Are wall cracks serious? How to tell structural from cosmetic
Most wall cracks are cosmetic and not a dealbreaker. The ones that matter are wider than a few millimetres, run diagonally from window or door corners, step through brickwork, or come with sticking doors and sloping floors — those point to movement in the footings. The Australian Standard for residential footings, AS 2870, classifies cracking by width into damage categories: fine cracks up to about 1 mm are negligible, while cracks beyond about 5 mm are treated as defective and warrant a structural engineer's assessment.
How serious is it? The damage categories
Australia has a Standard for exactly this question. AS 2870 (Residential Slabs and Footings) classifies cracking in walls into damage categories by width — the framework building inspectors and engineers actually use. It is the most objective starting point you have, and it travels well: a 2 mm crack is a 2 mm crack whether the vendor calls it 'settling' or not.
| Category | Crack width | What it means |
|---|---|---|
| 0 | Less than 0.1 mm (hairline) | Negligible — no action |
| 1 | Up to about 1 mm (fine) | Very slight — does not affect strength |
| 2 | About 1–5 mm | Slight — noticeable, usually filled and monitored |
| 3 | About 5–15 mm | Moderate — doors and windows stick, pipes may fracture; investigate and repair |
| 4 | About 15–25 mm | Severe — frames distort, walls lean or bulge; major repair |
As a rough rule that follows the Standard: Category 0–1 is cosmetic, Category 2 is worth monitoring over time, and Category 3 and above is treated as defective — it needs investigation and rectification, not paint. But width is only the first signal. A narrow crack in the wrong place, moving in the wrong way, can matter more than a wider one that has stopped.
How to read a crack
Width tells you how much; direction, location, company and time tell you why — and the why is what decides whether it is cosmetic or structural.
| Signal | Usually cosmetic | Possibly structural |
|---|---|---|
| Direction | Vertical, fine, along joints | Diagonal from window/door corners; stepped through brick |
| Company it keeps | Just the crack | Sticking doors and windows, sloping floors, gaps at cornices |
| Where | Plaster or render surface | Through the structure — external brickwork, lintels |
| Over time | Stable, seasonal | Progressive — getting longer or wider |
Why cracks appear
Reactive clay and moisture
Many cracks are simply materials moving — seasonal shrinkage, a new build settling, plaster on its own cycle. The concerning ones reflect movement in the footings. The most common cause in much of Australia is reactive clay soil, which swells when wet and shrinks when dry, lifting and dropping the footings with the seasons. AS 2870 classifies sites from Class A (stable sand or rock) through to Class E (extremely reactive). Melbourne and much of southern Australia sit on reactive clay, which is why footing movement is a recurring theme in older homes here.
Drainage, trees and original footings
Moisture is the lever, so anything that concentrates it matters: poor drainage directing water at the foundations, a leaking service pipe, or downpipes discharging beside the wall. Large trees draw moisture unevenly from one side and their roots can disturb footings directly. And in older homes the footings themselves may simply be shallower or less engineered than a modern slab — built to the standards of their day, not today's.
The single most useful test: time
If you take one thing from this, take this: a crack that is stable has usually done its moving; one that is visibly progressing is the one to investigate. A house breathes with the seasons, so a hairline that opens slightly in summer and closes in winter is normal. A crack that is longer or wider than it was six months ago is not. This is why a single glance is a poor diagnosis — the telling thing about a crack is how it changes, which you only learn by watching it.
When to bring in a structural engineer
If the signals point structural — Category 3 width, diagonal direction, sticking doors, sloping floors, or visible progression — do not guess. A building inspection will flag concerning cracking (see what a building inspection costs); a structural engineer's report then diagnoses the cause and scope. That fee is small against the remediation it might reveal: underpinning or restumping to correct footing movement can run into the tens of thousands. The engineer's diagnosis is what tells you whether you are looking at a monitor-and-fill job or a major capital repair — and lets you price it before you commit. Movement is also era-linked, so read it alongside red flags by era.
Frequently asked questions
Are cracks in walls a dealbreaker when buying?
Usually not — most cracks are cosmetic. The ones to take seriously are wider than a few millimetres, diagonal from window or door corners, stepped through brickwork, or accompanied by sticking doors and sloping floors. Under AS 2870, cracks beyond about 5 mm (Category 3) are treated as defective and warrant a structural engineer.
How wide does a crack need to be to worry?
AS 2870 classifies wall cracks by width: hairline under 0.1 mm and fine cracks up to about 1 mm are negligible; 1–5 mm is slight and worth monitoring; 5–15 mm is moderate and treated as defective; 15–25 mm is severe. Width is only one signal, though — direction, location and whether the crack is progressing matter just as much.
What causes structural cracks?
Most commonly movement in the footings — reactive clay soils swelling and shrinking with moisture, poor drainage at the foundations, leaking pipes, large trees, or shallow original footings. Much of southern Australia sits on reactive clay, which makes footing movement a recurring issue in older homes.
Should I get an engineer or is a building inspection enough?
A building inspection will flag concerning cracking; if it does, a structural engineer can diagnose the cause and scope. The engineer is the one who tells you whether it is a monitor-and-fill job or a major footing repair — and the fee is small against remediation like underpinning or restumping, which can run into the tens of thousands.
Is a hairline crack anything to worry about?
Generally no. Hairline cracks under about 0.1 mm and fine cracks up to roughly 1 mm fall into the negligible categories under AS 2870 — they are part of a house moving with the seasons. The test is whether they stay stable: a fine crack that is steadily growing is worth watching even if it is still narrow.
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