The short answer
Subsidence cracks are typically diagonal, run at roughly 45 degrees, and are wider at the top than the bottom (or wider at one end than the other). They usually appear near the weak points of a building — the corners of doors and windows — and are commonly wider than 3mm, often growing over time. A genuine subsidence crack tends to be visible both inside and outside in the same location, runs through the bricks and mortar rather than just the surface plaster, and may have a stepped or zig-zag path following the mortar joints in brickwork. By contrast, fine vertical hairline cracks confined to plaster are almost always harmless. Width is judged using the BRE crack categories, where anything above about 5mm is treated as serious.
Most homeowners can learn to read a crack reasonably well once they know what to look for. The sections below break down the four features that distinguish a subsidence crack — direction, width, depth and location — and explain the crack-width scale surveyors use.
The fingerprint
- DirectionDiagonal, ~45°, tapering
- Width that concernsWider than ~3mm
- DepthThrough brick & mortar, not just plaster
- LocationCorners of doors and windows
- VisibleInside and outside, same spot
The four features of a subsidence crack
- Direction: diagonal cracks, often running up and away from the corner of a window or door, are the classic signature. Vertical cracks are more often shrinkage or settlement; horizontal cracks can signal a different problem such as wall-tie or cavity issues.
- Width and taper: a subsidence crack is usually wider than 3mm and tapers — noticeably wider at one end. A crack that is the same width along its whole length, and very fine, is less likely to be structural.
- Depth: genuine structural cracks pass through the full thickness of the wall — you see them on the brickwork outside and the plaster inside at the same point. Surface crazing in plaster alone is cosmetic.
- Location: they cluster at the weakest points — around openings, at the junction of an extension and the main house, and near bay windows or porches.
How surveyors classify crack width
UK surveyors generally assess structural cracking against the categories set out by the Building Research Establishment (BRE), which group damage by crack width and the repair likely to be needed. The table below is an indicative summary used widely in the industry.
| Category | Typical crack width | What it usually means |
|---|---|---|
| Negligible / very slight | Up to ~1mm | Hairline; cosmetic, fill and redecorate |
| Slight | ~1–5mm | Easily filled; monitor, usually not structural |
| Moderate | ~5–15mm | Serious; repair needs some brickwork; investigate cause |
| Severe | ~15–25mm | Structural repair, possible re-bricking; specialist input |
| Very severe | Over ~25mm | Major structural work; risk to stability |
Indicative summary of the BRE crack-damage categories used in UK surveys. Always have cracks assessed in context by a chartered professional.
Reading a crack: a worked example
To see how the features combine, picture two cracks in the same house. The first runs diagonally up from the top corner of a ground-floor window, is about 6mm wide at the top and tapers to nothing near the sill, shows on the brickwork outside and the plaster inside at the same point, and has grown over the last two months. Every feature points one way — diagonal, tapering, over 3mm, through the wall, near an opening, and active. That is a textbook subsidence crack and warrants investigation. The second is a fine vertical line, under 1mm, where the kitchen extension meets the original wall, unchanged since you moved in. That is almost certainly the harmless junction of two structures. The point is that no single feature decides it; you weigh direction, width, taper, depth, location and movement together, which is exactly what a surveyor does.
Cracks that look alarming but usually aren't
Several common cracks mimic subsidence without being it. A vertical crack where an extension meets the original house often just marks two structures moving independently at their joint. Stepped cracks in a free-standing garden or boundary wall are not structural to the house at all. Fine map-like crazing on a rendered or plastered surface is shrinkage in the finish, not the wall behind it. And plenty of older homes carry long-standing, stable cracks that have not changed in decades. The deciding factor is movement: a crack that is widening, especially one with the diagonal, tapering, through-wall signature above, is the one worth a surveyor's attention.
Frequently asked questions
Are diagonal cracks always subsidence?
No. Diagonal cracks are the most common pattern for subsidence, but they can also result from thermal movement, lintel failure, or normal settlement in a newer build. It is the combination of a diagonal direction, a width over 3mm, a taper, and the crack passing through the full wall and visible inside and out that points more firmly to subsidence.
What colour or texture are subsidence cracks?
Subsidence cracks are usually clean fractures through brick and mortar rather than discoloured or damp-stained. If a crack is wet, stained or crumbling, that can indicate water ingress or a different defect, which still warrants investigation but is not necessarily subsidence.
Should I fill a crack while I monitor it?
It is best not to fill a crack you are actively monitoring, because filling hides whether it is still moving. Wait until a professional has assessed the cause and confirmed the structure is stable, then repair properly. Temporary filling for appearance can be done once movement has stopped.
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.