Most cracks in a wall are completely normal and nothing to worry about. According to the Royal Institution of Chartered Surveyors (RICS), there is rarely any cause for real concern unless a crack appears suddenly and is more than 3mm wide. That single guideline is the most useful thing to know before you worry any further, and it comes directly from RICS’s own consumer guide to subsidence.
The RICS guidance on crack width
RICS puts it plainly: establishing whether or not there is a genuine problem can take time, sometimes with a crack monitored for as long as 12 months, but as a starting point, cracks under 3mm wide that have not appeared suddenly are rarely a structural concern. A simple way to track a crack yourself is to note the date and width at each end with a pencil mark or a photo next to a ruler, and check back every few weeks to see whether it is actually growing.
Other signs of subsidence, not just the crack itself
A crack on its own rarely tells the full story. RICS lists these as signs that there could be genuine movement in the ground beneath a property:
- New or expanding cracks in plasterwork
- New or expanding cracks in outside brickwork
- Doors or windows sticking for no obvious reason
- Rippling wallpaper
- New sloping or dipping of solid ground floors
One or two of these on their own, especially the crack alone, is common and often harmless. Several appearing together, or any of them getting worse over a few weeks, is a stronger signal to get a proper opinion.
What actually causes subsidence
The most common cause, according to RICS, is shrinkable clay soil that loses moisture during a long dry spell, sometimes because nearby trees and shrubs are drawing water out of it. Other causes include water leaking into the ground from a broken drain or water main, which softens or washes away the soil under the foundations, and in some areas, historical mining activity causing ground movement well after the fact.
When it’s genuinely just cosmetic
Most cracks homeowners notice fall into this category:
- Hairline cracks in fresh plaster or paint, especially in a home built or extended in the last year or two, as materials naturally settle and dry out.
- Cracks that are not growing. If a crack has looked the same for months when you check it, that is a good sign.
- A crack confined to one small area, with none of the other warning signs above present anywhere else in the property.
- Cracks along a plasterboard joint, which is typically movement in the plaster or drywall itself rather than the structure of the building.
What to do next
If a crack is narrow, stable and isolated, keep an eye on it and get a plasterer to make it good once you are confident it has settled. If it is wider than 3mm, appeared suddenly, or comes with any of the other signs above, get an RICS-regulated chartered surveyor to assess it before anyone starts cosmetic repair work, since covering up a genuine structural crack just delays the real fix and can hide how it is progressing.
Fewer than 5% of investigated subsidence cases actually need underpinning, which is the most invasive and expensive fix, so a proper diagnosis is worth having before you assume the worst. Once the cause is identified and addressed, or ruled out entirely, a plasterer can do the cosmetic work. Look for TrustMark accreditation, and we apply the same checks when we build our Top 5 lists, so a page for your town is a quick way to find a plasterer who already meets that bar for when the wall is ready to be made good.
Frequently asked questions
How wide does a crack need to be before it's a concern?
According to the Royal Institution of Chartered Surveyors (RICS), there is rarely any cause for real concern unless a crack appears suddenly and is more than 3mm wide. Cracks narrower than that, especially ones that have been there a while and are not growing, are usually cosmetic.
What other signs suggest subsidence, not just a cosmetic crack?
RICS lists new or expanding cracks in plasterwork or outside brickwork, doors or windows sticking for no obvious reason, rippling wallpaper, and new sloping or dipping of solid ground floors as signs there could be movement in the ground beneath your home.
What causes subsidence?
The most common cause is shrinkable clay soil losing moisture in a long dry spell or having water drawn out of it by nearby trees and shrubs, according to RICS, which is also why a large proportion of subsidence cases are linked to tree roots. Other causes include water leaking into the soil from a broken drain or water main, and historical mining activity causing ground movement.
Does home insurance cover subsidence?
Most standard UK buildings insurance policies cover subsidence, but always check your own policy rather than assume, and notify your insurer as soon as you suspect a problem rather than waiting. RICS notes that most insurers aim to be as helpful as possible when dealing with a genuine claim.
About the author
Stu Clark
Stu spent over 15 years in the trades, working as a plumber before running his own plumbing and bathroom renovation company with a team of eight. He founded Top5Trades to give homeowners the kind of shortlist and straight advice you would get from someone who has actually done the work. More about us →