Wall crack severity checker
Engineers grade cracks on a six-point scale, and most of what frightens buyers sits in the bottom two categories. Measure yours and see where it falls.
Last updated July 2026 · built by working UK builders
Based on BRE Digest 251, the classification of visible damage used by structural engineers, loss adjusters and insurers in the UK. It is a guide to severity, not a diagnosis of cause — only opening up and, where necessary, monitoring can establish that.
The BRE damage categories in full
| Category | Width | What it means | Typical cost |
|---|---|---|---|
| 0 — Negligible | Hairline, under 0.1mm | Not a defect. Present in almost every plastered wall in Britain. | £0 |
| 1 — Very slight | Up to 1mm | Fine cracking, typically shrinkage or thermal movement, usually internal only. | £150–£400 |
| 2 — Slight | Up to 5mm | Visible externally as well as internally. Doors and windows may stick slightly. Repointing may be needed where it passes through mortar. | £300–£1,500 |
| 3 — Moderate | 5mm to 15mm, or several over 3mm | Brickwork may need cutting out and replacing. Doors and windows stick. Service pipes may have fractured. Weather-tightness may be affected. | £1,500–£10,000 plus £500–£1,200 for the report |
| 4 — Severe | 15mm to 25mm | Extensive repair needed — sections of wall rebuilt, beams losing bearing, windows distorted, floors sloping noticeably. | £10,000–£40,000 |
| 5 — Very severe | Over 25mm | Major structural damage. Walls out of plumb or bulging, beams losing bearing, risk of instability. | £40,000+ |
How to monitor a crack properly
Before spending money on an engineer, spend six months gathering evidence. It costs almost nothing and it is exactly what an engineer will ask for.
- Photograph the crack with a ruler or tape measure held against it.
- Mark each end with a fine pencil line and write the date next to it.
- Measure the width at three points and write the figures down.
- Repeat monthly, and make sure the period covers a change of season.
- For anything above category 2, fit tell-tale gauges — around £10 each — across the crack.
Cracks that open in summer and close in winter are almost always clay shrinkage driven by moisture, often with a tree involved. Cracks that only ever get wider are the ones that need an engineer.
What causes cracks that are not subsidence
- Shrinkage — new plaster, new render and new blockwork all shrink as they cure. Fine, straight, usually within the first two years.
- Thermal movement — materials expanding and contracting. Common at junctions between different materials and at long unrelieved runs of masonry.
- Lintel deflection — a horizontal crack above a window where the lintel has sagged or a steel one has corroded.
- Wall tie failure — regular horizontal cracks every four to six courses, following the mortar bed.
- Differential settlement at an extension — the new build settles at a different rate from the old. Extremely common, usually stabilises.
- Sulphate attack — expansion in old render or chimney brickwork, producing horizontal cracking.
When to get an engineer rather than a builder
Category 3 and above, anything that is measurably growing, anything where doors have stopped closing, and anything a lender or insurer has asked about. A structural engineer's report costs £500–£1,200 and is the only document that will satisfy an insurer, a lender or a seller. A builder's opinion, however experienced, will not.
Frequently asked questions
What size crack is serious in a house?
Under 1mm is very slight and cosmetic. Up to 5mm is slight and normally filled during redecoration. Between 5mm and 15mm is moderate and needs a professional opinion. Above 15mm is severe and warrants a structural engineer immediately. These are the BRE Digest 251 categories that engineers and insurers actually use.
Are diagonal cracks always subsidence?
No, but diagonal cracks that are wider at one end, run through both brick and mortar, appear on both the inside and outside in the same place, and are close to a window or door are the classic pattern. Thermal and shrinkage cracking is usually finer, straighter, and follows a joint or a change of material.
How do I know if a crack is still moving?
Mark each end with a pencil and the date, photograph it with a ruler alongside, and repeat monthly for six months through at least one seasonal change. Tell-tale gauges cost about £10. An engineer will ask for exactly this evidence, and having it saves you money.
Should I tell my insurer about a crack?
If you believe it is subsidence, yes — that is what the policy is for, and delay can prejudice a claim. Be aware that a subsidence claim stays on the property history and affects future insurance. For clearly cosmetic cracking there is nothing to report.
Can I buy a house with cracks?
People do, constantly. What matters is whether the movement is historic and stopped, or live and continuing. Get an engineer’s report rather than a builder’s opinion or a damp company’s, establish the cause, and check with an insurer that cover is available at a normal premium before you exchange.