What Do Subsidence Cracks Look Like?
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Quick answer
Subsidence cracks are typically diagonal (about 45°), wider than 3mm (thicker than the edge of a £1 coin), and tapered — wider at the top than the bottom. They start at the corners of doors and windows, the weakest points in a wall, and can usually be seen on both the inside and outside of the same wall. The defining feature is that they are progressive — they keep widening over time. Fine, stable, straight or vertical cracks are usually cosmetic.
Subsidence crack vs a harmless crack
Almost every home has some cracks. This table shows how the features of a subsidence crack differ from an everyday cosmetic one.
| Feature | Subsidence crack | Cosmetic crack |
|---|---|---|
| Width | Wider than about 3mm (thicker than a £1 coin edge); often 5mm or more | Hairline to ~1mm |
| Direction | Diagonal, typically running at roughly 45° | Straight, vertical or along a plaster join |
| Location | Starts at the corner of a door or window opening — the weakest point in a wall | Mid-wall, random, or where old and new plaster meet |
| Shape | Tapered — wider at the top than the bottom | Roughly even width along its length |
| Inside and outside | Visible on both faces of the same wall, often lining up | Usually only on the internal plaster surface |
| Over time | Progressive — keeps widening over weeks or months | Stable — unchanged for years |
How to read crack width: the BRE Digest 251 scale
UK surveyors classify structural cracking against BRE Digest 251, the Building Research Establishment's standard for assessing damage in low-rise buildings. It runs from category 0 (negligible) to category 5 (very severe). Use it as a guide to how seriously to take a crack — width alone is not a diagnosis.
| Category | Typical width | What it means |
|---|---|---|
| 0 — Negligible | Up to 0.1mm | Not significant |
| 1 — Very slight | Up to ~1mm | Fine cracks, easily filled |
| 2 — Slight | Up to ~5mm | Cracks noticeable; filling and redecoration |
| 3 — Moderate | ~5–15mm | Repointing / some brickwork replacement; doors may stick |
| 4 — Severe | ~15–25mm | Extensive repair; walls lean or bulge |
| 5 — Very severe | Over ~25mm | Major work, possibly partial rebuild; structural instability |
Widths are indicative and follow the BRE Digest 251 classification used in RICS surveys. Category 2 and above near door and window openings warrants a professional assessment.
Why does the crack widen towards the top?
When one part of a foundation drops, the wall above it rotates slightly around the point that stays put. That opens the crack more at the top than the bottom, giving the classic tapered, wider-at-the-top shape. Heave — the ground swelling upward, often after a large tree is removed and clay rehydrates — tends to do the opposite, so a crack that is wider at the bottom can point to heave rather than subsidence. A structural engineer reads the direction and taper together.
Why look at both the inside and the outside?
A crack caused by ground movement usually passes right through the full thickness of the wall, so it appears on the external brickwork and the internal plaster of the same wall, and the two often line up. A crack that shows only on internal plaster, or only at the surface, is more likely to be cosmetic — thermal movement or plaster shrinkage. Checking both faces is one of the quickest ways to separate a structural crack from a decorative one.
“Cracks caused by subsidence are usually diagonal and wider at the top than the bottom. They often appear suddenly and are visible both inside and outside the property.”
Where do subsidence cracks tend to appear?
The most common places are the corners of windows and doors, where the wall is weakest; the junction between an extension, bay window or porch and the main house, where a more lightly founded structure moves differently; and around the base of external walls. Because most UK subsidence is driven by shrink-swell clay, cracks often open in a dry summer and partly close again after wet weather — another clue that points to the ground rather than the building.
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Frequently asked questions
Related guides
Sources
- Subsidence — advice for homeowners — Association of British Insurers (ABI)
- Home surveys — levels and what they cover — RICS
- Shrink–swell: the most damaging geohazard in Britain — British Geological Survey (BGS)
- GeoSure — ground stability data — British Geological Survey (BGS)
Crack descriptions and the BRE Digest 251 categories are general guidance, not a diagnosis. Any crack over about 3mm, or one that is widening, should be assessed by a RICS surveyor or structural engineer.