Hertfordshire has one of the highest rates of subsidence claims per property in England. The reason is geological: much of the county sits on clay-rich soils that shrink when they dry out and swell when they get wet. This "shrink-swell" cycle is the single most common cause of subsidence damage to homes in the county.
Understanding how clay behaves — and where in Hertfordshire the risk is highest — puts you in a much better position to protect your property.
How Clay Causes Subsidence
Clay particles are flat and plate-like. When water molecules sit between the plates, the clay expands. When the water evaporates, the plates pack closer together and the clay shrinks. This volume change can be substantial — some London Clay deposits can shrink by 10% or more of their volume during a dry summer.
When clay shrinks beneath one part of a foundation but not another, the building experiences differential settlement. This is what causes the diagonal cracking, sticking doors, and sloping floors that characterise subsidence.
The key factors that determine severity are:
- Plasticity — how much the clay swells and shrinks. London Clay has high plasticity; boulder clay is moderate.
- Moisture change — driven by weather, drainage, and vegetation
- Foundation depth — shallower foundations are closer to the active zone where moisture changes occur
- Tree proximity — trees extract moisture faster than weather alone
Where Is the Risk Highest in Hertfordshire?
Not all of Hertfordshire is equally at risk. The geology varies significantly from south to north:
| Area | Dominant Geology | Shrink-Swell Risk |
|---|---|---|
| Watford, Bushey, Potters Bar | London Clay | High |
| Borehamwood, Cheshunt | London Clay | High |
| St Albans, Harpenden | Clay-with-flints over chalk | Moderate-High |
| Welwyn Garden City, Hatfield | Mixed clay, gravel, chalk | Moderate |
| Hertford, Ware | River alluvium over London Clay | Moderate (with washout risk) |
| Hemel Hempstead, Berkhamsted | Chalk with clay-with-flints | Moderate |
| Stevenage, Hitchin, Letchworth | Boulder clay over chalk | Low-Moderate |
| Royston, Baldock | Chalk | Low (but dissolution risk) |
The Seasonal Pattern
Clay subsidence follows a very predictable annual cycle:
- Spring: Soil is saturated from winter rain. Foundations are stable.
- Early summer: Trees and plants start extracting moisture. Soil begins to dry.
- Late summer/autumn: Maximum soil drying. Foundations are at their lowest point. Cracks are at their widest.
- Winter: Rain rehydrates the soil. Cracks partially close. Some recovery.
This cycle repeats year after year, and each cycle causes a little more cumulative damage. A property that has survived 30 dry summers without problems can start showing cracks after a particularly severe drought — like the summers of 2018 and 2022, which triggered thousands of new claims across Hertfordshire.
The Role of Trees on Clay
Trees are the single biggest accelerating factor for clay subsidence. They extract vast quantities of water from the soil through their roots, drying out the clay much faster and to a greater depth than weather alone.
The critical number is the tree's influence zone — the radius within which its roots can affect soil moisture. As a rule of thumb, this extends to about 1 to 1.5 times the tree's mature height.
Important: If a tree is causing subsidence, do not simply cut it down. Removing a tree from clay soil causes the clay to rehydrate and swell, pushing foundations upwards — a phenomenon called heave. Heave can be just as damaging as subsidence and takes years to resolve. Crown reduction (pruning to reduce the canopy by 30–50%) is usually the better approach.
What Can You Do to Protect Your Property?
If your property is on clay soil in Hertfordshire:
- Know your soil type. You can check the British Geological Survey's Shrink-Swell map for free online. This will tell you the risk level for your specific postcode.
- Manage trees responsibly. Keep trees pruned. If planting new trees, position them at least one mature height's distance from the building.
- Maintain your drains. Leaking drains can wash out soil beneath foundations, creating voids. Have your drains CCTV-surveyed if they're old or you suspect leaks.
- Fix your guttering. Blocked or broken gutters dump water next to foundations, which softens the ground on one side while the other dries out — exactly the conditions for differential settlement.
- Monitor cracks. Mark them with a pencil, date them, and check them every few months. If they're progressing, get a professional assessment.
Climate Change Is Making This Worse
Hotter, drier summers mean more severe and prolonged clay shrinkage. The Met Office projects that summer rainfall in south-east England will decrease by 20–40% by 2070, while temperatures will increase. For properties on clay soil, this means:
- More frequent subsidence events
- Deeper soil drying, affecting even well-founded properties
- Longer dry periods pushing the cycle further each year
Properties that have been stable for decades may begin to show movement for the first time. If your property is on clay, proactive monitoring is now more important than ever.