Most cracks in walls are cosmetic, and a few are an early sign that the footings are moving. Structural Consultancy offers professional wall cracking engineering services across Australia, backed by 35+ years of structural engineering experience: we find the cause, tell you whether it matters and design the repair.
When an engineer looks at a crack, the first two questions are how wide it is and whether it's growing. This guide shows what you can check yourself and when it's time to call a structural engineer.
When to worry about cracks in walls: what the width tells you
The damage scale most engineers use groups cracks by how much they affect the building. Width gives a first idea of the category, but it isn't the whole story: whether a crack keeps getting wider matters too.
| Damage category | Typical crack width | What it usually means |
|---|---|---|
| 0 | Up to about 0.1 mm (hairline) | Cosmetic |
| 1 to 2 | Up to about 5 mm | Cosmetic: fill and redecorate |
| 3 to 4 | About 5 to 25 mm | Affects how the house works: water can get in, doors and windows may stick |
| 5 | More than 25 mm | Affects stability: likely needs structural repair |
So a hairline crack you can barely see is usually nothing to worry about, while a wide crack, or one that keeps growing, deserves a closer look.
Why walls crack
Various factors can cause cracks in walls, for example poor design, soil movement, temperature change, water damage and inadequate construction. Cracks can run vertically, horizontally or diagonally, or step along the mortar joints of a brick wall, and they appear in brick, block and plaster walls as well as in retaining walls.
Clay soils that move
Many Australian homes sit on reactive clay, which shrinks when it dries out and swells when it gets wet. Before a house is built, the site is given a class based on how much the ground surface is expected to move:
| Site class | Soil | Expected ground movement |
|---|---|---|
| A | Mostly sand and rock | Little or none |
| S | Slightly reactive clay | 0 to 20 mm |
| M | Moderately reactive clay or silt | 20 to 40 mm |
| H1 | Highly reactive clay | 40 to 60 mm |
| H2 | Very highly reactive clay | 60 to 75 mm |
| E | Extremely reactive clay | More than 75 mm |
| P | Problem sites (for example abnormal moisture conditions) | Not classified by movement |
On more reactive sites, anything that changes the moisture under the footings can show up as cracks: a dry summer, a leaking pipe, poor drainage or a large tree close to the house. That's why cracks often open in summer and close again in winter. Good house drainage helps keep the soil moisture steady.
Problems caused by wall cracks
Cracks in walls spoil the look of a property and can reduce its value. They can let water in, causing dampness and mould in the walls. If cracks are deep and wide, they can threaten the safety of the people living in a home or working in a commercial building.
How to check a crack yourself
- Measure the crack at its widest point with a ruler marked in millimetres, or a crack width gauge.
- Take a photo with something for scale and note the date.
- Make a small pencil mark at each end of the crack.
- Check it again each month. If it gets longer past your marks or wider, that's movement.
When to call a structural engineer
Book an inspection if:
- the crack is wider than about 5 mm;
- it keeps getting longer or wider;
- doors or windows near it have started to stick;
- water is getting in through it.
Why Do You Need A Structural Engineer For Wall-Cracking Engineering Services?
A structural engineer inspects the cracks, works out what is causing the movement and whether it is still active, and recommends the right fix: from simple crack repair and drainage work through to underpinning in serious cases. We can also help prevent future problems, and check that the repair work is done correctly and in line with local building codes and regulations, which helps property owners avoid legal or financial issues down the road. You can also book a site inspection.
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Cracks in walls: FAQs
Are hairline cracks normal?
Cracks up to about 0.1 mm wide are in the lowest damage category of the widely used scale and are treated as cosmetic. Mark the ends and check them from time to time.
Can I just fill a crack and paint over it?
If the crack is under about 5 mm and it isn't growing, filling and repainting is usually all it needs. If it reopens or gets wider, have it assessed before patching again.
Why do cracks open in summer and close in winter?
Reactive clay soils shrink as they dry out and swell as they get wet, so the footings move a little with the seasons. Cracks that come and go with the weather are a sign of that movement.