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 building compressing the ground under its own weight, the small, expected movement that happens in the first years after a house or extension is built and then stops. Insurers and the Financial Ombudsman generally treat movement within the first ten years of construction as settlement, and initial settlement should never produce more than hairline cracks. The practical test an engineer applies is simple: would the ground have moved even if the house were not there? If the answer is yes, it is subsidence.

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. Nationally, about 70% of subsidence is clay-related and roughly half of all cases involve a tree or large shrub, a share that climbs in a drought year, which is why hot, dry summers produce "surge years" with more than 50,000 claims across the UK. Almost all of it happens in the top 4 to 5 m of ground, the zone a house's foundations sit in. The reverse process, the clay swelling and lifting the house after a tree is removed, is heave. Where you are building near one of these trees, it is also why foundations near trees on clay have to go deeper.

Drains are the other big London cause, covered in detail in our guide to leaking drains and subsidence. Houses built before about 1970 have drains with rigid joints that crack and leak readily; newer flexible-jointed drains leak less, but a broken or badly laid pipe is still common, so a newer house is not immune. A leak does damage in one of two ways: it softens the ground under the footing until it can no longer carry the load, or, where the pipe runs close to and below the foundation, it washes fine soil out into the drain and leaves a void. Other causes include made-up ground consolidating, fine sands washing out near soakaways and downpipes, 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 that taper, wider at one end than the other
• Cracks that start at a corner of the house or the corner of an opening
• Cracks that pass through brick and plaster, with a matching crack on the other face of the wall
• Cracks that continue below the damp-proof course
• 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 have a signature. They are usually diagonal, following the mortar joints in a staircase pattern. They taper, wider at one end than the other, and the wide end is always at a "free edge": the corner of the building, the top of a wall, or the side of a door or window. They travel around openings rather than through the masonry, so a crack arriving at the bottom corner of a window often continues from the opposite top corner, slightly wider. They frequently run on below the damp-proof course towards the footing, which shrinkage cracks in render or plaster never do. And a crack on the outside of a wall will have a matching crack somewhere on the inside, even if it has been filled and painted over; in a cavity wall the inside crack can sit some distance along. Sometimes the same movement produces a fan of several narrower cracks from one point rather than a single wide one; engineers add the widths together and treat the fan as one crack.

Two checks you can do without digging. Stand back and sight along the horizontal brick courses: they will dip towards the point that has dropped. And put a long spirit level on the floor nearby, remembering that timber floors sag for plenty of other reasons. Look from outside first, because internal decoration hides history. 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, and our guide to types of cracks in walls sets out the width categories engineers use.

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.

Old cracks or new? Reading the decoration

Cracks in fresh decoration mean recent movement. Cracks in decoration or pointing that is five or more years old, with nothing fresh alongside, usually mean the movement is historic and has stopped. A hairline crack reopening exactly where an old crack was filled is normal: filler never rejoins the brickwork, and a building flexes slightly with the seasons, so a couple of millimetres of seasonal opening and closing at an old crack is not, on its own, a concern. What matters is the trend, which is why photographs with a date and a tape measure, repeated over months, are so useful.

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 require prompt notification; the insurer often appoints or agrees the investigating engineer and, on an accepted claim, normally funds the investigation and monitoring. Our guide to subsidence insurance claims explains what is covered and how the claim runs.
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 with fixed studs read by a digital calliper, and often level monitoring of the brick courses, typically over six to twelve months, shows whether movement is seasonal, progressive or has stopped; see our guide to crack and level monitoring.
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. For what each stage costs, from the initial inspection through to underpinning, see our guide to subsidence repair costs in London.