The short answer
Most cracks in a home are cosmetic, but a crack is more likely to be serious if it is wider than about 5mm, diagonal, growing over time, visible inside and out, and located near doors, windows or where an extension meets the house. Other red flags include doors and windows that suddenly stick, cracks running through brickwork (not just plaster), and movement that appeared quickly after a dry summer. Surveyors grade cracks using the BRE categories, where damage above roughly 5mm is treated as serious and above 15mm as severe. By contrast, fine hairline cracks under about 1mm — especially in fresh plaster or at extension joints — are almost always harmless. The single most useful test is whether the crack is still moving: a stable crack rarely needs urgent action, while a widening one does.
Knowing which cracks to ignore and which to act on saves both worry and money. The sections below give a simple five-point check, the width scale professionals use, and the specific signs that mean you should stop monitoring and pick up the phone.
Severity checklist
- Concerning widthWider than ~5mm
- Concerning shapeDiagonal, tapering
- Concerning depthThrough brick, inside & out
- Concerning behaviourStill growing
- ReassuringFine, stable, plaster-only
The five-point check
Run any crack through these five questions. The more 'yes' answers, the more seriously it should be taken:
- Width: is it wider than about 5mm (roughly the width of a pencil)? Hairline cracks under 1mm are rarely structural.
- Direction: is it diagonal and tapering, rather than a fine vertical line? Diagonal cracks from the corners of openings are the classic structural pattern.
- Depth: does it pass through the brickwork and show on both the inside and outside of the same wall, rather than sitting only in the plaster?
- Location: is it near a door, window, bay or the join between an extension and the main house — the weak points where structural movement concentrates?
- Growth: is it getting wider? Mark and date the ends, then check monthly. Movement is the strongest single indicator.
The professional width scale
Surveyors in the UK commonly assess cracks against the Building Research Establishment categories, which link crack width to the likely repair. Use this as a guide, not a diagnosis — context always matters.
| Category | Width | Action |
|---|---|---|
| Very slight | Up to ~1mm | Cosmetic; fill and redecorate |
| Slight | ~1–5mm | Monitor; usually not structural |
| Moderate | ~5–15mm | Serious; investigate the cause |
| Severe | ~15–25mm | Structural repair; specialist needed |
| Very severe | Over ~25mm | Major work; possible stability risk |
Indicative BRE-style categories used in UK surveys. Have any moderate-or-worse crack assessed professionally.
Cracks in different parts of the home
Where a crack appears changes how seriously to take it. A few common locations:
- Internal plaster walls: fine cracks here are frequently cosmetic, caused by drying, shrinkage or minor thermal movement, and are usually the least concerning.
- External brickwork: diagonal, stepped cracks through brick and mortar — especially near openings — are more significant because they involve the structure itself.
- Around extensions and bays: vertical cracks at these junctions are often just two structures meeting, though widening diagonal cracks still warrant a look.
- Ceilings and floor-to-wall junctions: cracks that open up at these meeting points can indicate movement, particularly if accompanied by sloping floors.
- Boundary and garden walls: these are not part of the house structure, so cracks here are far less serious, though they can occasionally hint at ground movement nearby.
The same width means different things in different places: a 3mm crack in fresh plaster is routine, while a 3mm diagonal crack through external brickwork near a window is worth monitoring closely.
When to stop monitoring and call someone
Some signs warrant prompt professional advice rather than continued watching: a crack wider than about 15mm; a crack that has grown noticeably within a few weeks; multiple cracks appearing together with sticking doors and windows; a wall, floor or chimney that looks visibly out of plumb; or any crack you can see daylight through or that lets in water. In these cases, contact your buildings insurer (subsidence is a standard insured peril) or instruct an independent chartered surveyor or structural engineer. For anything milder — a single fine, stable crack — monitoring over a few months is usually enough, and a great many such cracks never amount to anything more than a redecoration job.
Frequently asked questions
Are hairline cracks anything to worry about?
Usually not. Fine hairline cracks under about 1mm, especially in newly plastered walls or where an extension meets the house, are typically cosmetic and result from drying, shrinkage or minor thermal movement. They are generally fixed by filling and redecorating once stable.
When does a crack become a structural concern?
A crack is more likely to be structural when it is wider than about 5mm, diagonal and tapering, passing through brickwork and visible inside and out, located near openings, and still growing. The BRE scale treats anything above 5mm as serious enough to investigate the underlying cause.
Should I claim on insurance for a crack?
Only if investigation points to an insured cause such as subsidence. For a single stable hairline crack, a claim is unnecessary. If you see several serious-crack warning signs together, contact your buildings insurer, who will usually arrange a surveyor to establish the cause before deciding on any work.
Sources & further reading
- RICS — subsidence and your home
- HomeOwners Alliance — subsidence guide
- Checkatrade — subsidence signs and costs
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.