The short answer
The signs most associated with subsidence and possible underpinning are diagonal cracks wider than about 3mm, often wider at the top than the bottom, that appear suddenly and run from ground level upwards and are visible both inside and outside. Other indicators include doors and windows sticking as frames warp, sloping or sinking floors, and wallpaper creasing at joints. These point to foundation movement — but it is worth keeping calm: many cracks are harmless settlement, and it is estimated that only about 10% of properties with subsidence actually need underpinning. The right next step is not panic but a structural engineer's assessment to confirm the cause and whether underpinning is needed at all.
Cracks are alarming, but most are not subsidence and most subsidence does not need underpinning. Knowing which signs genuinely matter helps you act calmly and get the right assessment rather than the most drastic fix.
Signs to note
- Crack widthmore than ~3mm is notable
- Crack shapediagonal, wider at the top
- Whereinside and outside, ground up
- Doors & windowssticking as frames warp
- Floorssloping or sinking
The signs that point to subsidence
- Cracks over ~3mm: fine hairline cracks are usually harmless; sudden diagonal cracks wider than about 3mm, often wider at the top, are more telling.
- Inside and out: a crack visible on both faces of a wall, running from the ground upward, is more significant than a surface crack.
- Sticking doors and windows: frames warping out of square as the structure moves.
- Sloping or sinking floors: a classic sign of foundation movement.
- Wallpaper creasing: rippling at joints or where walls meet ceilings.
Why most subsidence doesn't need underpinning
It helps to keep perspective. A great many cracks are simple settlement, seasonal movement in clay soils, or thermal expansion — not structural subsidence at all. Even where subsidence is confirmed, it is estimated that only around 10% of affected properties need underpinning; the rest are often resolved by addressing the cause, such as removing a nearby tree, fixing a leaking drain, or monitoring the movement over time. The sensible response to worrying cracks is a structural engineer's report, which establishes the cause and whether any structural work — let alone underpinning — is actually required.
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We'll match you with a vetted structural/underpinning contractor who can arrange a structural engineer's assessment to establish the cause and tell you honestly whether underpinning is needed.
Frequently asked questions
What are the signs you need underpinning?
Look for diagonal cracks wider than about 3mm, often wider at the top, that appear suddenly and show both inside and outside, plus sticking doors and windows, sloping floors and creasing wallpaper. These point to foundation movement, but only a structural engineer can confirm whether underpinning is needed.
Do all cracks mean subsidence?
No. Most cracks are harmless settlement, seasonal clay movement or thermal expansion. Subsidence cracks tend to be diagonal, over about 3mm, and visible both inside and out. If in doubt, a structural engineer's assessment is the calm next step.
Does every subsidence case need underpinning?
No — it is estimated that only around 10% of properties with subsidence actually need underpinning. Many cases are resolved by addressing the cause, such as a tree or a leaking drain, or by monitoring movement over time.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.