A client in Perth once sent me a photo of a hairline crack above a doorway with the subject line “is my house sinking,” and honestly, that reaction is the norm rather than the exception. Nearly every house older than a decade has some cracking somewhere. The job isn’t deciding whether cracking exists — it’s telling normal settlement, which every structure goes through, apart from active movement, which keeps getting worse and eventually needs engineering intervention. Those two things get lumped together constantly, and the confusion costs buyers money in both directions: some walk from houses that are fine, others buy houses that need real work without knowing it.
Settlement is a one-time event; movement is ongoing
Settlement happens as a structure’s weight compresses the soil beneath it, and on most sites that process is largely finished within the first few years after construction. The cracking it leaves behind is typically fine, stable, and doesn’t reappear once repaired. Active movement is different — it keeps happening, often tied to a seasonal cycle of soil moisture, and cracks from active movement tend to return within months of being patched, sometimes in the same spot, sometimes slightly wider.
What separates the two on inspection
Crack width and direction matter less on their own than whether the crack is growing. A technician will often mark a crack with a dated pencil line or a small dab of filler specifically to check on a return visit — if the gap has widened past that mark weeks or months later, that’s active movement, not historic settlement. Doors and windows that have been re-planed more than once, or that stick seasonally rather than permanently, are another strong signal of ongoing movement rather than a one-time event. Gaps that open and close with the seasons — wider in a dry summer, tighter after winter rain — point toward soil moisture cycling as the driver, which is a very different repair conversation than a single crack that simply appeared once during construction and never moved again.
Soil type sets how much movement to expect
Sandy sites like much of the Perth basin behave differently to clay sites — sand drains quickly and doesn’t swell with moisture the way reactive clay does, so foundation movement here is more often tied to erosion around footings or poorly compacted fill than to seasonal expansion and contraction. The USGS explanation of expansive soil behaviour is a useful baseline for understanding why the same crack pattern can mean something completely different depending on what’s underneath the slab.
Cut-and-fill sites need their own read
On sloped blocks that were cut and filled during subdivision, the fill side of a building often settles at a different rate than the cut side, because compacted fill and undisturbed natural ground behave differently under load even when built at the same time. A crack pattern that runs diagonally across a wall, tracking from the fill side toward the cut side, is a classic signature of differential settlement across that boundary rather than uniform movement across the whole footprint. Knowing which side of the house sits on fill versus original ground, something a site plan or a conversation with the builder can usually establish, changes how a technician reads every crack on that elevation.
When it’s time to bring in an engineer
A general building inspection can describe the pattern, but confirming whether movement is active and quantifying it properly is outside that scope. Cracks wider than about 5mm, cracking that runs through more than one material (say, from brick into a window frame), or any doors that have stopped closing entirely are reasonable triggers for a structural engineer’s assessment before settlement, since an engineering report typically costs AUD 600 to 1,200 and is far cheaper than guessing wrong on a purchase.
Frequently asked questions
Does every crack need an engineer to look at it? No. Most hairline cracking in plaster or masonry is cosmetic and tied to normal settlement or minor thermal movement, not structural concern.
Can settlement start up again years later? It can, usually triggered by a change nearby — a new pool excavation, a burst pipe saturating soil under a footing, or a large tree removed that had been drawing moisture from the ground.
How is movement under the slab actually confirmed if I can’t see it? Getting eyes on the subfloor or crawl space directly is usually the next step, and it’s exactly why inspectors push so hard for proper subfloor access even when a seller would rather they didn’t bother.