Cracked walls? Don't panic! How to tell which cracks are only in the plaster and which are structurally dangerous

1. Which cracks are only skin-deep and nothing to worry about

Take a look: if the cracks on the wall are thin lines criss-crossing like a crazed pattern, or short cracks scattered all over, those are "hairline cracks" caused by the plaster shrinking, plaster mixed in the wrong proportions, or plastering in very strong sun.

These cracks don't affect the strength of the structure and won't make the house collapse. You can easily repair them yourself with a filler such as acrylic filler — fill the crack, sand it smooth and repaint, and it looks as good as new.

2. Dangerous cracks! Warning signs of a structural problem

If the cracks in your house match any of the following, treat them as a critical warning you must not ignore:

• Diagonal cracks at 45 degrees: usually in the middle of a wall or running out from the corners of door and window frames — a clear sign that the foundation or piles are settling unevenly
• Vertical or horizontal cracks in columns and beams: show that the main structure is carrying more load than it can take, or is twisting
• Large cracks wider than 0.5 mm, or concrete spalling to show the steel inside: caused by the reinforcing steel rusting and pushing the concrete apart

3. A checklist to assess the situation yourself

Before you repair anything, follow these simple risk-assessment steps:

• Mark it and monitor: draw a pencil mark at each end of the crack and write the date next to it; check every 1-2 weeks whether the crack is getting longer or wider
• Check the depth: if you probe with a trowel or the tip of a cutter and find the crack goes into the concrete block or brick inside, not just the outer plaster layer, assume there is a risk
• Watch for noises and debris: if bits of plaster keep falling, or you hear cracking sounds from the house, be extra careful right away

4. Safety warning and work that needs a tradesperson

Cracks in the plaster surface can be safely filled and repainted by the homeowner. But if your assessment shows it is a structural crack , the one thing you must never do is cover it with chemicals or mortar to make it look nice. Not only does it not solve the problem, it hides how the crack is growing and makes the danger harder to assess.

If you find a dangerous crack, contact a structural engineer or specialist tradesperson right away to investigate why the foundation is settling and to shore up or repair it to engineering standards, for the safety of everyone in the house.