Almost every Australian home cracks somewhere. Most of it is harmless — plaster shrinking, materials moving with the weather, a house settling into itself. But some cracks are the first visible sign that the ground beneath the footings is moving, and those are worth taking seriously.
The difficulty is telling one from the other. Here's how structural engineers actually assess cracking, which patterns matter, and when it's time to get an inspection.
Most cracks are cosmetic. What engineers look at isn't just the crack itself, but four things together: how wide it is, what pattern it follows, where it is, and whether it's growing. A 1 mm hairline crack that's been there for ten years is a very different thing from a 4 mm crack that appeared last summer and is still opening.
Australian Standard AS 2870 includes a classification of damage to walls that gives engineers (and builders and insurers) a common language. It runs from Category 0 to Category 4:
| Category | Typical crack width | What it means |
|---|---|---|
| 0 | Under ~0.1 mm (hairline) | Negligible. No action required. |
| 1 | Up to ~1 mm (fine) | Easily covered by normal decoration; generally doesn't need repair. |
| 2 | Up to ~5 mm (noticeable) | Easily filled. Doors and windows may stick slightly. |
| 3 | ~5–15 mm (or several around 3 mm) | Needs opening up and patching by a tradesperson. Doors and windows stick; service pipes may fracture; weather-tightness affected. |
| 4 | ~15–25 mm | Extensive damage. Doors and windows distorting; significant repair required. |
As a rough guide, Categories 0–2 are usually appearance issues, while Category 3 and above is generally treated as a defect requiring investigation, monitoring and rectification.
One important caveat: width alone doesn't determine severity. A 2 mm crack that's widening steadily can matter more than a 6 mm crack that stabilised years ago. Cause and behaviour over time matter as much as the measurement.
Pattern is often more revealing than width:
Fine map or crazing cracks in render and plaster. Usually shrinkage of the finish itself. Cosmetic in almost all cases.
Vertical cracks. Frequently caused by shrinkage or thermal movement, particularly in long, unarticulated masonry walls. Often minor — but check whether they're consistent in width top to bottom.
Stepped (stair-step) cracks following the mortar joints in brickwork. This is the classic signature of ground or footing movement — the wall is responding to one part of the building rising or dropping relative to another. Worth an engineer's eye.
Diagonal cracks running from the corners of windows and doors. Openings concentrate stress, so movement tends to show up here first. Common with reactive soil movement.
Horizontal cracks in masonry. Treat these with more caution. They can indicate a wall being pushed laterally — for example by soil pressure behind a retaining structure — or issues with wall ties. These warrant assessment rather than filling.
Cracks visible in the same location both inside and outside. A crack that passes right through the structure is more likely to be structural than one confined to internal linings.
Book an inspection if you're seeing any of these:
In Australia, the leading cause is reactive clay soil. Clay swells when wet and shrinks when dry, lifting and dropping the ground beneath the footings through the seasons. If the footing system wasn't designed for the site's reactivity, that movement transfers into the structure. (Our guide to [site classification] explains how reactive soils are assessed.)
Other common causes:
Notice how many of these come back to water management. Consistent moisture around a house is one of the most effective things an owner can control.
If you've spotted a crack and want useful information before calling anyone:
Cracks that open in a dry summer and close again after rain point strongly to reactive soil movement. Cracks that only ever widen suggest something progressive. Either way, record before you repair — filling a crack destroys the evidence an engineer needs.
An inspection is more than looking at the crack. An engineer will typically:
Those options might be as simple as fixing drainage, installing a root barrier or adding articulation joints — or as involved as underpinning. The critical principle is the same either way: fix the cause, not just the crack. Patching cracking that's still moving guarantees it comes back.
QED Engineers provides structural inspections and reports for homeowners, buyers and builders across [QLD, NSW, VIC & ACT] — assessing what's causing the movement and setting out what actually needs to be done.
Request a Free Quote | Call: 0421 157 963
This article is general information only. Cracking must be assessed on site by a qualified structural engineer, as classification and cause depend on your specific building, soil and history.
When should I worry about a crack in my wall? Get it assessed if it's wider than about 5 mm, growing over time, stepped through brickwork, horizontal in masonry, or accompanied by sticking doors and windows or uneven floors.
Are hairline cracks normal in a new home? Generally yes. New homes settle and materials shrink as they cure and dry out, so fine cracking in plaster and render is common in the first couple of years. It's the wide, growing or stepped cracks that warrant attention.
Do cracks mean my house is subsiding? Not necessarily. Most cracking comes from seasonal reactive soil movement rather than genuine subsidence. An engineer distinguishes between the two by looking at the pattern, behaviour over time and site conditions.
Should I just fill the crack? Not until you know why it's there. Filling a crack that's still moving simply hides it temporarily — and removes useful evidence. Establish the cause first, fix that, then repair.
How much does a structural inspection cost? Typically a few hundred dollars up to around $1,000+ for a standard residential inspection and report, depending on complexity, access and how much investigation is required.
Will my insurance cover cracking? It depends entirely on your policy and the cause. Many policies exclude gradual ground movement while covering sudden events such as a burst pipe. Check your product disclosure statement, and note that an engineer's report is usually required to support a claim.
