Categories
Footings and Slabs New Homes

The Inevitable Failure of Waffle Slabs

I took the time to read right through the Softley v Metricon Homes Pty Ltd decision that was published by VCAT on 11 December 2014. Metricon was ordered to pay costs to knock down and rebuild the Softley’s house.

I was editing some of my waffle slab posts when I suddenly realised that the amount of slab movement in the Softley’s home was a lot less than the amount of slab heave and movement I have inspected and reported on in Queensland – even recently.

The VCAT Decision

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

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

The Investigation

The information in this decision describes the important steps in diagnosing and determining the cause of slab heave.

The thing is, the Softley’s house exhibited only 44mm of uneven slab surface movement.

Say What?

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

So how is it that 44mm of movement gets a house rebuilt and 115mm only gets an engineer’s investigation and report?

Waffle Slab Builders Should Be Nervous

I’d like to reiterate.

If I was a builder of waffle slab homes I would be very, very nervous.

It’s maybe lucky that the suburb of Melton is quite some way from Bellbird Park; because when Brisbane and Sydney and Adelaide and Emerald (Central Queensland) homeowners discover that the precedence has been set for demolition and reconstruction of poorly performing waffle slab homes (with a measly 44mm of differential slab surface movement), then the flood gates of building tribunals and court cases is going to open.

Poor moisture maintenance around a waffle slab house is probably inevitable. There are so many things that could go wrong.

  • Movement leading to broken pipes
  • Porous backfill around the house
  • Slight subsidence of fill resulting in tilting of pathways towards the house.
  • Overland flow and gutters overflowing.
  • Patio areas are setdown creating soakage trenches under the house

Flood Gates have Opened

Just remember where you were when you discovered that the cracks in your house could be enough that a claim against the builder might result in the builder being forced to rebuild your home.

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

You didn’t hear it from me.

Matt Cornell
Cornell Engineers

10 replies on “The Inevitable Failure of Waffle Slabs”

Hi Matt,
Since coming across some issues with the Waffle Pod Slab recently (horse has bolted) poured on my block, I informed the builder that my site was classed as a H2 (highly reactive clay) as per their Geotech report and they have cut into the clay level on the high side and there is evidence of top and subsurface runoff after heaver rain. They engineered the slab with a doubtful 85mm top slab being presumably a H1 class and not 100mm H2 class construction. I suggested that maybe the slab should have been the Hybrid WPS (Raft edge beams trenched below ground level) for improved stiffening. and I am awaiting their response. To alleviate their problem I have also suggested that not only are they having 3 pits and grates positioned on the high side for surface runoff (part of their engineering plan)but at their expense they should put Agi slotted piping and aggregate for subsurface runoff, parallel to slab and tapped into stormwater downstream of this slab with fall to at least comply with the so-called Warranty clause that the builder should provide an adequate initial diversion to comply with their guarantee signoff and the owner is responsible for diverting stormwater (retaining walls with Agi piping) after the build, preventing ingress under the WPS and subsequent heaving and shrinking. Very disillusioned Matt.

Hi David
We hear this all too often and very often after the proverbial horse has bolted.
I guess that’s why I have created my Youtube videos on drainage and sloping blocks and waffle slabs – to try and get in front of the issue. Sadly many of the suggested fixes are beyond the scope of a homeowner to implement themselves – but maybe they can be used to force the builder’s hand in the resolution of the problems.
I wish you well in your attempts to get what you deserve – a performing, trouble-free house. Homeowners shouldn’t be left to solve these problems on their own.
Best Regards
Matt Cornell

Hi Matt, i’m not 100% sure but i think the differential movement was 71mm mate in this case as stated in section 28 wasn’t it?

Matt, I am very interested in knowing would the Softley’s house have performed any better if it was a regular raft slab as opposed to the Waffle Pod design??
From my experience Waffle pods if designed according to AS2870 perform equally or better than standard rafts??
your view on this.. thanks.

I have seen Pier and beam slabs fail on the Darling Downs. I have build only around a dozen or so homes using WP slabs with no problems. Done a further 50 odd slabs for other builders and only a few did have movement Mostly caused by not building the site up prior of preparing the slab and engineers so called soil tests.
I was doing these slabs since the late 80,s and was one of the first concretors in Qld using this system. I had and old guy walk past my site once and he told me he used this system over 50 years back, only using bails of straw and besser blocks between the bails.

Matt,

I have attended a seminar conducted by FFSV in Victoria. Based on their findings especially in the west side of Melbourne that the soil can change from a class H to a P or E during change of season. In addition, I have also, been informally informed that a waffle slab can only span a max of 7m in both directions.. I believe what we need is to get all our engineering brains together and come up with a better solution to this matter. I also believe that we need to set up an organisation that will focus and provide proper training in assessing crack failure. I read in England that they actually have one and there are books published to properly diagnose a crack and provide proper solution.

J

Hi Jonathon
In Queensland, QBCC panel engineers get a fair bit of guidance on report structure and investigation procedures for house cracking / subsidence claims. We also have some wonderful engineers on the AS2870 committee that love to share their knowledge.
That same level of in-house experience is probably available in each state, but engineers are a funny bunch and aren’t always open to sharing their knowledge. So years of knowledge and experience can be lost when an engineer retires or moves on.
I’m sure engineers in each state could learn from each other and from overseas engineers (eg Texas has reactive clays). More open sharing would definitely advantage consumers.
Then we just have to teach consumers to stop searching out and taking the lowest price for footings.
All the best
Matt Cornell

Hi Charlie
Thanks for your comment. I agree. The risk is just too high. Which isn’t to say raft slabs have never failed – just that sometimes bigger really is better.
Matt Cornell

Wow. Sure glad I haven’t done any waffle pods. Won’t ever be doing them either. Give me the full depth large footings full of reo.

Leave a Reply

Your email address will not be published.