Most people assume that when a house moves, it sinks. Slab heave is the opposite — the ground swells and lifts part of your slab. It produces very similar symptoms to settlement, which is exactly why it's so often misdiagnosed, and why the wrong repair can make things worse.
Here's what slab heave actually is, how engineers identify it, and the order of steps that fixes it properly.
Slab heave happens when reactive clay beneath your slab absorbs moisture, swells, and pushes part of the slab upward. It's driven by moisture change, not by weight.
It's usually fixable — but the critical rule is fix the cause before you fix the damage. Repairing cracks while the ground is still moving simply buys you the same repair again next season.
These get used interchangeably in conversation, but they're opposites:
They can produce similar cracking, sticking doors and uneven floors. But the remedies differ, and a repair designed for settlement can be actively counterproductive on a heaving slab. Establishing which one you have is the first job of any competent assessment.
Where the moisture is determines the shape of the movement:
Edge heave is the more common pattern. Dry soil around the perimeter of the slab gets wet — from rain, poor drainage, a downpipe, a garden bed or irrigation — and swells. The edges lift while the centre, still relatively dry, stays put. The floor ends up falling toward the middle.
Centre heave occurs when moisture builds up beneath the middle of the slab, often from a plumbing leak or trapped moisture. The centre rises, producing a noticeable hump through the middle of the house.
Both are usually seasonal and cyclical — often more pronounced after a prolonged wet period, easing during dry months.
Reactive clay soil. The precondition. Without expansive soil there's nothing to swell. (See our guides to [site classification] and [reactive clay in Brisbane].)
Plumbing and stormwater leaks. A slow leak under or beside a slab is one of the most common triggers of localised heave.
Poor surface drainage. Ground falling toward the house, or downpipes discharging at the footings, saturates the perimeter.
Garden beds and irrigation against the slab edge. Consistently wet soil hard against the footing while the rest of the site dries out is a recipe for differential movement.
Tree removal. This one surprises people. A large tree draws significant moisture from the soil. Remove it, and the ground rehydrates over the following seasons — and can heave.
Construction issues. Edge beams shallower than the site classification requires, or inadequate site preparation, can leave a slab less able to tolerate normal movement. These often only become apparent once the ground starts moving.
Slab heave frequently ends in disagreement about who's responsible, so this point is worth knowing.
AS 2870 governs residential slab and footing design and site classification. Stormwater and subsoil drainage are dealt with separately under the relevant plumbing and drainage standards (AS/NZS 3500.3).
That means a statement that "the slab was designed to AS 2870" doesn't, on its own, answer whether the site's surface water, stormwater and subsoil drainage were properly designed and constructed. In a heave investigation, both need looking at.
(This is general information, not legal advice. If you're in a dispute, get independent engineering assessment and speak to a qualified lawyer.)
A proper investigation typically includes:
Step 1: Fix the cause. Repair leaks, correct drainage and grading, move garden beds and irrigation away from the slab edge, and address problem trees. In many cases this alone stabilises the situation.
Step 2: Monitor. Give the ground time to re-equilibrate — often across at least one full seasonal cycle — with levels and crack widths recorded.
Step 3: Rectify the structure. Only once movement has stabilised, and with the cause addressed.
Reversing steps 1 and 3 is the most common and most expensive mistake in this area.
Drainage correction and subsoil drainage. The cheapest intervention, and frequently sufficient on its own.
Root barriers, where trees are a contributing factor.
Resin (polymer) injection re-levelling. Injected geo-polymer can stabilise and re-level with far less disruption than traditional underpinning, and allows incremental lift with survey monitoring — useful when the aim is restoring serviceability rather than forcing a building back to perfectly level.
Traditional underpinning — with caution. This is important: on reactive clay, rigid underpins can create differential movement between the underpinned zones and the rest of the structure. Underpinning is a settlement remedy first and foremost, and applying it reflexively to a heaving slab on reactive soil can worsen the problem. It has its place, but it should be a considered engineering decision, not a default.
Partial rebuild in severe cases where movement has caused unrecoverable damage.
Newly built homes affected by heave may be covered by statutory warranties, and builders and engineers owe a duty of care. What's worth doing early is obtaining an independent structural assessment documenting the movement, the likely cause and the rectification required — that record is valuable regardless of how the matter is ultimately resolved.
QED Engineers provides structural inspections, floor level surveys and reports for homeowners and builders across [QLD, NSW, VIC & ACT] — identifying whether you're dealing with heave or settlement, what's driving it, and what actually needs to be done.
Request a Free Quote | Call: 0421 157 963
This article is general information only and is not legal advice. Slab movement must be assessed on site by a qualified structural engineer, as diagnosis and remedy depend on your specific building, soil and history.
What is slab heave? Movement where reactive clay beneath a concrete slab absorbs moisture, swells and lifts part of the slab upward. It's driven by moisture change rather than load, and typically produces uneven floors and cracking.
How is slab heave different from settlement? Heave lifts the slab as soil swells; settlement drops it as soil consolidates or erodes. They cause similar symptoms but require different remedies, so correct diagnosis matters before any repair.
Is slab heave covered by insurance? It depends entirely on your policy and the cause. Many policies exclude gradual ground movement while covering sudden events such as a burst pipe. Check your product disclosure statement — an engineer's report is usually needed to support a claim.
Can slab heave be fixed permanently? The soil stays reactive, so the goal is management rather than elimination. Correcting drainage, leaks and vegetation, combined with appropriate structural rectification, can stabilise a home long-term.
Does removing a tree cause slab heave? It can. A large tree draws substantial moisture from the soil; once removed, the ground rehydrates over subsequent seasons and may swell. Get engineering advice before removing established trees close to a house.
How long does slab heave take to stabilise? Often at least one full seasonal cycle after the cause is corrected, sometimes longer. This is why engineers commonly monitor movement before committing to structural repairs.
