Why are there cracks in my walls?
Which cracks are normal, which are worth watching, and the specific features that mean get someone in.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 3 min
- Safety
- warning
Short answer
Fine hairline cracks in plaster are normal and cosmetic. Worry about width over about 5mm, diagonal cracks from window and door corners, cracks that are wider at one end, and anything visible on both sides of the wall.
Almost every house has cracks and almost all of them are nothing. But the ones that matter have recognisable features, and the difference between a decorating job and a structural problem is worth knowing rather than worrying about.
Safety
Step by step
- Measure the width.Under about 1mm is hairline and cosmetic. Up to 5mm is usually still fine and fillable. Above 5mm, and particularly above 15mm, needs professional assessment.
- Look at the direction.Vertical cracks along a plasterboard joint, or fine cracks where a ceiling meets a wall, are usually shrinkage and movement. Diagonal cracks running from the corner of a window or door are the ones that suggest structural movement.
- Check whether it is tapered.A crack noticeably wider at one end than the other suggests one part of the structure has moved relative to another. Parallel-sided cracks are far less concerning.
- Check both sides of the wall.A crack visible on both faces goes right through the structure rather than through the plaster. That is a meaningful difference.
- Look for the accompanying signs.Doors and windows that have started sticking, floors that have gone out of level, gaps opening at skirting. A crack plus any of these is worth a professional opinion.
- Consider what is new.New plaster cracks as it dries, and a new extension settles against an older house. Cracks appearing after building work, a very dry summer, or a nearby tree being removed all have likely explanations.
- Monitor before you fill.Mark the ends with a pencil and date them, or photograph with a ruler alongside. Re-check monthly. A crack that is not moving over six months is behaving. Filling first destroys the evidence.
- Fill the cosmetic ones properly.Rake out loose material, fill with flexible filler for hairline cracks or a proper plaster repair for wider ones, and expect fine ones to reappear seasonally.
Tips
- Seasonal cracks that open in summer and close in winter are usually clay soil shrinking and swelling. Common, and not the same as subsidence.
- Trees near a house on clay soil affect ground moisture significantly. Both planting and removing a large tree can cause movement.
- If you are buying, a surveyor should assess cracks. If you already own, an engineer's report costs far less than the anxiety and is what an insurer will want anyway.
If it doesn't work
Fine cracks where ceiling meets wall
Cause: Normal seasonal movement between materials — Fix: Flexible decorator's caulk. Expect it to reappear occasionally.
Straight vertical cracks on a plasterboard wall
Cause: Joint between boards — Fix: Scrim tape and filler when redecorating.
Diagonal crack from a window corner, tapered
Cause: Possible structural movement — Fix: Do not fill. Monitor with dated photographs and get a structural engineer's opinion.
Cracks plus sticking doors and sloping floors
Cause: Structural movement — Fix: Structural engineer, and notify your insurer before doing any work.
Questions people ask
When should I worry about a crack in a wall?
Wider than about 5mm, tapered, diagonal from a window or door corner, visible on both sides of the wall, growing, or accompanied by sticking doors and uneven floors. Any of those warrants an engineer.
Are cracks in a new house normal?
Yes. New plaster and new buildings both dry and settle, and fine cracks in the first couple of years are expected. Report them to the builder within any warranty period.