Subsidence: the signs to look for and what to do next
Not every crack is subsidence, but some patterns are hard to ignore. Here is how to recognise the warning signs, what to do first, and what an investigation and repair usually involve.
What are the signs of subsidence?
The main signs of subsidence are diagonal cracks that are wider at one end, cracks wider than about 3 mm (roughly the thickness of a pound coin), cracks running from the corners of windows and doors, doors and windows that start to stick, and cracking that shows on both the inside and outside of the same wall. If cracks reopen after redecoration or widen over the summer, the ground under part of the house is probably moving.
Subsidence means the ground beneath a foundation has moved downwards, taking part of the building with it. In London the usual cause is clay shrinking as it dries, often helped by tree roots, which is why it is a familiar problem in clay boroughs such as Harrow and Enfield. Leaking drains are the other common culprit.
- Diagonal cracks, often stepping through the brick joints
- Cracks around bay windows, doorways and where an extension meets the house
- Doors and windows that jam, or gaps above them that were not there before
- Wallpaper creasing at the junction of walls and ceilings
- Cracks that open in late summer and partly close in winter
Subsidence, settlement and heave are not the same
Settlement is the initial compression of the ground under a new building's weight. It happens in the first few years and then stops, and most old London houses finished settling long ago. Cracks from historic settlement are usually old, stable and often filled during earlier redecoration.
Heave is the opposite of subsidence. The ground swells and lifts the foundations, typically after a large tree has been removed from clay, or when a leaking water pipe saturates the soil. Heave cracks often taper the other way, wider at the bottom, and floors may lift.
Plenty of cracking has nothing to do with the ground. Thermal movement, roof spread, overloaded lintels, corroding wall ties and poorly supported alterations all produce cracks that look alarming. That is why diagnosis comes before repair, and why our guide on whether cracks in your house are serious is worth reading alongside this one.
What to do first if you suspect subsidence
Do not fill or paint over the cracks yet. They are evidence, and your insurer and engineer will want to see how they have changed. Start a simple record instead:
- Photograph each crack with a ruler alongside it and note the date
- Pencil-mark the ends of each crack so you can see if it grows
- Note any doors or windows that have started to stick
- Look for trees, large shrubs or hedges near the affected wall, including next door
- Check for damp patches, blocked gullies or slow drains nearby
- Contact your buildings insurer before arranging any repairs
Talking to your insurer
Most buildings policies cover subsidence, usually with a higher excess than for other claims. Insurers prefer to be told early and will normally appoint a loss adjuster to manage the claim. Carrying out repairs first, or leaving it until the cracks are much worse, can make the claim harder to resolve.
An independent engineer's view is still useful. If you are not sure whether the cracking is subsidence at all, a short inspection before you claim avoids a claim being opened for something that turns out to be thermal movement or a failed lintel. If a claim is already under way, an engineer's report can help you follow the investigation and question its conclusions where needed.
How an engineer investigates subsidence
A structural engineer starts by mapping the cracks: where they are, which way they taper and how wide they are. Recognised industry guidance grades damage in categories from negligible to very severe, based mainly on crack width and how easily the damage can be repaired. Most domestic subsidence sits in the lower to middle categories.
The pattern of cracking points to the part of the foundation that has moved, and the surroundings usually explain why. A tree near a bay window, a corner above an old clay drain, or a rear addition on shallower footings than the main house are typical findings. Our subsidence report for insurers, from £550 + VAT, sets out the likely cause in a form a loss adjuster can act on, and like all our inspection reports it is signed off by a chartered engineer who is a Member of the Institution of Civil Engineers (MICE).
Further investigation may follow: trial pits to expose the foundations and soil, laboratory tests on the clay, root identification, and a CCTV survey of nearby drains. Each answers a specific question, so not every case needs all of them.
Why monitoring often comes before repair
If the cause is not clear, or the movement may already have stopped, the next step is monitoring. Small studs are fixed either side of key cracks and measured at intervals, and sometimes level readings are taken along the walls. Clay shrinkage follows the seasons, so a programme usually needs to run through at least one summer and winter to give a reliable answer.
Monitoring answers the questions that matter. Is the movement continuing? Is it seasonal or progressive? Has the action taken, such as removing a tree or repairing a drain, stabilised it? Our crack monitoring baseline report records the starting position so every later reading is compared on a fair basis.
How subsidence is usually fixed
Most subsidence in London is fixed without underpinning. Once the cause is dealt with, by managing the tree, repairing the drains or both, the clay recovers and the movement settles down. The cracks can then be repaired, often by bonding stainless steel helical bars into the mortar joints and repointing, followed by redecoration.
Underpinning is kept for cases where the cause cannot be removed, where the foundations are clearly inadequate for the ground, or where movement continues despite other measures. Options include traditional mass concrete underpinning dug in short sections, mini piles with beams, and resin injection in suitable ground. Each needs a proper design and Building Control approval.
If you are buying a house with a history of subsidence, ask for the engineer's reports, any monitoring results and the certificate of structural adequacy issued at the end of the claim. Together they show whether the cause was dealt with and whether the house has been stable since.
Frequently asked questions
Is subsidence covered by home insurance?
Most buildings insurance policies in the UK cover subsidence, heave and landslip, though the excess for these claims is usually higher than for other damage. Check your policy wording and tell your insurer as soon as you suspect a problem. Insurers normally appoint a loss adjuster, and may want an engineer's report to establish the cause before agreeing the next steps.
How long does subsidence monitoring take?
Long enough to see a full seasonal cycle. Because clay shrinks in summer and swells back in winter, readings taken over only a few weeks can be misleading. Many monitoring programmes run for around a year, with readings at regular intervals. Where the cause has been removed, monitoring confirms when the building has stabilised and repairs can safely go ahead.
Can I sell a house that has had subsidence?
Yes. Many London houses have had past movement and sell normally. Buyers and lenders will want to see the history: the engineer's reports, what caused the movement, what repairs were carried out and any certificate of structural adequacy issued when the claim closed. Clear paperwork showing the cause was dealt with makes a sale far more straightforward.
What size of crack should I worry about?
Hairline cracks under 1 mm are usually cosmetic. Cracks of a few millimetres are worth watching, and anything over about 5 mm, cracks that taper noticeably, or cracks that keep growing should be looked at by a structural engineer. Width alone does not tell the whole story, though. Where the crack is, its pattern and whether it is changing matter just as much.
Do I need underpinning if I have subsidence?
Not usually. Most subsidence is resolved by dealing with the cause, typically a tree or a leaking drain, and then repairing the cracks once monitoring shows the building is stable. Underpinning is a significant intervention and is normally reserved for cases where the cause cannot be removed or the existing foundations are clearly inadequate for the ground.