Why is there a crack above my door or window?

Every opening in a wall is a hole in something that is carrying load, so the load has to travel around it. The corners are where that concentration lands, which is why cracks start there and nowhere else.

What you'll need

• A tape measure or a crack gauge
• A pencil and a camera
• A torch
• A spirit level

Step by step

1. Note where the crack starts.Almost always the top corner of the opening, running up and away at 30 to 45 degrees. That is the classic signature of stress around an opening.
2. Measure the width.At its widest point. The widely used scale runs from hairline under 0.1mm through fine at 1mm, moderate at 5mm, to serious above 15mm. Most domestic cracks are under 2mm.
3. Check whether it tapers.A crack markedly wider at one end than the other indicates rotation or differential movement, which is more significant than a uniform crack.
4. Look at the pattern in the brickwork outside.A crack that steps through the mortar joints in a diagonal staircase pattern on the external wall is following the weakest path and indicates real movement.
5. Check for a lintel.In older properties, timber lintels rot and steel ones corrode and expand. A crack directly along the top of the opening with the plaster bulging suggests the lintel rather than the wall.
6. Ask whether it is new.A crack that has been there for decades and has not moved is very different from one that appeared this year. Photograph and date it now regardless.
7. Monitor before repairing.Mark each end with a pencil line and the date, or fit a simple tell-tale gauge. Three to six months of readings across a change of season tells you far more than any single inspection.
8. Look for other cracks.One crack over one door is common. The same crack over several openings on the same elevation, plus doors sticking and gaps at skirtings, is a pattern worth taking seriously.
9. Consider the obvious causes.A tree close to the house on clay soil, a recent extension, a leaking drain, or a removed chimney breast are the usual explanations for genuine movement.
10. Repair a stable crack properly.Rake it out, undercut it, and fill with a flexible filler. Where it has recurred, scrim tape and skim over it.
11. Get advice for anything significant.A chartered surveyor or a structural engineer, not a damp company or a builder offering a free survey. Cracks over 5mm, stepped external cracks, or anything that is growing.

Tips

• Photograph every crack with a ruler or a coin in frame and the date visible. It is the most useful thing you can do and it costs nothing.
• Seasonal cracks that open in a dry summer and close in winter, particularly on clay soil, are usually shrinkage rather than subsidence.
• Do not fill a crack you are monitoring. Filler hides the very information you need.

Common mistakes

• Filling and decorating over a crack that is still moving — The crack reappears within months, and worse, you have destroyed the evidence of how fast it was moving. If a claim or a survey ever becomes necessary, that history is what determines the outcome.
• Calling a specialist firm that offers a free survey and also sells the remedy — The organisation diagnosing the problem should not be the one selling the underpinning or the injection. An independent surveyor or engineer has no commercial interest in the answer.

If it doesn't work

Questions people ask