Dry summers and clay soil: how subsidence risk builds over a season
Subsidence rarely has one cause you can put a date on. On shrinkable clay, it is the result of a whole season of drying, and the cracks often appear weeks or months after the weather that caused them.
Why clay moves
Clay soil swells when it is wet and shrinks as it dries. Shallow foundations on clay move with it. In a normal year, winter rain replaces what the summer took out, and the movement stays small. In a long, dry summer, the soil keeps drying deeper, the shrinkage builds, and walls start to crack.
Shrinkable clay is most widespread in south-east England, including London and much of the surrounding area, but clay soils are found across the UK.
Trees make it worse. A large tree draws water from well beyond its canopy, and species with a high water demand, such as oak, willow and poplar, dry clay out further and faster. That is why mature trees close to a property raise its subsidence risk.
Rainfall alone doesn't measure it
A dry summer is hard to pin down from rainfall alone. Two summers with similar rainfall can dry the soil very differently, depending on how warm they were and how much water evaporated.
A soil moisture deficit is the standard way to measure it. It is a running total through the year: water lost to evaporation is added, rainfall is taken away. Winter rain brings it back towards zero. A warm, dry summer drives it steadily up. The higher the deficit, the drier the ground.
What 51 years of UK weather show
We rebuilt the soil moisture deficit for every year from 1975 to 2025 at 10 locations across the UK. We then compared it with the years known for drought and subsidence (1976, 1995, 2003, 2006, 2010, 2011, 2018, 2022 and 2025) and with 2007 and 2012, two years that were wet across the whole UK.
Three findings shaped how subsidence watch works:
- No single UK-wide threshold works. A deficit that is routine in a dry part of the country is exceptional in a wet one. One national number either fires in ordinary years in wetter regions or misses real droughts in drier ones.
- Each location is measured against its own history. Subsidence watch compares a property's deficit with the history of its nearest location, and alerts when it passes a level reached in about three years in ten. A second, higher level marks an escalation.
- The known years show up, and the wet years don't. At that level, the alert fired in 71% of the known drought years across the 10 locations, and in none of the wet years at any location.
When the damage appears
Soil moisture deficits generally peak in late summer. Cracking often becomes visible around then or later in the autumn, and subsidence claims rise after hot, dry summers. By the time a claim is made, the weather behind it can be months in the past.
That gap is where a dated record matters. A claim is easier to support when you can show when conditions turned risky, rather than recalling that it was a dry summer. For how insurers handle subsidence claims, see Is subsidence covered by landlord insurance?
What to look for at the property
- New diagonal cracks near doors and windows, especially any wider than about 3mm.
- Doors and windows that start to stick.
- Cracks that open in summer and close again over winter.
- Gaps opening where an extension or bay meets the main building, or between walls and ceilings.
If you see these, contact the property's insurer promptly rather than waiting to see whether they grow.
Add your properties and note whether each one is on known clay and has mature trees nearby. Both change how a subsidence alert for that property is worded.