Cracks in Walls: Which Ones Actually Matter

You’ve found a crack, you’ve looked at it several times since, and you want to know whether it’s a decorating job or a structural one.

Most cracks in most houses are not structural problems. Some are. What separates them isn’t mainly how big they are — it’s their direction, where they start, whether they’re moving, and what the rest of the house is doing.

Here’s how to read yours.

Cosmetic, or affecting how the house works?

The Australian Standard covering residential footings draws a distinction that’s more useful to a homeowner than any measurement, and it’s worth borrowing.

An appearance defect is cracking where the only consequence — now or expected — is that the thing looks worse. The wall still does its job.

A serviceability defect is cracking where the function of the building element is affected. The window no longer closes properly. The wall is no longer weatherproof. A pipe running through it has been stressed.

That’s the real question, and you can usually answer it yourself. Walk the house and ask, of each crack: is anything not working because of this?

Things that suggest you’ve crossed from appearance into serviceability:

Doors or windows in the same wall have started binding
Water is getting in through the crack
You can see daylight, or the crack goes right through the wall
The wall is visibly out of plumb, not just cracked
Tiles or fittings attached to that wall are cracking or lifting

A hairline crack in plaster above a doorway that hasn’t changed in ten years is an appearance defect and always has been. That’s most cracks.

How much does width matter?

Less than people expect, and it’s the wrong first question — but it isn’t irrelevant.

For context: cracks caused by foundation and ground movement are commonly in the range of about 1 to 3 millimetres, and only rarely exceed 5 millimetres. So a crack you can just fit a fingernail into is squarely within the normal range for a house on reactive clay, and is not by itself evidence of anything serious.

What width tells you and what it doesn’t:

A wide crack that isn’t changing is often less concerning than a narrow one that’s growing. Progression beats size.
Several cracks of moderate width can indicate more distortion than one wide one, because the movement is distributed.
Width varies along a crack. Measure at the widest point and note where that is — a crack wider at the top than the bottom tells you which way the wall is moving.
Width changes with the season on reactive clay. A summer measurement and a winter measurement of the same crack can differ noticeably, and that difference is itself the diagnosis.

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Direction and shape: the actual diagnosis

This is where cracks tell you something. Different movements produce different crack geometry.

What it looks like	What it usually means
Diagonal, stepping along the mortar joints in brickwork	The classic differential settlement crack. One part of the wall has moved relative to another. The direction of the step points towards the side that’s dropped
Diagonal, running up and out from a window or door corner	Same cause. Openings are the weak points in a wall, so movement expresses itself there first
Vertical, straight, often at a junction between old and new work	Frequently two structures moving independently — an extension against an original wall. Often stable and long-standing
Vertical through the middle of a long unbroken wall	Can be expansion and contraction of the masonry itself rather than ground movement, particularly on long walls with no control joint
Horizontal, along a mortar course	Less common and less likely to be footing-related. Worth having looked at properly rather than self-diagnosed
Fine random cracking across a rendered or plastered surface	Usually the surface finish itself — shrinkage in render or plaster, not the structure behind it
Hairline cracks at plasterboard joints and cornice lines	Normal movement in the lining. Extremely common and almost never structural

The pattern worth taking seriously is diagonal stepped cracking that concentrates around openings on one side of the house, and is getting wider. That combination is describing a footing or ground problem.

The material matters as much as the crack

Where you’re seeing the crack changes what it’s likely to mean, and people frequently panic about the wrong one.

Internal plaster and plasterboard. The most alarming to look at and the least diagnostic. Linings crack readily from normal seasonal movement, from timber framing shrinking, and from being knocked. A crack in plaster with no corresponding crack in the brickwork outside is usually the lining, not the house.

Lath and lime plaster in a period house. Gold-era Bendigo cottages often have original plaster on timber lath. It cracks. It has been cracking for a century, and much of what you can see was patched at some point already. Age-related cracking in a 140-year-old lining is not news about the footings.

External brickwork. The most diagnostic surface on the building. Brickwork is stiff and doesn’t accommodate movement, so when the footing moves the brickwork records it — stepping along mortar joints, concentrating at openings. If you want to know whether the house is moving, look here.

Render. Deceptive both ways. Render can crack independently of the wall behind it, and it can also hide brickwork cracking underneath. A crack in render should be read alongside what the rest of the house is doing rather than on its own.

The most useful comparison you can make: is there a crack in the external brickwork in the same place as the internal one? If yes, something structural moved. If no, it’s very likely the lining.

The question a general inspection won't ask

In most Australian cities, diagonal cracking in brickwork means reactive clay or a footing problem, and that’s the end of the list.

In Bendigo there’s a third possibility. The goldfield — roughly 16 kilometres by 4, more than 5,000 registered mines, 37 gold-bearing quartz reefs — sits underneath residential suburbs, not beside them. More than 1,600 shafts were filled or capped between the 1930s and 1950s, the shallower ones with uncompacted mullock that continues consolidating. Bendigo’s clay also carries deep-movement classifications, meaning seasonal moisture reaches further into the soil profile here than it does south of the Great Divide.

So the same diagonal crack has three possible explanations locally, and they cost very different amounts to resolve. That’s not a reason to assume the worst — most cracked walls in Bendigo are reactive clay doing exactly what reactive clay does. It’s a reason to establish which one before anyone quotes a fix.

Reactive clay, the goldfield, and how to tell them apart →

Is it getting worse?

The most important question about any crack, and the cheapest one to answer. Mark the ends in pencil, photograph it with a ruler against it and note the date, then repeat in three and six months — and compare across seasons rather than weeks, because on reactive clay a crack that widens through summer and closes after a wet winter is telling you something quite different from one that only ever widens.

The full monitoring method, and how to interpret what you find →

When to get it looked at

Worth acting on:

Diagonal stepped cracking in external brickwork that’s widening
Cracks accompanied by doors or windows binding where they didn’t before
Floors sloping in the same part of the house
Water getting in, or the crack passing right through the wall
A newer house or a slab — less history means movement is more likely to be current
You’re selling or buying, and it’s on an inspection report

Usually not urgent:

Hairline cracking in internal plaster with nothing matching it outside
Fine random cracking in render
Cracks that have been there as long as you can remember and haven’t changed
Cracking that reverses with the seasons
A vertical crack where an extension meets the original house, not changing

Filling and painting a crack that’s still moving is a waste of a weekend — it’ll reopen. Establishing whether it’s moving comes first.

What it takes to fix properly

Patching is the last step, not the first. If the wall is cracking because something below it is moving, the sequence is:

Find the water. A downpipe discharging beside the house, a cracked stormwater line or ground falling towards the wall drives clay movement, and it’s the cheapest thing on this list by a wide margin. Drainage and subfloor causes →
Establish whether it’s still moving, using the method above.
Diagnose the cause — stumps, clay or ground. Floor levels across the whole house, and a look underneath where there’s a subfloor.
Structural work only if the diagnosis calls for it — restumping where stumps have failed, underpinning where the ground won’t carry load. Typically $12,000–$30,000 and $15,000–$40,000+ respectively.
Then patch and paint, once the movement has stopped.

Full cost ranges for every method →

Find out whether it's structural

Five questions — what you’re seeing, what the house sits on, roughly when it was built, why you’re looking into it now, and how to reach you. If it looks like plaster rather than footings, that’s what you’ll be told.