Cracks in your walls: when to worry
The short answer
Most cracks are cosmetic: hairlines under 1mm in plaster are normal seasonal and drying movement. The worry list is specific - cracks wider than 5mm, stepped diagonal cracking through brickwork, cracks visible inside and outside at the same spot, doors and windows newly sticking, or any crack that keeps growing. One from that list: photograph, measure, monitor. Two or more, or rapid change: structural engineer.
Reading a crack like a professional
| What you see | Likely meaning | Action |
|---|---|---|
| Hairline (<1mm) in plaster, straightish | Drying, thermal and seasonal movement | Decorate, forget |
| 1-5mm, isolated, stable | Settled movement, often historic | Photograph with a ruler, re-check quarterly |
| Over 5mm, or widening over months | Active movement | Engineer's inspection |
| Stepped diagonal through mortar joints outside | Foundation-level movement pattern | Engineer's inspection |
| Same crack inside and out; sticking doors nearby | Through-wall movement | Engineer, promptly |
| Around a new extension's junction | Differential settlement of new against old | Common; engineer if it exceeds a few mm or keeps moving |
The pattern says more than the width: vertical hairlines above a door frame are plaster doing plaster things; a diagonal staircase of cracked mortar joints running from a corner window toward the ground is a wall rotating on its foundations. Direction, location and change over time are the diagnostic trio - which is why the cheapest monitoring tool is a pencilled date line across the crack's end and a photo with a ruler, repeated monthly.
What the engineer does with it
For cracks outside the insurance route, a specific defect inspection around £500 to £900 gives cause, severity and remedy - and for the genuinely ambiguous, monitoring over months with tell-tales that measure movement in tenths of a millimetre. Most inspections end undramatically: historic movement, long stable, repoint and redecorate. The engineer's letter saying exactly that is also the document that calms buyers, surveyors and lenders at sale time - crack anxiety discounts houses more than cracks do.
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Compare engineer quotesCommon questions
The crack appeared suddenly after a dry summer - subsidence?
Clay-soil shrinkage in drought is the classic trigger, especially with trees nearby, and some of it recovers when the ground rewets. Photograph, measure, monitor - and if it's wide, stepped or growing, that's the insurer conversation, since subsidence is exactly what buildings policies cover.
Should I just fill and repaint it first?
Only after it's diagnosed or dismissed - filler hides the diagnostic evidence and, on an active crack, reopens within months anyway. The reopened filler line is itself useful data: it proves movement is live.
Are cracks worse in old houses?
Old houses have more cracks and worry less about them - a Victorian terrace has been moving gently for 140 years, and its lime-built fabric tolerates it. The same 3mm stepped crack means more in a 1990s cavity wall than in an 1890 one; context is why the engineer's eye beats the ruler alone.
What does crack monitoring involve?
Calibrated tell-tales or studs fixed across the crack, read periodically for six to twelve months - movement direction, magnitude and seasonality measured rather than guessed. It feels slow, but it's how engineers separate live subsidence from settled history without a spade in the ground.
Sources and further reading