Tool · crack register · BRE Digest 251
Cracks in walls checker. When to worry, and when not to.
Measure the crack, answer five questions, and see where it sits on the BRE Digest 251 damage scale that surveyors, engineers and insurers use — plus what its pattern usually means and what to do next. Most cracks are harmless. This helps you tell which ones aren’t.
Check a crack
The scale professionals use
BRE Digest 251 damage categories, in plain English.
Building Research Establishment (BRE) Digest 251, Assessment of damage in low-rise buildings, is the standard way surveyors, structural engineers, loss adjusters and insurers describe cracking. It sorts damage into six categories by how easy it is to repair. Crack width is a useful guide to each category, but it isn’t the whole story: the number of cracks, where they are and whether they are moving matter as much.
Categories 0 to 2 are usually aesthetic, 3 and 4 affect how the building works (serviceability), and 5 affects stability.
| Category | Typical crack width | What it usually means | Type |
|---|---|---|---|
| 0 · Negligible | Under about 0.1 mm | Hairline cracks. Usually no action beyond normal decoration. | Aesthetic |
| 1 · Very slight | Up to about 1 mm | Fine cracks, easily dealt with during normal redecoration. Perhaps isolated slight fracturing; cracks may be visible externally on close inspection. | Aesthetic |
| 2 · Slight | Up to about 5 mm | Cracks easily filled; redecoration probably needed. Some repointing may be needed outside. Doors and windows may stick slightly. | Aesthetic |
| 3 · Moderate | 5–15 mm, or several up to about 3 mm | Cracks need opening up and patching by a builder; repointing and possibly replacement of some brickwork. Doors and windows sticking, service pipes may fracture, and weathertightness is often impaired. | Serviceability |
| 4 · Severe | 15–25 mm, depending on number | Extensive repair, including breaking out and replacing sections of wall, especially over doors and windows. Frames distort, floors slope noticeably, walls may lean or bulge, and service pipes are disrupted. | Serviceability |
| 5 · Very severe | Usually over 25 mm | A major repair job, possibly partial or complete rebuilding. Beams lose bearing, walls lean badly and may need shoring, windows break with distortion, and there is a danger of instability. | Stability |
Reading the pattern
What crack shapes usually mean.
Fine vertical cracks, inside only
Often plaster shrinkage, or movement where two materials meet — at a wall-ceiling junction, around a new extension, or over a lintel. Common in new or newly plastered houses as they dry out.
Diagonal or stepped cracks from doors and windows
Openings concentrate stress, so movement shows there first. A stepped crack following the mortar joints away from a window corner can point to foundation movement or a failing lintel, and deserves a closer look.
Tapered cracks, wider at one end
Taper suggests part of the wall is rotating. A crack wider at the top often means the end of the building is dropping — a classic sign of subsidence worth investigating promptly.
Horizontal cracks in brickwork
Regular horizontal cracks along mortar joints, every few courses, on an older cavity wall can indicate corroding wall ties expanding in the joints. Wall tie failure has a known fix, but it needs confirming by inspection.
A web of fine cracks in render
Crazing in render or plaster is usually a surface issue from shrinkage or weathering, but cracked render lets rain in, so it matters for damp even when it doesn’t matter structurally.
Measuring and monitoring
How to watch a crack properly.
- Measure the widest point with a crack gauge or ruler, and note the length.
- Photograph it with the ruler in shot, from the same position each time.
- Mark and date both ends in pencil so you can see if it lengthens.
- Consider a tell-tale — an inexpensive plastic crack monitor fixed across the crack that shows movement to a fraction of a millimetre.
- Check over months, not days, including a change of season. Clay soils shrink in dry summers and swell in wet winters, so seasonal opening and closing is itself useful evidence.
- Don’t fill it yet if you’re unsure. Filler hides the evidence.
If the crack is growing, tapered, or wider than about 5 mm, get it looked at. Our guides towhat causes cracks in walls,the signs of subsidence andwhether insurance covers itgo further. For a professional view, our subsidence assessmentand defects investigation focus on cause.
Common questions · cracks
Asked about cracks.
Are hairline cracks in walls normal?
Very often, yes. Hairline cracks under about 1 mm are common in plaster, at the junction of walls and ceilings, and where different materials meet. They are usually caused by drying shrinkage or small seasonal thermal movement, and fall into BRE Digest 251 categories 0 or 1 — treated during normal redecoration. What matters is whether they are growing.
When should I worry about cracks in walls?
Get professional advice if a crack is wider than about 5 mm, is getting wider over weeks or months, is tapered (wider at one end), runs diagonally from the corners of doors or windows, is visible on both the inside and outside of the same wall, or comes with sticking doors and windows. Those are the signs of structural movement rather than surface cracking.
Do cracks in walls mean subsidence?
Usually not. Most cracks are shrinkage, thermal movement or old, stable movement. Subsidence cracks tend to be diagonal, tapered, wider than a few millimetres, visible inside and out, and active — often with a cause nearby such as a tree on clay soil or a leaking drain. A professional assessment is the only way to be sure.
Should I fill a crack before getting it checked?
Not if you are unsure what caused it. Filling a crack hides the evidence a surveyor or insurer needs, including whether it is growing. Photograph it with a ruler, mark and date the ends, and monitor it first.
Will my home insurance cover cracks in walls?
Buildings insurance usually covers damage caused by subsidence, heave and landslip, with a higher excess, but not cracking from settlement of new buildings, shrinkage or general wear and tear. If you suspect subsidence, tell your insurer early.
How do I measure a crack at home?
Use a crack width gauge if you have one; otherwise a ruler and a common object help. A bank card is about 0.8 mm thick and a pound coin about 2.8 mm. Measure at the widest point, photograph it with the ruler in shot, and date it so you can compare over time.
Sources and further reading
Where this comes from.
- BRE Digest 251: Assessment of damage in low-rise buildings — the damage classification used on this page (BRE).
- Assessing cracks in houses — BRE guidance for homeowners.
- Access coal mining information and data — for properties in former mining areas (GOV.UK).
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