Subsidence season: clay soils, dry summers and the trees nearby

Subsidence is a summer problem that only shows itself in autumn. Through a long dry spell the clay under a building loses moisture and shrinks, the ground moves, and by the time a crack opens up above a window the weather that caused it has already passed. It is the least visible of the land risks a manager carries, and one of the most expensive when it goes wrong.

The scale is not small. Following the UK's hottest summer on record in 2025, domestic subsidence payouts reached a record £307 million, up ten per cent on the year before, according to the Association of British Insurers (ABI, 2025). In the first half of that year alone insurers supported almost 9,000 households, with an average payout of £17,264 per claim. The pattern is a familiar one: claims spike during and after hot, dry summers, as they did in 1976, 2003, 2018 and 2022.

Why clay and trees sit at the centre of it

Large parts of England, and the South East in particular, sit on shrinkable clay soils that expand when wet and contract when dry. During a drought the clay pulls away from foundations, and a building that has stood level for a century starts to move. Add a mature tree drawing water from the same ground and the effect concentrates. Thirsty species close to a building, poplars, willows, oaks, are a common factor in clay subsidence, which is why the tree and the foundation have to be thought about together rather than separately.

That is where it gets awkward for a manager. The obvious response, removing or heavily pruning the tree, can be the wrong one. Sudden removal of a large tree on shrinkable clay can trigger heave as the ground recovers moisture, and the tree itself may be protected. A Tree Preservation Order or a conservation area makes it an offence to fell, top or lop without written consent. So the decision is rarely quick, and it is always better made from a record than from memory.

What a manager can actually record

You cannot control the weather, and you cannot dry-proof a clay site. What you can do is know your ground and keep the evidence that shows you were paying attention.

Start with the land. Knowing which of your sites sit on shrinkable clay, and which buildings have large trees within falling-distance of a foundation, turns a vague worry into a short watchlist. Holding the trees and the grounds against the site, with species and distance noted, means the person who orders the pruning and the person who signs it off are working from the same facts.

Then record the movement. When a crack appears, a dated photograph, its location on the site and land record, and a note of the width lets you tell seasonal movement that opens and closes from progressive damage that keeps growing. A single crack means little. The same crack measured across two summers is evidence. That is the difference between a reassuring inspection and a claim that drags because nobody can show when the damage started.

Finally, keep it in one place. A subsidence claim is one of the moments a portfolio's record is really tested. An insurer will ask what you knew, when, and what you did about the trees and the drainage nearby. Answering from a folder that holds the tree survey, the crack monitoring and the maintenance history, all dated and against the right building, is a very different conversation to reconstructing it after the fact.

None of this stops the clay from shrinking. What it does is make sure that when the ground moves under one of your buildings, the record moves with it, and the questions that follow have answers you can point to rather than remember.