Hertfordshire consistently ranks among the top counties in England for subsidence claims, largely due to the London Clay and boulder clay deposits that underlie much of the county. Recognising the signs early can be the difference between a straightforward repair costing a few thousand pounds and a major structural project running into tens of thousands.
This guide will help you distinguish between harmless cosmetic cracking and genuine subsidence — and tell you exactly what to do if you spot it.
1. Diagonal Cracks from Window and Door Corners
This is the single most reliable indicator. Subsidence cracks run diagonally because foundations drop unevenly — one side moves while the other stays put, creating a shearing force through the wall.
What matters: Width and progression. Cracks under 2mm that haven't changed in years are usually thermal movement or normal settlement. Cracks over 3mm that are widening — particularly if they follow a stepped pattern along mortar joints — need investigation.
Hertfordshire context: Properties in St Albans, Watford, and Borehamwood on London Clay are especially prone to these cracks during dry summers when the clay shrinks.
2. Doors and Windows That Suddenly Stick
When foundations move, the rectangular frames in your walls become parallelograms. A door that closed perfectly in March but jams every August is a classic clay-shrinkage pattern.
Key distinction: If every door in the house sticks in humid weather, that's moisture swelling the timber — not subsidence. Subsidence tends to affect doors and windows in one area of the property, not uniformly.
3. Cracks Where an Extension Meets the Main House
Extensions almost always have different foundations from the original building. They're often shallower, and they settle at different rates. A crack running vertically along the junction between old and new is extremely common — and doesn't always mean subsidence.
When it is subsidence: The crack is widening over time, or is accompanied by other signs like sticking doors in the extension. If the crack is stable and has been the same width for years, it's likely just differential settlement and can be monitored.
4. Rippling or Detaching Wallpaper
When a wall moves even slightly, wallpaper crinkles or lifts at the seams. This is often the earliest visible sign, appearing before cracks are noticeable.
Rule out: Damp. Moisture causes wallpaper to bubble and peel differently from structural movement. If the wall behind feels damp, the issue is likely water ingress, not subsidence.
5. Stepped Cracks in External Brickwork
Stepped cracks follow the mortar joints in a staircase pattern. They occur because mortar is weaker than the bricks, so movement concentrates at the joints.
| Crack Width | Likely Cause | Action |
|---|---|---|
| Under 1mm | Thermal movement or normal settlement | Monitor with pencil marks |
| 1–5mm | Possible subsidence or settlement | Get a professional survey |
| 5–15mm | Probable structural movement | Urgent specialist assessment |
| Over 15mm | Severe structural damage | Immediate professional help |
6. Sloping or Uneven Floors
Place a marble or a ball on the floor. If it rolls consistently in one direction, the floor is sloping. In older properties this can simply be age-related settlement, but in post-war housing it strongly suggests ongoing movement.
Hertfordshire note: Many Stevenage and Hatfield new-town properties from the 1950s–70s have relatively shallow foundations. These are more vulnerable to ground movement than deeper modern foundations.
7. Gaps Between Walls and Skirting Boards or Ceilings
When the structure moves differentially, gaps open up where rigid elements meet. A gap at the top of a wall (between wall and ceiling) suggests that part of the building is dropping. A gap at the bottom (between wall and floor) can suggest heave — the ground pushing upwards.
8. One Part of the Building Sitting Lower
Stand back from your property and look at it critically. Is one corner or one end lower than the other? Check the roofline, the window sills, and the damp-proof course line. If any of these are noticeably out of level, the foundations have moved.
9. Cracks That Open in Summer and Close in Winter
This seasonal pattern is the hallmark of clay shrinkage subsidence. During dry summers, clay soils lose moisture and shrink, pulling away from the foundations. When it rains in autumn and winter, the clay rehydrates and swells back.
This is the most common type of subsidence across southern Hertfordshire — particularly in Bushey, Potters Bar, and Rickmansworth where London Clay dominates.
10. Large Trees Close to the Building
Trees don't directly cause subsidence, but they accelerate it dramatically. A mature oak extracts over 40,000 litres of water per year from the surrounding soil. On clay, this drying effect can extend to a radius of one-and-a-half times the tree's height.
| Tree Species | Mature Height | Influence Radius | Water Demand |
|---|---|---|---|
| Willow | 15–25m | Up to 40m | Very high |
| Oak | 20–30m | Up to 30m | High |
| Poplar | 25–35m | Up to 35m | High |
| Ash | 20–25m | Up to 20m | Moderate-high |
| Cherry | 8–12m | Up to 10m | Moderate |
If you have mature trees within these distances and you're seeing any of the other signs above, the two are very likely related.
What to Do Next
Don't panic — but don't ignore it. Most subsidence is entirely fixable, especially when caught early. Modern methods like resin injection can stabilise foundations in a single day without excavation.
Do this now: 1. Photograph all cracks with a ruler for scale and date them 2. Mark the ends of each crack with a pencil and date 3. Check again after 4–6 weeks for any progression 4. If cracks are growing, or are already over 5mm, request a free survey immediately
We provide free property surveys across all of Hertfordshire's towns and villages. Our engineers will tell you honestly whether the issue needs treatment — or whether it's nothing to worry about.