Underpinning advice
What is slab heave?
Published
Ask most homeowners what’s wrong with a cracking house and they’ll say the house is “sinking.” Across Melbourne’s western growth corridor, that’s frequently backwards. The ground isn’t dropping away from the house — it’s swelling and pushing part of the slab up. That’s slab heave, and it’s one of the more common and more misunderstood foundation problems in the city.
The basic mechanism
Reactive clay changes volume with moisture. Add water and the clay particles swell; remove it and they shrink. Under a concrete slab, that swelling doesn’t happen evenly — it depends on how much water is reaching the soil at each point along the slab’s perimeter and beneath it. Where moisture builds up, the clay swells more, and the slab above it lifts. Where the soil stays drier, the slab doesn’t move as much. The difference between those two states is what damages the house — not the movement itself, but the fact that it isn’t uniform.
The most common pattern is edge heave: moisture concentrates around the slab’s perimeter — from a downpipe, a garden bed, a leaking pipe — while the centre of the slab, insulated by the house above it, stays comparatively dry. The edges lift relative to the middle, and the brickwork built on top of that edge cracks to accommodate the movement it was never designed to absorb.
What triggers it
Heave needs a moisture source, and in practice it’s almost always one of these:
- A downpipe discharging at the slab edge instead of into stormwater drainage — the single most common cause we see.
- A leaking sewer or water line running near or under the slab edge, keeping the soil there permanently wetter than everywhere else.
- Garden beds watered daily against one side of the house, especially drip irrigation left running through summer.
- A change in drainage or paving that redirects surface water toward the house instead of away from it.
- A tree removed near the house — counterintuitively, this can cause heave rather than fix a problem: the tree was drawing moisture out of the clay, and once it’s gone, the soil rehydrates and swells.
That last one surprises people, and it’s worth sitting with, because the instinct is usually “the tree is the problem, remove it.” Sometimes it is. Sometimes removing it is what starts the heave.
How to tell it’s heave, not settlement
Heave and settlement are opposite movements that can look identical from inside the house, and telling them apart changes the entire repair approach.
Heave signs: cracking above windows and doors that widens toward the top, internal doors that bind at the top corner while the bottom still clears the frame, a slab edge that looks slightly higher than it used to relative to a fixed reference like a step or path.
Settlement signs: cracking that widens toward the bottom, doors binding at the bottom corner, a visible dip or gap opening up under a slab edge.
Neither is reliably obvious without a proper floor level survey — levels taken across the whole slab, which is the only way to actually map which direction each part of the house has moved, and by how much.
Why it matters more in Melbourne’s west
Caroline Springs, Melton, Tarneit, Werribee and the suburbs around them sit on basalt-derived clay with a high shrink-swell capacity — site classifications commonly come back M, H1 or H2 under AS 2870. That’s the soil half of the equation. The housing half is that these are almost entirely young estates built on slab-on-ground, with compact lots, minimal setbacks, and downpipes, paving and garden beds all concentrated in a narrow strip right against the footings. Put highly reactive clay next to a slab with a badly placed downpipe, and heave isn’t a remote possibility — it’s close to the default failure mode for that combination.
What actually fixes it
Underpinning is not automatically the answer, and installing piers against a slab that’s still actively heaving can make things worse — you’ve now anchored part of a structure the ground is still trying to lift. The sequence that actually works:
- Find and fix the moisture source. A downpipe redirected into stormwater, a garden bed moved off drip irrigation, a leaking pipe repaired — this alone resolves a meaningful share of heave cases.
- Monitor. Once the moisture source is corrected, the soil needs time to re-equilibrate. Measuring floor levels across a season shows whether the movement has stabilised.
- Repair the cosmetic damage — crack filling, repointing — once movement has genuinely stopped, not before. Filling a crack that’s still active just means doing it again next year.
- Underpin only where movement has caused permanent, non-reversing displacement — which a stabilised, monitored slab will not show.
The takeaway
If your Melbourne home is cracking and you’ve assumed it’s sinking, it’s worth asking the opposite question first. In a suburb with reactive clay and a young slab, heave is at least as likely a cause — and it’s often fixable with a downpipe and a garden hose timer, not $20,000 of piers.
Not sure whether your cracking is heave, settlement, or something else? Our underpinning and foundation repair page covers how we tell them apart, or book a free assessment and send us a few photos.
Common questions
Can slab heave be reversed?
Partly, sometimes, and slowly. If the moisture source driving the heave is removed — the downpipe corrected, the leak repaired, the irrigation moved — the clay gradually gives that moisture back up and the heaved section can settle back toward its original level over months to a couple of years. How much recovery you get depends on how long the heave has been active and whether the structure above has taken a permanent set. This is precisely why the correct sequence is to fix the water first and monitor across a season before committing to structural work.
Is slab heave covered by home insurance?
Generally not as gradual ground movement, which most Australian home policies exclude explicitly. Where a claim can succeed is when the heave results from a sudden escape of water from a pipe rather than seasonal soil behaviour, which many policies do cover. The distinction between 'sudden' and 'gradual' is where these claims are won and lost, so document when you first noticed it, keep the plumber's report and any CCTV footage, and lodge promptly. Read your own product disclosure statement rather than relying on a general answer.
Does slab heave mean the builder did something wrong?
Not necessarily. A compliant slab on reactive clay is designed to tolerate substantial ground movement, not to prevent it, so heave can occur on a perfectly well-built house when the moisture regime around it becomes uneven. Where a builder question does arise is if the site classification used at design was wrong for the ground actually present, or if drainage was not installed as designed. If the home is under ten years old, Victoria's statutory structural warranty makes that worth investigating with an independent engineer's report before you pay for repairs yourself.
Think this is happening at your place?
We assess before we quote — floor levels measured, cracks mapped, and a straight answer on whether you need work at all. Read more aboutunderpinning & foundation repair, or get in touch.