Underpinning advice
Wall cracks: when to worry, and when not to
Published
Almost every Melbourne home has a crack somewhere. Most of them are nothing — plaster shrinking as it cures, paint flexing over a hairline, timber moving with the seasons. A small number are the visible edge of real structural movement. The trouble is telling one from the other without paying for an inspection every time a new line appears on the wall.
Three things separate a crack worth watching from a crack worth calling someone about: direction, width, and behaviour over time. None of them require special equipment, and this guide will get you most of the way to knowing which category you’re in.
Start with direction
Cracks that run in a straight line, either dead vertical or dead horizontal, following a single plane, are usually the least concerning. This is typically shrinkage — plaster or render curing, or a timber frame settling slightly as it dries out — and it’s extremely common in homes under a few years old and after a first hot summer in an older one.
Diagonal cracks are the ones that matter. A crack running at 30–60 degrees, especially one that steps along mortar joints in brickwork (a “stair-step” pattern) rather than punching straight through bricks, is the classic signature of differential movement — one part of the footing or slab moving relative to another. These commonly radiate from the corners of windows and doors, because that’s where a wall is weakest and concentrates the stress first.
If you can trace a diagonal crack back to a door or window corner, that’s worth paying attention to, particularly if it’s on an external wall.
Then check width — but don’t stop there
Width alone is a weaker signal than most people assume, but it’s still useful:
- Under 1mm (hairline): Usually cosmetic, especially if it’s vertical or horizontal and stays that width along its whole length.
- 1–5mm: Worth monitoring. Could be seasonal movement, could be early structural movement — width alone won’t tell you which.
- Over 5mm, or wide enough to see daylight through: This is the range where it’s not cosmetic. If it’s also diagonal or stair-stepped, treat it as a sign of genuine movement.
A crack that’s wider at one end than the other — a taper — is a stronger signal than a uniform crack of the same maximum width, because a taper means one point is moving more than another, which is exactly what differential settlement looks like.
The single most useful test: behaviour over time
Direction and width tell you where you are right now. Behaviour tells you whether it’s getting worse — and that’s the question that actually matters.
Mark each end of the crack with a pencil line and the date. Check again in three months. Two outcomes:
It’s the same size, or it’s closed up slightly. This is the pattern of seasonal movement — reactive clay under a slab swelling in wet weather and shrinking back in a dry summer, common across Melbourne’s western growth corridor. It’s normal, if occasionally alarming to look at, and generally doesn’t need structural repair, though the moisture source behind it (a downpipe, a leaking pipe, garden watering hard against the wall) is worth fixing regardless.
It’s visibly wider than your pencil mark, and keeps growing every time you check. This is the pattern that indicates ongoing structural movement rather than seasonal flex, and it’s the point to get a proper assessment — not necessarily because the house is in immediate danger, but because the cause needs identifying before it’s allowed to run further.
This simple record — two dates, two measurements — tells an engineer more than a single photo ever will, and it costs nothing but three months of patience.
What the pattern usually points to
If your home is a pre-1970s weatherboard or brick veneer on timber stumps, diagonal cracking that’s worsening often traces back to failed or settled stumps rather than the ground itself — see signs your house needs restumping for the fuller picture, particularly in stump-era suburbs like Yarraville, Footscray and Coburg.
If your home is a slab-on-ground brick veneer, especially in Caroline Springs, Point Cook, Tarneit or Melton, diagonal cracking from window and door corners is the classic signature of reactive clay moving under the slab — either settlement or slab heave, and the direction of movement changes which repair applies.
For the formal classification engineers actually use to describe crack severity — the AS 2870 damage categories — see our companion guide to AS 2870 soil classes explained.
When it’s genuinely urgent
A small number of signs warrant a same-week call rather than a three-month monitoring period: a crack you can see daylight through, a door or window that has jammed shut or won’t latch where it used to close fine, a crack that’s visibly widened within weeks rather than months, or any crack accompanied by a sloping floor you can feel by eye or with a marble. None of these mean the house is about to fail, but they mean the movement is active enough that waiting three months to check a pencil mark isn’t the right call.
Not sure which category your cracks fall into? Book a free assessment — we’ll map every crack, take floor levels, and tell you honestly whether you’re looking at seasonal movement or something that needs fixing.
Common questions
How do I monitor a crack properly?
Mark each end of the crack with a fine pencil line and write the date beside it, then measure the width at a marked point with a ruler or feeler gauge and write that down too. Photograph it with something for scale. Repeat every three months for at least a year, so you capture a full seasonal cycle. That simple record is worth more to an engineer than any single inspection, because it answers the question a photograph cannot: is this still moving, and in which direction? Tell-tale gauges glued across a crack do the same job more precisely and cost very little.
Are cracks in a brick veneer wall structural?
The brickwork in a brick veneer house is a non-structural skin over a timber or steel frame, so cracking in it is not itself a failure of the structure. That does not make it unimportant — the veneer cracks because the slab or footing beneath it has moved, and it is often the most visible evidence of that movement. So the cracks are a symptom to be read rather than a structural problem to be patched. Repointing them without addressing the movement means repointing them again.
Should I repair cracks before selling the house?
Cosmetically patching active structural cracks before a sale is a poor idea on two counts. Practically, they reopen, often within months, and a fresh patch over a stair-step crack is something building inspectors are specifically looking for. Legally, concealing a known defect creates real exposure in Victoria. The stronger position is to have the cause diagnosed, keep the engineer's report and any monitoring record, and either repair it properly or disclose it with the documentation. Buyers deal much better with a known, quantified problem than with a discovered one.
Think this is happening at your place?
We assess before we quote — floor levels measured, cracks mapped, and a straight answer on whether you need work at all. Read more aboutunderpinning & foundation repair, or get in touch.