Short answer: Look at the pattern, the width and whether the crack is changing. Cosmetic cracks are usually hairline to about 1 mm, in plaster or at joints between materials, and do not grow. Settlement cracks are fine cracks in a new house or extension, often at window corners, that appear in the first years and then stop. Structural movement, such as subsidence or a failing lintel, tends to show cracks that are tapered (wider at one end), diagonal or stepped through brickwork, visible inside and out, and that keep getting wider or come back after filling, often with doors or windows that stick. A crack that opens in summer and closes in winter is usually seasonal movement on clay.
The types at a glance
| Type | What it looks like | How it behaves | What it usually needs |
|---|---|---|---|
| Cosmetic (shrinkage and temperature) | Hairline cracks in plaster, at ceiling and wall junctions, or between different materials | Stays the same, or opens very slightly with the seasons | Filling and redecoration |
| Settlement | Fine cracks in a new building or extension, often at window corners or where it meets the old house | Appears early, then stops | A watching brief, then cosmetic repair |
| Seasonal movement on clay | Cracks near trees that open in dry summers | Opens in summer and closes in winter, ending the year roughly where it started | Usually monitoring; if it is wider each year, it may be subsidence |
| Structural movement (subsidence, heave, lintel or roof problems) | Tapered, diagonal or stepped cracks; through the wall; distorted openings; sloping floors | Gets wider, spreads, or returns after filling | An inspection by a structural engineer |
Clues that point to structural movement
- The crack is wider at one end than the other: one part of the building is moving relative to another.
- It runs diagonally or steps through the mortar joints, often from the corners of windows and doors.
- It shows on both sides of the wall.
- It is wider than about 3 mm, or getting wider.
- It returns after being filled.
- Doors or windows have started to stick, floors slope, or a wall bulges or leans.
- A bay window or extension is separating from the main house.
- There is a large tree or a suspected leaking drain close by.
How engineers grade cracks
Engineers in the UK commonly describe visible damage using BRE Digest 251. Crack width is a guide, not the whole story:
- Categories 0 to 2 (up to about 5 mm): aesthetic damage, usually filled and redecorated.
- Categories 3 and 4 (about 5 to 25 mm): damage affecting serviceability, such as weathertightness and how doors and windows work.
- Category 5 (usually more than 25 mm): damage that can affect stability.
A narrow crack can still be the first sign of subsidence or a failing lintel, so the cause matters more than the number. Our insurance structural reports page has the full BRE table.
How to monitor a crack at home
- Mark both ends of the crack with a pencil line and the date.
- Measure the widest point with a ruler or a plastic crack gauge, and write it down.
- Photograph it with the ruler beside it, from the same spot each time.
- Repeat every few weeks for several months, ideally through a summer.
- Note any doors or windows that start to stick.
If the crack grows longer, gets wider, or ends each year wider than it started, have it inspected. Readings like these are also useful evidence for an engineer or an insurer.
Common causes in London homes
Much of London is built on shrinkable clay, so trees and dry summers cause many cracks. Leaking drains can soften or wash away the soil under a foundation. Victorian and Edwardian bay windows often sit on shallower foundations than the main house. Cracks rising from the corners of a window often mean a sagging lintel, a local repair rather than a foundation problem. Heavier roof coverings, and walls or chimney breasts removed without proper support, can cause cracking higher up the building. Our guide to common structural defects shows each of these.
If you are unsure, a structural engineer can tell you which type it is. Our Specific Structural Inspection for one crack starts from £420*, and reports usually arrive within 5 working days. See structural survey and structural inspection.
* Prices are a guide only and VAT may be added.
Common questions
Are hairline cracks in a new extension normal?
Yes. Fine cracks from drying shrinkage and settlement are common in the first year or two. If they keep growing, are tapered or run diagonally, have them checked.
My crack opens in summer and closes in winter. Is that subsidence?
Usually it is seasonal movement on clay, often near trees. If the crack ends each year wider than it started, it may be subsidence and is worth an inspection.
Is a 3 mm crack serious?
By width alone, a 3 mm crack is BRE category 2, slight damage. What makes it matter is its shape and behaviour: tapered, diagonal, through the wall, growing or coming back after filling. Any of those is worth a structural engineer's view.
Can I tell for certain myself?
No. Monitoring tells you a lot, but only an inspection by a structural engineer can establish the cause of a crack and what it needs.