The house slab is down – waffle pod learnings

So we selected waffle pod slabs because they were a good choice for the house thermal rating for our area and they are cheaper. At the moment, we don’t regret it, but we did learn a bit while it was being installed.


The polystyrene turned up in packaged up blocks, but little bits started breaking off, and we ended up with small lumps of it flying around. We could collect the lumps, and did daily pick ups, but as the photo below shows, it started to breakdown into individual pellets – impossible to get them all.

Not sure that we could have done much more to avoid the mess – a definite downside that we hadn’t considered. Try to keep them plastic wrapped as much as possible! And be prepared for frequent clean up.


So I’d read a lot about potential issues with soil type (don’t do well with reactive clays) but all our geotech said they were fine for our conditions, so we weren’t too concerned.

But when the pods were laid out we noticed that we had cut outs where our fireplaces were to be installed. What the? Talked to the concreter and he said they didn’t normally do it, but that the engineers had specified it, so he had cut them out and there would be a huge amount of concrete sitting under the wood heaters. So the heaters are heavy, but not as much as a car – and the load is distributed across the hearth. So how come the garage was waffle pod? Would we have a weight issue with the really large fish tank – heavier than the wood heaters when filled with water? What about the large bath? So didn’t make sense.

Well, after lots of slightly panicked research it still doesn’t make sense. We spoke to the building inspector (who had to come pre and post pour to make sure everything was OK), and he agreed that it was very odd, although he did say there could be issues with waffle pod slab strength.

The Australian standard requires a minimum of 85mm concrete on top of the waffle pods. He said he thought this was too low because if the waffle pods weren’t installed dead level then it was possible for the concrete to be thinner in places reducing the strength. He said he’d seen a slab punched through in a garage when a car had been jacked up (fine when the load was distributed across the 4 wheels, just not when the jack was carrying the load). So he’d certainly been on the look out for that during the inspection – he frustrated the contractors a bit by being very nitpicky (yay – do like that we selected and employed the inspectors) and making them rejig things and dig deeper ring beams around the edges.

Ours were actually specified to be 100mm thick, so above the minimum – nice that our builder does that as standard. Although the inspector said he would prefer an even higher margin – a further 10 or 15mm.

So if you’re going waffle pod, ask about your concrete thickness, and check how level the pods are. Obviously the extra concrete costs more, but I’m glad that we’ve at least got some margin…

Services Check

The day before the concrete was poured we turned up to find a plumber and his girlfriend at the site. They’d planned a day out on the beach, but stopped to do the pre concrete pour inspection on the way – running a camera down all the plumbing to make sure no damage was done to it by installing the waffle pods and rebar on top. He says he’s done the check hundreds of times and never found an issue. So he came on Sunday with his girlfriend thinking he’d only be there for a little while. But somehow a stake had been driven through the toilet plumbing without anyone noticing, and it needed to be replaced.

We’re so so glad he did the check. Backed up toilet, sewage running under the slab, and then having to dig up the tiles and slab and redoing it after the house was finished would have been a disaster.

Make sure that the checks are done!

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