Why Houses Sink: Clay, Drainage and Dry Spells

Reactive clay, the main culprit

Clay particles absorb water and swell; when the water leaves, they shrink. On reactive clay — which underlies large parts of Melbourne, Adelaide, western Sydney, Brisbane and most inland towns — the ground surface can rise and fall meaningfully between a wet winter and a dry summer. Site classifications used in residential footing design run from slightly to extremely reactive for exactly this reason.

Uniform movement would be harmless: the whole house rides up and down together. The damage comes from differential movement — one corner over dry soil, another over damp. On a stumped house, the stumps above the shrinking soil drop, the bearers follow, and you get the sloping floors, cracks and sticking doors catalogued in the signs guide. Add the fact that many older stumps are timber, and that moisture cycling also drives rot at the ground line, and clay is behind most of the work restumpers do. The stump materials guide covers how modern concrete and steel stumps handle it.

Droughts and the demand spike

Seasonal shrink-swell reaches perhaps the top metre of soil. A multi-year drought dries clay much deeper, and soil that has never dried before shrinks for the first time — which is why long dry spells are followed by waves of settlement damage, and why restumpers and underpinners see demand spike after extreme seasons. The same thing happens in reverse: an unusually wet year swells clay that had settled into a dry equilibrium, and houses heave.

The practical lesson is that some movement is cyclical. A floor that dropped during a drought may partially recover when the rains return, which is why measured monitoring beats panic: cracks of 1–5 mm are worth tracking monthly before they are worth paying to fix, while anything past 5 mm or visibly worsening deserves professional eyes.

Drainage faults around the house

If clay is the powder, water management is the match. The most common drivers of localised movement are boringly domestic: blocked or overflowing gutters, downpipes that discharge beside the footings instead of into stormwater, paths and garden beds graded so water runs toward the house, garden watering concentrated against one wall, and slow leaks from old pipework or air-conditioner condensate lines. Each one wets a patch of clay while the rest of the site stays dry — differential movement made to order. A walk around the house during heavy rain will show you in ten minutes what a report costs hundreds to say.

Trees, fill and compaction

Large trees are pumps. A mature gum can dry the clay through its root zone far below seasonal depth, dropping the nearest corner of a house millimetre by millimetre over years. A common rule of thumb keeps large water-hungry trees at least their mature height away from footings on reactive sites. The reverse is also true and regularly surprises people: remove a big tree, and the soil it kept dry slowly rehydrates and swells for several years afterwards.

The other soil problem is fill. Houses built on poorly compacted fill — common on cut-and-fill sloping blocks and old filled gullies — settle as the fill consolidates under load, often more on one side than the other. Fill settlement is not moisture-driven, so drainage fixes will not stop it; it usually needs deeper founding when the house is eventually restumped.

Why fixing water comes first

Here is the sequence that saves money: fix the water, then fix the house. A $15,000–$30,000 restump over an unfixed drainage fault treats the symptom and leaves the cause running — the new stumps sit in the same wet-dry cycling that moved the old ones, and you risk buying the same problem twice. The same logic applies to underpinning on slab homes.

None of this means the stumps never need doing — rot, termites and end-of-life timber are one-way streets. It means water first, structure second, and monitoring in between.