Cracks in brick walls: when to worry — and when to relax

Every older Australian house has cracks. Most mean nothing. A few mean the footings are losing a fight with the ground. The difference isn't mysterious — engineers classify it by width, pattern and behaviour, and you can read the same signals yourself. Here's the honest translation.

Width: the first and best filter

Australia's residential footing standard (AS 2870) and the CSIRO's homeowner guidance grade crack damage in categories, and width does most of the sorting:

• Hairline, under 1mm: negligible to very slight. Plaster shrinkage, paint film, normal seasonal movement. Cosmetic in almost every case.
• 1–5mm: slight to moderate. Worth attention — not panic. These are the cracks to photograph, date and monitor, because the trend matters more than the width.
• 5–15mm (or several cracks of 3mm+ clustered together): moderate to severe territory. Doors and windows usually misbehave by now; weathertightness can be compromised. Get a professional assessment.
• Over 15mm: severe. This bracket typically involves distorted walls, and repair means structural work, not filler. Nobody with a 15mm crack needs convincing — the house is telling them loudly.

Pattern: what the shape tells you

• Vertical, uniform width: often shrinkage or minor differential movement — the least alarming pattern, especially if the crack is old and stable.
• Diagonal, radiating from door and window corners: the frame is racking as footings move unevenly. Inside on plaster this is the classic early sign; a few are common in any old house, but fresh ones — or old ones re-opening — mean current movement.
• Stepped or zigzag, following the mortar joints through brickwork: the signature of footing movement. Masonry doesn't flex — it cracks along its weakest lines, and a staircase crack is the wall folding around a sinking or heaving footing. Of all patterns, this one most reliably earns an inspection.
• Horizontal cracks along a mortar bed, especially with bowing: less common, potentially serious (wall tie corrosion or lateral pressure) — professional assessment, promptly.

Behaviour: the trend beats the snapshot

A 2mm crack that has been 2mm for a decade is history. A 2mm crack that was hairline last spring is a process. Two questions sort almost every crack:

1. Does it open and close with the seasons? Reactive clay swells in winter and shrinks in summer, so cracks that breathe — wider in late summer, tighter after rain — are usually the soil cycling, not the footing failing. Manage moisture (downpipes, garden watering, trees) and monitor.
2. Is it ratcheting wider each year? Progressive, one-way growth is the pattern that matters. That's ongoing subsidence or heave, and it doesn't fix itself.

How to monitor a crack properly

• Photograph it with a scale reference — a coin or ruler in frame — and write the date on the photo. Repeat every couple of months and after big weather swings.
• Mark the ends with a light pencil line and date. Cracks grow at their tips; a crack extending past your mark is growing.
• Cheap crack-width gauges (a few dollars from hardware stores) or a dated filler line across the crack both work: if the filler cracks again, it moved.
• Check the company it keeps: a crack plus sticking doors, sloping floors or gaps at skirtings is a movement story, not a plaster story.

When it's structural

Treat a crack as structural — and get it assessed — when any of these are true: it's wider than 5mm; it's stepped through external brickwork; it's growing month on month; it clusters with other cracks at one corner of the house; or doors and windows near it are jamming. On a house with a concrete slab or strip footings, that assessment is underpinning territory. On a house with stumps, cracking usually traces back to stump settlement — a different, usually cheaper fix.

Fixing cracks in the right order

The most common crack-repair mistake in Australia isn't bad filler — it's good filler applied to a wall that's still moving. Patch and paint before the cause is fixed and the crack returns within a season, now with a visible repair scar beside it. The right order: cause first (drainage, trees, leaks, or the footing/stump repair itself), then stabilise, then patch. For seasonal hairlines you choose to live with, flexible gap fillers outperform rigid plaster; for cracked brick mortar, repointing after movement stops restores the wall's weather seal. And if you're underpinning or restumping anyway, patch afterwards — a properly levelled house closes many of its own cracks, and what remains patches once, permanently.