Signs of Subsidence in London Homes

What subsidence actually is

Subsidence is the downward movement of the ground supporting a building's foundations. As the soil loses volume or strength, part of the structure sinks, and because the movement is uneven it pulls the building apart along its weakest lines. This is different from settlement, which is the small, expected downward movement that happens soon after a building or extension is built and then stops.

In London the most common cause is clay shrinkage. Much of the city sits on London Clay, which swells when wet and shrinks when dry. Long dry summers, leaking drains, and the roots of thirsty trees such as oaks, willows and poplars all change the moisture in the clay and can trigger movement. Other causes include broken drains washing out fine soils, made-up ground, and historic mining or old building work.

The warning signs, in order of importance

No single sign confirms subsidence on its own. Engineers look at the whole pattern. The signs below are the ones worth taking seriously.

• Diagonal cracks wider than about 3mm
• Cracks wider at the top than the bottom
• Cracks that pass through brick and plaster
• Cracks near doors, windows and extensions
• Doors and windows that suddenly stick
• Rippling or tearing wallpaper at corners
• Floors that have started to slope
• Gaps opening between walls and skirting

Cracks: what to look for

Subsidence cracks are typically diagonal and follow the line of the mortar in a staircase pattern, often radiating from the corners of openings where the masonry is weakest. They tend to be wider at the top, taper towards the bottom, and run continuously through both the brickwork outside and the plaster inside. Hairline cracks in plaster alone, or fine cracks that have not changed for years, are usually cosmetic. If you are unsure whether a crack is serious, our guide on when cracks are structural explains the difference in more detail.

Doors, windows and floors

As a building distorts, frames go out of square. Doors and sash windows that used to open freely begin to catch or jam, and the gaps around them become uneven. Floors may develop a noticeable slope, and you might feel a dip when walking across a room. These signs together with diagonal cracking are a strong indication that the structure is moving rather than simply ageing.

What to do if you suspect subsidence

Do not panic, and do not rush into underpinning. The right first step is diagnosis. Subsidence is often slow, and an experienced structural engineer can establish whether movement is active, what is driving it, and whether it can be resolved by addressing the cause rather than rebuilding foundations.

1. Record the evidence. Photograph cracks with a ruler or coin for scale, and note the dates so changes can be tracked.
2. Notify your insurer. Most policies cover subsidence and the insurer often appoints or agrees the investigating engineer.
3. Get an engineer's assessment. A subsidence assessment identifies the likely cause and whether the movement is active.
4. Allow for monitoring. Crack monitoring over several months shows whether movement is ongoing or has stabilised.
5. Investigate the cause. This may include trial pits, drain surveys and root identification before any repair is designed.

How subsidence is fixed

Often the cure is simpler than people fear. If a tree or leaking drain is the cause, managing the tree or repairing the drain may allow the ground to recover. Where foundations genuinely need deepening or strengthening, underpinning is one option, but it is not always necessary. Cracks themselves are repaired only once movement has stopped, using crack stitching, repointing and making good as part of an engineering-led repair strategy. The key principle is to treat the cause first and the symptoms second.