The Inevitable Failure of Waffle Slabs

I took the time to read right through the Softley v Metricon Homes Pty Ltd decision, published by VCAT (the Victorian Civil and Administrative Tribunal) on 11 December 2014. Metricon was ordered to pay the cost of knocking down and rebuilding the Softleys’ house.

I was editing some of my waffle slab posts when it suddenly hit me: the slab movement in the Softleys’ home was a lot less than the slab heave and movement I have inspected and reported on in Queensland, including recently.

The VCAT Decision

For those interested, the full decision is here: Softley v Metricon Homes Pty Ltd.

If I were a builder of waffle slabs, I would be reading this decision very, very thoroughly.

It is also a good read for homeowners. It sets out the steps that matter in diagnosing slab heave and working out what caused it: measuring the movement, investigating the soil and moisture conditions, and tracing the movement back to its cause.

Say What?

Here’s the thing. The Softleys’ house had about 44 mm of uneven slab surface movement.

In contrast, in the Ipswich suburb of Bellbird Park I recorded slab surface unevenness in the order of 110 mm in a newish home.

So how is it that 44 mm of movement gets a house rebuilt, and 110 mm only gets an engineer’s investigation and report?

Part of the answer is that the amount of movement isn’t the only thing a tribunal looks at. The outcome of a building dispute depends on the cause of the movement, who was responsible for it, the contract, the legislation in that state, and whether a repair is actually practical. Each case turns on its own facts, and a tribunal decision in one state doesn’t automatically decide a case in another.

But it does show what is possible when movement is properly investigated and the cause is traced back to the way the house was built.

Waffle Slab Builders Should Be Nervous

I’d like to reiterate. If I were a builder of waffle slab homes, I would be very, very nervous.

Melton is a long way from Bellbird Park. But when homeowners in Brisbane, Sydney, Adelaide and Emerald (Central Queensland) discover that a tribunal has ordered a builder to demolish and rebuild a poorly performing house with a measly 44 mm of differential slab movement, I expect the tribunals and courts to get a lot busier.

Why Waffle Slabs Are So Vulnerable

Poor moisture control around a waffle slab house is probably inevitable. There are so many things that can go wrong:

• movement leading to broken pipes, which then leak under the house;
• porous backfill placed around the house;
• slight subsidence of fill, tilting paths back towards the house;
• overland flow, and gutters overflowing beside the slab; and
• set-down patio areas that act as soakage trenches under the house.

Every one of these lets water reach the reactive clay under the slab unevenly, and that is what drives slab heave. I go through why waffle slabs are particularly exposed in beware waffle slabs, waffle slabs on H2 class soil and water around a waffle slab.

The Floodgates Have Opened

Just remember where you were when you found out that the cracks in your house could be enough for a claim against the builder to end with the builder rebuilding your home.

I’m not ambulance chasing. The ambulance has already left.

You didn’t hear it from me.

Matt Cornell
Cornell Engineers

Is Your Waffle Slab House Moving?

Whether a claim is worth pursuing starts with good evidence: how much the house has moved, why, and whether the cause traces back to the design or construction. Cornell Engineers investigates slab heave and cracking in waffle slab and raft slab houses, and prepares independent expert reports for disputes, including in QCAT. See our forensic structural engineering page for more.

Send us the property address, photographs of the cracking and of the ground around the house, a short description of what has happened, and any soil reports, engineering drawings or previous reports. We will review the information and let you know whether the matter is a good fit for Cornell Engineers before preparing a fee proposal.

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This article is general commentary on a published tribunal decision and is not legal advice. If you are considering a claim against a builder, speak to a lawyer about your circumstances.