House subsidence: the early signs and what's actually causing them

"Subsidence" is the word that makes homeowners' stomachs drop, but it describes something mundane: part of your house's footing system is going down (or being pushed up) relative to the rest, because the ground beneath it has changed. Catch it early and it's a manageable, quotable repair. Ignore it and it compounds. Here's how to catch it early.

Subsidence, settlement and heave — the three words

Worth thirty seconds of vocabulary, because they get used interchangeably and shouldn't be. Settlement is the normal, mostly-even compression of ground under a new building's weight — every house does some in its first years, and even settlement is harmless. Subsidence is the ground itself giving way or shrinking under part of the house after that — the problem case, because it's differential: one corner moves, the rest doesn't, and the structure racks between them. Heave is the reverse — ground swelling and pushing part of the house up, classically after a big tree is removed or a leak saturates reactive clay. The symptoms of subsidence and heave look near-identical from the living room; the fix and the cause differ, which is one reason a proper assessment beats guessing.

The four causes behind most Australian subsidence

1. Reactive clay doing its thing. Large parts of Victoria and Queensland sit on clays that swell when wet and shrink when dry. A drought year can shrink clay enough to drop a footing; a wet year heaves it back. Movement that cycles is soil behaviour; movement that ratchets one way is subsidence proper.
2. Trees. A large eucalypt can dry the soil well beyond its dripline — root systems commonly reach as far sideways as the tree is tall. A thirsty tree near a clay-founded wall creates a permanent dry (shrunken) zone under that wall, and the footing steps down toward it. The reverse also bites: remove a big old tree and the soil it kept dry slowly rewets and heaves for years.
3. Drainage and downpipes. A downpipe discharging at the base of a wall, blocked stormwater, or paving graded toward the house keeps one patch of ground wet — soft in any soil, swollen in clay. One wet corner moving differently from three dry ones is the classic pattern we find.
4. Leaking pipes. A cracked sewer or dripping water line quietly washes fines out of the soil or soaks a footing for years. If subsidence appears with no visible cause — no tree, no drainage fault — a plumber's pressure test and CCTV drain inspection is money well spent, and it's where an honest assessment starts.

Early signs, in the order they usually appear

• Doors and windows going out of true — sticking, swinging open on their own, latches missing their plates.
• Fresh diagonal cracks at door and window corners inside; fine cracks re-opening through old patches.
• Stepped cracks in external brickwork, usually starting near one corner — the single most reliable external sign.
• Floors sloping toward one corner (roll a marble), or a dip developing in a slab floor.
• Gaps opening between walls and skirtings, cornices, or between the slab edge and paving outside.
• Cracks wider at the top than the bottom — a wall rotating as its end sinks.

One of these alone, stable, in an older house is normal wear. Two or more, appearing or worsening together, clustered around one part of the house — that's a pattern, and patterns deserve attention.

What to do — in this order

1. Document, don't panic. Photograph every crack with a coin for scale and a date. Mark the crack tips in pencil. Subsidence decisions are trend decisions, and you're starting the trend record today.
2. Fix the water this weekend. Extend downpipes into stormwater, clear gutters, re-grade garden beds away from the wall, fix dripping taps. This is cheap, and in a surprising number of cases movement slows or stops once the moisture insult stops.
3. Audit the vegetation. Large trees within their own height of the affected wall are suspects. Get arborist advice before removing anything big — sudden removal can trade shrinkage for heave.
4. Test the plumbing if there's no obvious cause: pressure test plus drain camera.
5. Get a structural assessment if cracks exceed 5mm, step through brickwork, or keep growing. On slab and strip-footing homes the fix is underpinning ($10,000–$40,000+ for most jobs); on stump homes it's usually restumping ($9,000–$22,000 typical) — different problem, different price.