.

How Tree Roots Really Grow

The image of a root system mirroring the branches, plunging deep beneath a house, is one of the most persistent myths in gardening. In truth, most roots spread outwards rather than downwards. For a typical tree, the roots extend roughly two to three times the radius of the canopy, and the vast majority sit in the top 600mm of soil. The fine feeder roots that do the work of absorbing water and nutrients are concentrated in the upper 150–300mm, where oxygen, moisture and organic matter are plentiful.

Roots also grow opportunistically. They do not seek out drains or foundations with intent; they simply follow moisture and the path of least resistance. If a pipe is already cracked or a mortar joint already failed, roots will happily exploit the gap. They are followers of weakness, not creators of it. Deep structural roots do exist, but they are typically a small proportion of the whole system and taper quickly with depth.

Why Foundations Usually Escape Unharmed

Most UK house foundations are built to a depth of around a metre or more, often below the zone where the bulk of tree roots operate. Even when roots pass near a foundation, they exert remarkably little force. A growing root tip can generate pressure, but nothing close to the force required to break sound concrete or brickwork.

Where things go wrong, the cause is usually soil movement rather than direct root pressure. A tree can affect a foundation indirectly by removing water from the soil beneath it, causing that soil to shrink. This is a soil problem, not a root problem — and it only becomes significant in particular conditions.

Clay Soils, Subsidence and Heave

The UK has a great deal of shrinkable clay, particularly across London, the South East, the Midlands and parts of the North West. These soils swell in wet winters and shrink in dry summers. A large, thirsty tree growing close to a shallow foundation on such soil can tip that natural cycle out of balance, and the building may settle unevenly — the process we call subsidence.

Signs worth taking seriously include:

• Diagonal cracks, often wider at the top, appearing at the corners of windows and doors
• Cracks that widen noticeably in late summer and close again in winter
• Sticking doors and windows that free up after wet weather
• Damage concentrated on one side of the building, near a large tree

The reverse problem, heave, occurs when a mature tree is removed from clay soil and the ground swells as it re-wets. Heave cracks are typically wider at the bottom. This is a strong argument for taking advice before felling a large tree near a house, especially on clay.

Species matter enormously. Willows, poplars, oaks, ashes, elms, eucalyptus and leylandii are all high water demand trees. Yew, holly, hawthorn, rowan, magnolia and most ornamental cherries and plums are far more modest drinkers.

Cracked Paving, Lifted Patios and Blocked Drains

These are far more common than foundation damage, and they are usually surface-level issues. Shallow roots lifting a driveway are unsightly but rarely structural. Practical responses include re-laying paving on a compacted sand or gravel sub-base with flexible jointing, which tolerates slight movement far better than rigid mortar. For patios and paths, a root barrier installed at construction stage is far easier than retrofitting one afterwards.

Drains are a special case. Older clay and pitch-fibre pipes crack with age and ground movement, and roots then enter through those existing failures. Modern plastic pipework in good condition is rarely penetrated. If a drain is blocked, a CCTV survey will reveal whether roots are the cause or simply the symptom of a pipe that was already damaged.

Planting Sensibly: Distances and Species

A little forethought at planting time prevents decades of conflict. The NHBC guidance offers a useful rule of thumb for shrinkable clay: plant at least half the tree's mature height away from a foundation on low-plasticity clay, and around three-quarters on high-plasticity clay. On sand and gravel, the risk is much lower and distances can be reduced.

Also consider:

• Mature size, not the sapling in the pot — a willow can reach 20 metres and its roots travel further still
• Drainage runs, boundaries and neighbours' properties, not just your own walls
• Whether the site has a Tree Preservation Order or sits in a conservation area — check before any pruning or felling
• Right tree, right place: a hawthorn, rowan or Amelanchier will suit a small garden beautifully and cause no trouble

When to Call in an Arboriculturist

If you spot structural cracking, contact your insurer before doing anything to the tree. Subsidence claims often require an arboricultural report, and cutting first can complicate matters. If a tree needs root pruning or removal, an experienced arboriculturist can assess the root protection area, advise on the minimum intervention required and, where necessary, manage a gradual reduction rather than a sudden severance that could destabilise the tree.

Most trees and most houses coexist perfectly well for a century or more. A modest amount of care — mulching young trees, watering them through dry spells, avoiding disturbance of the root zone during building work and choosing the right species for the space — will keep that relationship healthy for generations.