Acceptable Crack Width: Your Quick Guide to Safe Cracks in UK Homes
Spotting a crack in a wall or floor can send anyone’s mind racing. Is it just a harmless hairline line, or is it a red flag waiting to turn into a costly repair? In the UK we have clear guidelines on what counts as an “acceptable” crack and when you should start worrying. Below you’ll get the numbers you need, simple ways to measure, and the tell‑tale signs that scream ‘call a professional now’.
How to Measure Crack Width
The first step is getting an accurate measurement. Grab a ruler or a crack‑width gauge – a cheap plastic gauge works fine for most homes. Place the ruler flat against the crack and read the opening at its widest point. In the UK, a crack under 0.3 mm (about the thickness of a human hair) is generally considered harmless. Anything between 0.3 mm and 0.5 mm is still usually fine, especially if it shows up in plaster rather than structural bricks. When the width hits 0.5 mm or more, it’s time to look closer.
Don’t just measure once. Cracks can grow, so note the width now and check back in a month. A stable width suggests movement has stopped; a widening crack signals ongoing settlement or stress.
When a Crack Needs Professional Attention
Even a narrow crack can be worrying if it appears in the right (or wrong) place. Look out for these red flags:
- Location: Cracks along floor joists, around doors and windows, or in load‑bearing walls are riskier than decorative plaster cracks.
- Pattern: Stair‑step cracks in brickwork, diagonal cracks, or a series of cracks forming a network often point to structural movement.
- Speed of growth: If a crack widens by more than 0.2 mm in a few weeks, call a structural engineer.
- Associated issues: Doors that stick, windows that won’t close, or damp patches near the crack add to the concern.
If any of these apply and the crack measures 0.5 mm or more, you’re in “call‑the‑expert” territory. A qualified engineer will assess the foundation, check for settlement, and advise on repairs – from simple epoxy filling to underpinning.
Remember, not all cracks are created equal. A tiny hairline in interior plaster rarely means structural trouble, but the same size in a concrete foundation could be a warning sign. Use the guidelines above, keep a simple log, and when in doubt, get a professional opinion. Your home will thank you for catching problems early before they become expensive fixes.
