What is heave and how is it different from subsidence?

The short answer

Heave is the upward movement of the ground beneath a building, the opposite of subsidence, which is downward movement. It occurs mainly on shrinkable clay soils that swell as they re-absorb moisture, pushing the foundations up. The most common trigger in the UK is the removal of a large tree that had been drying out the clay for years — once the tree is gone, the soil slowly rehydrates and expands, sometimes for several years. Heave can also follow a burst water main or seasonal rewetting after drought. Like subsidence, heave is a recognised insured peril on most UK buildings policies. The cracks can look similar, but heave often produces cracks that are wider at the bottom, plus lifting of floors, paths and door thresholds — clues that the ground is rising rather than sinking.

Heave is less talked about than subsidence but no less serious, and it is frequently self-inflicted by removing a tree without advice. The sections below explain the mechanism, how to tell it apart from subsidence, and the situations that most often cause it.

Heave at a glance

Movement directionUpward (ground swells)
Main soilShrinkable clay
Top triggerRemoving a large tree
Other triggersBurst main, post-drought rewetting
InsuranceRecognised insured peril

How heave works

Clay soil changes volume with its moisture content: it shrinks when dry and swells when wet. Subsidence happens when clay dries and shrinks, dropping the foundations. Heave is the reverse — when clay that has been kept dry for a long time regains moisture, it expands and pushes upward against whatever sits on it, including foundations, floor slabs and paths. Because soil rehydrates slowly, heave can develop over months or years rather than appearing overnight, and the upward pressure it generates is considerable. The effects are often most visible in ground-floor slabs, thresholds and external hard surfaces, which can lift, crack or tilt.

Telling heave from subsidence

Both are clay-related ground movements that crack buildings, and a professional is needed to confirm which is occurring. That said, some clues differ:

Feature	Subsidence	Heave
Ground movement	Downward	Upward
Common trigger	Drought, trees drawing water	Tree removal, ground rewetting
Crack taper	Often wider at top	Often wider at bottom
Floors / paths	May drop or settle	May lift or hump
Doors	May drop and stick	May jam at the top

Indicative differences for guidance. Confirmation requires investigation by a chartered professional.

What triggers heave

Heave needs two ingredients: shrinkable clay soil, and something that lets previously dried clay regain moisture. The most common triggers in UK homes are:

Removing a large tree that had been drying the clay for years — the classic and most frequent cause.
Severe pruning of a big tree, which sharply cuts its water uptake and lets the surrounding clay rehydrate.
A burst water main or leaking pipe introducing water into clay that had been dry.
Natural rewetting after a long drought, as wet winters restore moisture the clay had lost.
Building over previously vegetated ground, where the clay swells once the moisture-hungry plants are gone and the area is sealed by a slab.

Because clay re-absorbs water slowly, heave often develops gradually and can continue for several years, which makes it harder to spot early than a sudden event. This slow build-up is one reason engineers monitor before deciding on remedial work.

Why tree removal is the classic cause

The most frequent cause of heave is removing a mature tree near a building on clay. While the tree was alive, its roots kept the surrounding clay dry. Once it is felled, the clay gradually re-absorbs the moisture the tree was taking, swells, and lifts the foundations. This is precisely why arborists and structural engineers warn against felling implicated trees without advice: the cure for one problem (clay shrinkage and subsidence) can create the opposite problem (clay swelling and heave), and heave-related repairs can be as costly as underpinning. Where heave is diagnosed, remedies may involve allowing the ground to stabilise, redesigning affected slabs, or in serious cases specialist foundation work. As with subsidence, the right answer depends on the specific cause, which is established through monitoring and ground investigation.

The lesson on trees: never remove a large, established tree close to a house on clay without consulting a structural engineer and arboriculturist first. Staged crown reduction is often safer than outright felling, because it lets the ground adjust gradually rather than swelling all at once.

Frequently asked questions

Is heave covered by buildings insurance?

Generally yes. Heave, like subsidence and landslip, is a standard insured peril on most UK buildings policies. As with subsidence, the insurer will usually investigate the cause before agreeing remedial work, and a typical subsidence/heave excess applies to the claim.

Can heave damage be worse than subsidence?

It can be. The upward pressure from swelling clay is powerful and can lift floor slabs, thresholds and foundations, sometimes causing damage comparable to or worse than the original subsidence. Heave can also continue for years as the ground slowly rehydrates, which complicates repair timing.

How do I avoid causing heave?

The main safeguard is not to remove large, established trees near a house on clay without professional advice. If a tree must be managed, staged pruning under expert guidance is usually safer than felling. Any tree decision in a clay area near foundations should involve a structural engineer.

Sources & further reading

Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.