Finding a crack in your wall can be worrying, but most cracks in houses are not caused by subsidence. Plaster shrinkage, thermal movement and normal settlement in a new building all cause cracks that are cosmetic. Subsidence is less common, but when it does happen it needs to be dealt with properly, usually starting with your insurer and a structural engineer rather than a builder.
This guide explains how to tell the difference, what causes subsidence in Cornwall, how claims and investigations usually work, and what underpinning involves. It is general guidance only and is not a substitute for a structural engineer's inspection.
What is subsidence?
Subsidence is the downward movement of the ground supporting a building, which takes the foundations with it. It is different from:
Settlement: a new building or extension settling slightly under its own weight in its early years, which usually stabilises.
Heave: the ground swelling and lifting the foundations, for example when a large tree is removed from clay soil.
Landslip: sideways movement of ground, which can affect homes on steep slopes.
Each has different causes and remedies, which is why proper investigation matters.
Warning signs to look out for
Signs that may point to ground movement include:
Cracks that are wider at one end, often diagonal and running from the corners of doors and windows.
Cracks visible inside and outside in the same place.
Cracks wider than around 3 millimetres, or ones that are growing.
Doors and windows that start sticking for no obvious reason.
Wallpaper rippling at the junction of walls and ceilings, or gaps opening between an extension or bay and the main house.
Fine hairline cracks in new plaster, or cracks that open and close with the seasons, are often cosmetic. If you are unsure, take dated photographs and measurements so changes can be tracked.
Common causes of subsidence in Cornwall
Much of Cornwall is built on granite and slate rock, so the shrinking clay soils that cause many subsidence claims in south-east England are less widespread here. But there are local risks:
Old mine workings: shafts and shallow workings can collapse or settle, sometimes many years after mining stopped. Our guide to building on old mining land explains mining searches and ground checks.
Leaking drains: a broken drain can wash fine material out of the ground beneath foundations. This is a frequent cause of localised movement. See our septic tanks and drainage guide.
Trees: where there are clay or silty soils, trees and large shrubs can dry out the ground in summer. Removing a mature tree can cause heave. Our guide to building near trees covers foundations and Tree Preservation Orders.
Made ground and shallow foundations: older cottages often have very shallow foundations, and some plots are built on filled ground.
Slopes and water: steep sites, poor surface water drainage and failing retaining walls can all lead to movement. See our retaining walls guide.
Cracking in some Cornish homes can also be linked to deteriorating concrete blocks rather than ground movement. Our mundic block guide explains testing.
Insurers and structural engineers
If you suspect subsidence, contact your buildings insurer first, as most policies cover subsidence (usually with a higher excess) and will expect to manage the investigation. Avoid starting repairs before speaking to them, as it can affect a claim.
A typical process includes:
Investigation: a loss adjuster or structural engineer inspects the property and may arrange trial pits, borehole tests, drain surveys or tree surveys.
Monitoring: cracks or levels may be monitored over several months to confirm whether movement is ongoing.
Dealing with the cause: often this means repairing drains or managing trees. Once the cause is removed, many buildings stabilise without underpinning.
Repairs: cracks are repaired, and underpinning is only used where the foundations still need strengthening.
If you are not making a claim, appoint an independent chartered structural engineer to assess the problem and specify any repairs.
Underpinning methods
Underpinning means strengthening or deepening foundations so they bear on firmer ground. Common methods include:
Mass concrete underpinning: digging sections beneath the existing foundation in a set sequence and filling them with concrete. It is a traditional method suited to shallower depths.
Beam and base: a reinforced concrete beam transfers the load to concrete bases at intervals.
Mini-piled underpinning: small-diameter piles are driven or drilled down to firm strata, with a beam or needles carrying the wall. It suits deeper problems or restricted access.
Resin injection: expanding resin is injected into the ground to consolidate it, which is suitable for some situations.
The engineer chooses the method based on soil tests, the depth of firm ground, access and the building. Our groundworks and foundations guide explains how foundations are designed.
Building Regulations and the Party Wall Act
Underpinning is normally notifiable building work, so Building Regulations approval is likely to be needed. See our Building Regulations guide. If you share a wall with a neighbour, or are digging close to their foundations, the Party Wall Act may apply. Our Party Wall Act guide explains the notices.
Frequently asked questions
Does every crack mean subsidence?
No. Most cracks are cosmetic. Cracks that are wide, tapered, growing or accompanied by sticking doors and windows are worth investigating.
Will my house need underpinning?
Not necessarily. Many subsidence cases are resolved by fixing the cause, such as a leaking drain, followed by crack repairs.
Does past subsidence affect selling a house?
It can. You will usually need to disclose it, and buyers and lenders will want engineer's reports and certificates for any repairs.
Worried about cracks or foundations? Talk to us
Cornish Building Company carries out groundworks and foundations and property renovations across the whole of Cornwall, working to engineers' designs where structural repairs are needed. Call us on 01208 816991 or contact us to discuss your property.
