Guide
Cracks in walls: when to worry, the BRE 0 to 5 damage categories
Updated
Engineers grade cracking against the framework in BRE Digest 251, Assessment of damage in low-rise buildings. Knowing the categories tells you, before spending a fee, whether a crack is a decorating matter or an instruction.
The BRE damage categories
| Category | Typical crack width | What it generally means |
|---|---|---|
| 0 | Hairline, under 0.1mm | Negligible; no action |
| 1 | Up to 1mm | Fine cracks dealt with in normal decoration |
| 2 | Up to 5mm | Easily filled; may need some external repointing to keep weather out |
| 3 | 5–15mm, or several cracks around 3mm | Repointing and possibly some brickwork replacement; doors and windows may stick |
| 4 | 15–25mm | Extensive repair, replacing sections of wall; window and door frames distorted, floors sloping |
| 5 | Over 25mm | Structural damage requiring partial or complete rebuilding; beams lose bearing, walls lean |
Two cautions attach to the table. Width is measured on the crack itself, not the chipped plaster either side. And width alone is not the diagnosis: the pattern, the location and whether the crack is live matter as much as the millimetres.
Features that raise concern regardless of width
- Diagonal cracks, wider at the top than the bottom, particularly stepped through brickwork mortar joints: the classic differential movement pattern, see subsidence signs.
- Cracks around door and window openings combined with doors and windows that have started sticking.
- Cracks visible inside and outside at the same location: the crack passes through the wall, not just the finish.
- Movement in a crack you have previously filled: recurrence is information; a returning crack is live.
- Bulging or leaning masonry alongside any cracking.
What an engineer will do
For most domestic cracking the appropriate instrument is a specific defect inspection: typically £250–£500 in 2026 fees for an hour or so on site and a written report categorising the damage, identifying the probable cause and stating what, if anything, should follow. Where movement may be ongoing, the honest answer is often monitoring over months rather than an immediate verdict; be wary of anyone diagnosing a live structural problem, or selling underpinning, from a single visit. If the report does point to subsidence, the next call is usually to your buildings insurer rather than a contractor.