Why does clay soil make a house move?
Shrinkable clay changes volume as it wets and dries, and everything founded in that layer moves with it. What the seasonal cycle looks like in a house, how trees magnify it, and the difference between subsidence, heave and settlement.
- Difficulty
- intermediate
- Time
- 10 min
- Read
- 5 min
- Safety
- caution
Short answer
Clay holds water between its particles and swells; when it dries it shrinks. The upper few metres go through that cycle every year, so anything founded in it rises in winter and falls in summer. A tree accelerates the drying and reaches deeper, which is why cracking on clay so often correlates with a hot summer and a large tree. Depth below the moving layer is the engineering answer.
Roughly the whole of the London basin, much of Essex, Kent, Hampshire and a good deal of the Midlands sits on shrinkable clay, and the seasonal cracking that comes with it is one of the most misdiagnosed things in British housing. The distinction that matters is not how alarming a crack looks. It is whether the movement reverses.
Safety
Step by step
- Understand what clay does with water.Clay particles are tiny and plate-like, and water sits between them. Wet clay occupies more space than dry clay. The greater the plasticity of the clay, the larger the volume change between the two states.
- Picture the active zone.Only the upper part of the clay dries out and rewets through a year. Below that depth the moisture content barely changes. Anything founded inside the active zone moves seasonally; anything founded below it does not.
- Add a tree, and the zone deepens.A mature tree transpires a great deal of water in summer, drawing it from clay at depth and over a radius comparable with its height. Under and around a tree the active zone extends much deeper than it does in open ground.
- Learn the three words properly.Subsidence is downward movement of the ground beneath the building unrelated to the load. Heave is upward movement, usually clay recovering moisture. Settlement is the building bedding into the ground under its own weight, which happens to every new building and stops.
- Read the timing rather than the crack.Seasonal clay movement produces cracks that open in late summer and close through winter. Progressive movement produces cracks that widen and do not recover. A year of dated photographs distinguishes them and nothing else reliably does.
- Look for the standard aggravating causes.A leaking drain washing fines out of the ground, a newly planted or newly removed tree, a new hard surface changing where rain goes, or ground levels raised against the wall. These are the things that turn a stable situation into a moving one.
- Involve the insurer before anyone else.Subsidence is an insured peril on most buildings policies and the insurer manages the investigation. Commissioning your own structural report first can complicate the claim, and the insurer's appointed engineer will want to start from the beginning anyway.
Tips
- A very dry summer produces a wave of cracking across whole streets on clay. If several neighbours have the same thing at the same time, that is information.
- Poplar, willow, oak and elm are the species that appear most often in the NHBC tables at the greatest distances, because of how much water they move and how far their roots go.
- Fixing a leaking drain is one of the few interventions that is almost always worth doing on its own merits, whatever else is going on.
Common mistakes
- Felling the tree first — On clay, the ground under the building can then take up moisture again and swell for years. Heave lifts parts of a building and opens cracks that shrinkage never would, and it is harder to remedy than the original problem.
- Repointing and filling cracks in the same summer they appeared — It removes the evidence before anyone has seen whether the movement reverses, and the filler cracks again in the next dry spell.
- Assuming clay movement means underpinning — Most seasonal clay movement is managed by dealing with the cause — a drain, a tree's water demand, drainage away from the wall — and by monitoring. Underpinning is a last resort with its own risks.
If it doesn't work
Cracks appear every August and close by February
Cause: Seasonal clay shrink and swell in the active zone — Fix: Monitor with dated, measured photographs across at least one full cycle. Deal with drainage and any water demand near the wall. Redecorate only once you know the cycle, and use a flexible filler.
A crack has appeared since a large tree was taken out
Cause: Possible heave as the clay recovers moisture — Fix: Tell the insurer, and say explicitly that a tree was removed and when. Heave is investigated differently from subsidence and the tree history is the key fact.
Cracking concentrated at one corner near a gully or manhole
Cause: A leaking drain eroding or softening the ground locally — Fix: Get the drains surveyed by camera. This is one of the most common causes and one of the most fixable, and the survey is cheap relative to everything downstream of it.
Questions people ask
How do I know if I am on clay?
The British Geological Survey publishes mapping of superficial and bedrock geology that covers the whole country, and most conveyancing environmental searches report shrink-swell hazard. Local knowledge is also good: if the gardens crack open in summer, that is clay.
Is every crack on a clay site subsidence?
No, and most are not. Thermal movement, moisture movement in a wall, lintel deflection, plaster shrinkage and the junction between an extension and a house all produce cracking that has nothing to do with the ground.
Can I plant near the house on clay?
Small shrubs and herbaceous planting are not the issue. The species, the mature size and the distance are what matter, and NHBC's tables are the standard reference. A tree planted today is a foundation influence for the next fifty years.
Does subsidence make a house unsellable?
No, but it makes the paperwork important. A completed, documented and warrantied repair with a continuous insurance history is what buyers and lenders want to see. An unresolved, undocumented history is the difficult case.