Why does plaster crack diagonally from the corners of doors and windows?
The single most common crack in a British house, why it appears at exactly 45 degrees from an opening, and how to tell the harmless version from the one that needs an engineer.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
Short answer
Openings concentrate stress at their corners, so any movement in the wall shows there first. A fine, stable, hairline crack running diagonally up from a door or window corner is usually normal shrinkage or seasonal movement. A crack that is wider than a coin's thickness, is tapering, is stepping through brickwork outside, or is still opening, needs assessing rather than filling.
A rectangular hole in a wall creates four corners where the load has to redistribute, and those corners are where the material is under most stress. That is true whether the movement comes from timber shrinking, a lintel deflecting slightly, seasonal expansion, or something more serious. The pattern is so consistent that its presence tells you almost nothing on its own — the width, direction, behaviour over time and the view from outside are what carry the information.
Step by step
- Look at where the crack starts and where it goes.A diagonal from the corner of an opening, running up and away at roughly 45 degrees, is the classic pattern. A crack that runs vertically down the middle of a wall, or horizontally along a line, is telling a different story.
- Measure the width honestly.A hairline crack you can barely feel with a fingernail is in a different category from one you can fit a coin into. Note the widest point and where on the crack it is.
- Note whether it tapers.A crack that is wide at one end and closes at the other indicates rotation or differential movement. Uniform width usually indicates shrinkage.
- Check the same location from outside.A crack that shows only internally in plaster is a finish problem. A crack that also appears in the brickwork outside, particularly one that steps through the mortar joints diagonally, is a structural indication and changes the answer entirely.
- Check whether the door or window still works.A door that has started binding, a window that will not close, or a frame visibly out of square are all signs that the opening itself has moved rather than just the plaster on it.
- Monitor before repairing.Pencil a line across the crack at its widest point with the date. Photograph it. Look again after a full heating season. Filling a moving crack means filling it repeatedly.
- Repair a stable crack properly.Rake it out to open it up, dust it, and fill with a flexible filler in thin passes, or scrim tape and skim across it if it has come back before. Filling a hairline crack without opening it out is why it reappears.
- Get advice on anything that fails the tests.Wide, tapering, stepping through external brickwork, accompanied by doors that stick, or still moving after a year: a structural engineer, not a builder and not a damp company. A short report is cheap relative to the consequences of guessing.
Tips
- Cracks at the junction of an extension and the original house are usually two structures moving independently and are best treated with a flexible sealant rather than a rigid repair.
- Newly built and newly extended houses crack at openings as everything dries and settles for the first year or two. That is expected, and the developer's snagging period exists partly for it.
- A crack you can see daylight through, or that has debris falling from it, is not a decorating question.
If it doesn't work
Fine diagonal hairline that has never changed
Cause: Normal shrinkage or seasonal movement concentrating at the corner — Fix: Rake out, fill flexibly, redecorate. Expect it may show again faintly; scrim and skim if it does.
Crack reappears in the same place every winter
Cause: Seasonal movement, often timber lintels or moisture cycling — Fix: Scrim tape and a skim across the line, or a flexible decorator's caulk finished neatly, rather than rigid filler.
Crack above a window, running horizontally then diagonally
Cause: Lintel deflection or a failed or missing lintel — Fix: Stop. This is a structural assessment, and it is one of the few crack patterns that genuinely can get worse quickly.
Matching stepped crack in external brickwork
Cause: Foundation or ground movement — Fix: Structural engineer. Note that a stepped external crack is also what subsidence looks like, and insurers have a defined process for it.
New crack after a nearby extension, tree removal or drainage work
Cause: Ground conditions changed — Fix: Document it with dated photographs and get it assessed. Recent works are relevant to any claim.
Questions people ask
How wide is too wide?
There is no universal figure, and the professional assessment uses categories that consider width alongside pattern, location and whether it is progressing. As a householder threshold: anything you can insert a coin edge into, anything tapering, and anything visible outside deserves an opinion.
Will filling it make it worse?
No, but it may hide useful evidence. If you are unsure about a crack, photograph and measure it before you fill it, so a later assessment has a baseline.
Do I have to tell my insurer?
For ordinary hairline cracking, no. Where you suspect subsidence or structural movement, yes — claims for movement are handled through a specific process, and delaying can complicate it.