Structural Slab Heave: What It Is, Why It Happens, and What to Do About It

If you've noticed cracks forming in your internal walls, tiles that are starting to lift, floors that feel uneven, or doors that have suddenly started sticking, don't ignore them. These are things I look for on every inspection I do. They can be early signs that something is happening to the structure beneath your home.

Slab heave is one of the more serious structural defects I come across in my work as a building inspector across Melbourne and the Mornington Peninsula. Most homeowners don't know it's happening until the signs have already been building for some time. This post explains what it is, what causes it, how to spot the warning signs, and what you should do if you suspect it.

What Is Slab Heave?

Slab heave occurs when reactive soil beneath a concrete slab expands and lifts sections of the slab upward. That movement causes cracking and displacement throughout the home, in the slab itself, in the walls built on top of it, and in finishes like tiles and internal wall linings.

The concrete slab is a primary structural element. It is the foundation your home sits on. When it moves, even partially, the effects are felt through the entire structure above it.

Why Does Slab Heave Happen?

The cause is moisture getting into the soil beneath the slab.

Reactive soils expand when they absorb water. When that expansion happens unevenly, it pushes up sections of the slab rather than lifting it uniformly. That differential movement is what causes the damage you can see inside the home.

The moisture typically comes from one or more of these sources:

Poor site drainage. Water that isn't directed away from the building can pool around and beneath the slab. Stormwater needs to drain away from the base of walls, not towards them. On inspections of older properties, I've found subfloor soil that was excessively damp, in some cases muddy, with no adequate drainage in place at all.

Leaking pipes. A slow leak from a supply pipe or waste pipe can introduce water directly into the soil beneath the slab over time, without any visible signs above ground until the damage is already done.

Disconnected or blocked downpipes. Downpipes that aren't properly connected to stormwater drainage place concentrated water flow directly at the base of the footings. I've flagged this on both new build inspections and established property inspections. It looks like a minor issue but it can have serious long-term consequences for the foundation.

Water pooling under the slab. Any combination of the above can result in sections of subfloor soil becoming saturated. That sustained moisture is what allows reactive soil to expand and move.

What Are the Early Warning Signs of Slab Heave?

This is what I get asked most often. People want to know what to look for, and the honest answer is that the visible symptoms show up higher in the home, not at the slab itself.

Cracking in internal walls. Cracking on external walls can sometimes be explained by other factors, but cracking in internal walls generally points to structural movement. Wide cracks, or cracks that keep reappearing after being patched, are worth taking seriously. Step cracking in brickwork near windows, doors, or corners is another indicator of stress in the wall structure.

Lifting or cracked tiles. Floor tiles sit directly on the slab surface. When sections of the slab move, the tiles above them are placed under stress. Tiles that are lifting, or that have cracked across the body rather than just at the grout line, can indicate movement in the slab beneath them.

Floors that are out of level. If you're walking through a room and it feels like you're going up or downhill, that's worth investigating. On inspections, I use a spirit level to check floors. A floor that is significantly out of level over the length of a two-metre level is not normal. It points to foundation movement.

Doors that stick or won't close properly. When I walk through a property, I open and close every single door. Bedroom doors, lounge room doors, all of them. If they're sticking and jamming, that can mean there has been structural movement in the home. A door that sticks slightly might be nothing. A door that you can barely close is a different matter. That tells me there has been movement in the frame.

Recently repainted walls. If a property has been freshly painted, look more carefully at the doors and floors. Paint can cover patched cracks and make walls look fine when they're not. The doors and floors don't hide movement the way a fresh coat of paint can.

Is Slab Heave a Major Defect?

Yes. A major defect is defined as one where rectification must be carried out to avoid unsafe conditions, loss of utility, or further deterioration of the property. Slab heave meets that definition.

Left unmanaged, movement in the slab continues to damage the walls and structure built on top of it. The longer it goes without being addressed, the more extensive the damage becomes. In many cases, full demolition of the house may be required if slab heave is not detected early enough. A structural engineer needs to be engaged to assess the full extent of the damage and determine what remediation is required. A building inspector identifies and documents the defect. The structural engineer determines what needs to happen next.

Does a Building Inspection Cover Slab Heave?

A pre-purchase building inspection conducted in accordance with Australian Standard AS 4349.1 covers major defects in primary elements, including structural cracking and movement. Slab heave and the conditions that are conducive to it fall within that scope.

I inspect all readily accessible areas of a property, including subfloor spaces where accessible, and I use an electronic moisture meter to test for damp in areas that are known to be susceptible. If I find evidence of structural movement, soil moisture issues, or drainage problems that are likely to be contributing to or causing slab movement, I document it in the report with photos and a clear explanation of what I found and what should happen next.

If you're buying a property and the inspection report identifies a major structural defect, you can only legally exit the contract under REIV regulations if the report was issued by a Registered Building Practitioner. This is an important point that a lot of buyers don't know. If your inspector isn't registered, that legal protection doesn't apply, regardless of what they found.

I am a Registered Building Practitioner (Lic. DB-U 28872). Every report I issue carries that legal standing.

What Should You Do If You Suspect Slab Movement?

Get an inspection done by a Registered Building Practitioner before the damage gets worse. Slab movement tends to progress over time rather than stabilise on its own. The sooner it's properly assessed, the more options you have.

If slab heave is confirmed, a structural engineer will need to be engaged to assess the damage and determine what remediation is required. That process starts with a proper inspection and a documented report.

If you're in the process of buying a property in Melbourne or on the Mornington Peninsula and you're seeing any of the warning signs described in this post, contact me before you commit. A building inspection is a straightforward step that puts you in a far better position, whether that ends up being negotiating on price, requesting rectification, or walking away from the purchase.

Book a Building Inspection in Melbourne or the Mornington Peninsula

I'm Andrew Muling, The Building Inspection Doctor. I've been in the building industry since 1985 and have been performing building inspections since 2016. I'm a qualified carpenter, a Registered Building Practitioner (Lic. DB-U 28872), and I hold a Certificate IV and Diploma of Building and Construction.

Call me directly on 0425 725 497 or visit email andrew@inspectiondoc.com.au to book your inspection.