Crack patterns that matter
The signature of subsidence in brickwork is the stepped crack — following the mortar joints diagonally, like a staircase, usually radiating from a corner of the house or from a window or door opening. Width matters (the crack-width guide covers the millimetre thresholds), but pattern and freshness matter more: a crack with clean, sharp faces and no paint or dirt inside it is new, and new is worth attention. Cracks wider at the top than the bottom suggest one section of the house rotating away — classic corner subsidence. Matching internal and external cracks in the same spot mean the movement goes through the full wall, not just the render.
Beyond the cracks
Doors and windows going out of square on one side of the house — the gap over the door wider at one end. Gaps opening between the slab and skirting boards, or between brickwork and window frames. A slab edge you can see dropping away from a path or paving that used to sit flush. Sticky patches in tiled floors, or tiles cracking along a line. Externally: gaps under external walls, and paving or steps pulling away from the house.
What's usually harmless
Hairline render crazing (a shattered-glass pattern, not a line), cracks along plasterboard joints inside, seasonal hairlines that open in late summer and close in winter, and the first-year settling of a new build. Most houses on reactive clay flex a little annually — the Australian standard even classifies how much movement each site class is designed to tolerate. The question is never "is there a crack?" but "is there progressive movement?"
The usual causes
Reactive clay drying (drought, or a large tree drinking the moisture from one side), water softening the soil (leaking pipes, blocked drainage, decades of poor stormwater), and fill that was never compacted properly. Diagnosing the cause is half the fix — underpinning over an unfixed leak just gives the leak a more expensive victim. Why houses sink covers the soil story in full.
What to do next
Photograph, date and measure the cracks; re-check monthly. If cracks pass 5 mm, keep growing, or come with doors out of square, commission a structural engineer's report before talking to any underpinning contractor — here's how that works — and read "Do I actually need underpinning?" for the honest decision framework. If the movement is confirmed, the cost guide sets fair expectations before quotes arrive.
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Common questions
Are cracks worse in summer?
On reactive clay, often yes — soil shrinks as it dries, and cracks widen late in a dry summer, then partly close over winter. Purely seasonal breathing is usually manageable; a crack that ratchets wider each cycle without closing is the one to watch.
The crack is only in the render — does it count?
Check inside at the same location. Render can craze on its own harmlessly. A crack that shows on both faces of the wall at the same spot is structural movement, whatever the width.
My neighbour's tree is near my wall — is that the cause?
Large trees within roughly their mature height of a footing can dry clay enough to matter. It's a genuine and common factor, but let the engineer attribute it — tree-versus-house disputes escalate quickly, and you'll want the report before that conversation.