Key Takeaways:
- Most wall cracks are cosmetic. Under Australian Standard AS 2870, anything below 1mm is classed as negligible to very slight and needs no repair.
- Start worrying when a crack is wider than 5mm, runs diagonally or in a stepped pattern, appears at door or window corners, or keeps returning after patching.
- Melbourne’s reactive clay soil swells and shrinks with the seasons, which is why townhouses across the city crack more than homes built on sand or rock.
- Width is only the starting point. A crack’s direction, location, and whether it’s still moving matter just as much.
- The only reliable way to know if a crack is structural is a professional inspection of the subfloor, footings, and the pattern across the whole home, not just the crack in front of you.
You notice it one evening: a thin line creeping up the hallway wall of your Melbourne townhouse that you’re fairly sure wasn’t there last month. Cue the spiral. Is the place falling apart, or is it nothing?
Take a breath, because most cracks in walls are cosmetic. Knowing when to worry about cracks in walls comes down to a few specific signs you can learn to read in minutes. A few, though, are worth taking seriously, and they tend to give themselves away.
Fortunately, telling the two apart is easier than you’d think. Let’s start with what actually separates a harmless crack from a serious one.
What’s the Difference Between Cosmetic and Structural Cracks?
The simplest way to think about it: cosmetic cracks sit on the surface; structural cracks go deeper than that.
Cosmetic cracks live in the plaster, paint, or render. They’re usually caused by the wall drying out, temperature swings, or a new home settling into place in its first year or two. They’re thin, they stay put, and they’re an eyesore rather than a warning.
Structural cracks come from movement in the parts of the home that hold it up: the footings, the slab, or load-bearing walls. They tend to be wider, they often keep growing, and they usually bring other clues with them.
Australia’s residential building standard, AS 2870, actually grades wall cracks by width. It’s the same scale a professional inspector uses:
| Crack width | What it usually means |
| Under 1mm | Negligible to very slight; no repair needed |
| 1–5mm | Slight; noticeable but easily filled |
| 5–15mm | Moderate; likely movement, needs proper repair |
| Over 15mm | Severe; significant structural work likely |
Width is the easiest place to start. On its own, though, it can mislead, which is why the next few checks matter just as much.
When Should You Worry About a Crack in the Wall?
Knowing when to worry about cracks in walls is less about any single crack and more about the signs stacking up. Here’s what pushes a crack from “keep an eye on it“ to “get it checked“:
- It’s wider than 5mm, or you’ve got several cracks over 3mm grouped together
- It runs diagonally or in a stepped, staircase pattern, especially out of the corners of doors and windows
- It keeps reappearing after you’ve patched it
- Doors or windows have started sticking or no longer close square
Floors feel bouncy or look like they’re sloping - Gaps are opening up between the walls and the skirting boards or cornices
A single one of these isn’t proof of anything. Two or more together is your cue to act.
The 5-cent coin test
Want a quick gauge before you call anyone? Slide a 5-cent coin into the crack edge-on. A true hairline crack won’t take it. If the coin’s edge (around 1.5mm) slips in, the crack has moved past cosmetic and is worth monitoring. If it swallows a good part of the coin, treat that as your signal to book an inspection.
It’s rough, not scientific, but it’s a fast way to tell a cosmetic nuisance from something that deserves a closer look.
Types of Wall Cracks and What They Mean
The direction a crack runs is one of the biggest clues to what’s causing it. Here are the four types of cracks in walls you’re most likely to see:
| Crack type | What it usually means | Concern level |
| Vertical | Plaster and materials drying out, common in new townhouses; usually cosmetic | Low, unless wide or still growing |
| Horizontal | Sideways pressure from soil, water, or the load above; wall under stress | High, get it assessed |
| Diagonal | Uneven foundation movement, often at door and window corners | Medium to high, worth investigating |
| Stepped | Footing movement in brickwork, usually traced to clay soil in Melbourne | High, the clearest red flag |
Notice the pattern?
Vertical cracks are usually your home settling, while diagonal and stepped cracks are the ones that point underground. That’s where Melbourne’s soil becomes a part of the picture.
Of course, no single crack pattern confirms structural damage on its own, but it helps build the overall picture alongside the crack’s width, location and whether it’s changing over time.
Why Do Melbourne Townhouses Crack So Often?
If your townhouse seems to crack more than your parents’ old brick home ever did, you’re not imagining it. Two things are working against you: the ground underneath, and the way townhouses are built.
Much of Melbourne sits on reactive clay soil. Clay soaks up water and swells through the wetter, cooler months, then dries out and shrinks over summer. That swelling and shrinking lifts and drops the footings beneath your home, and the walls above move with them. The result is seasonal cracking that opens up, and sometimes partly closes again, as the year turns.
Townhouses tend to feel it more than older, established homes:
- They’re often newer builds still settling into their footings in the first year or two
- Many sit on subdivided or infill blocks where the ground has been cut, filled, or recently disturbed
- Smaller footprints and shared walls mean even modest movement shows up quickly as a visible crack
None of this means your townhouse is failing. It means cracking is common here, and worth understanding rather than panicking over. That leaves one thing to sort out: do you repair the crack, or just keep an eye on it?
Should You Repair a Crack or Just Monitor It?
For most hairline cracks, the answer is simple. Fill them, repaint, and move on. Cosmetic cracks in plaster are a normal part of owning a home, and a tube of flexible filler will handle them.
The trap is patching a crack that’s actually structural. Filler hides the symptom rather than the cause, so the crack comes straight back, often wider than before. A crack that keeps returning after repair is telling you something underneath is still moving.
If you’re not sure which you’ve got, monitor it before you patch.
- Mark each end of the crack with a pencil and note the date
- Measure the width and take a photo
- Check it again after a few weeks, then again once the season changes
A crack that doesn’t widen over time is more likely to be cosmetic or seasonal.
One that’s steadily widening needs a professional eye and won’t fix with a simple coat of paint.
A Professional Inspection Sees What You Can’t
The crack on your wall is the symptom. A professional inspection looks for the cause, and that’s the part you can’t assess from the hallway.
An inspector does more than just measure the crack in front of you. They read the whole picture, checking the things you can’t see from inside the room.
- The subfloor and footings holding the home up
- How the slab is sitting, and whether it has moved
- Whether cracks line up across multiple rooms in a pattern that points to movement
- Drainage, moisture, and the reactive clay soil your townhouse sits on
A single hairline crack means little on its own. The same crack alongside a dropped floor and a sticking door tells a very different story.
This matters most with newer townhouses, where catching movement early saves you from arguing about it after handover. A construction stage inspection picks up defects and cracking while they’re still easy to trace, long before they become your problem.
If you’ve got cracks you can’t confidently explain, a professional set of eyes is the reliable way to know exactly where you stand.
Book your inspection today with Landmark Inspections and take the guesswork out of protecting your home.
