Drainage Footings and Slabs

Water Around a Waffle Pod Slab

Water around a waffle pod slab house being built on a reactive clay site is a problem and it’s not an easy fix.

That’s why the Australian standard for footings and slabs AS2870 says that the surface drainage of a construction site shall be controlled from the start of site preparation until the end of construction.

Why? Because it is easier to get good drainage around a waffle pod slab house while it is being built than after the house is finished.

Check out our detailed video on why waffle slabs are so affected by site drainage and what to do about it.

Adding Water to Clay Makes it Swell

When water soaks into the ground, the reactive clay in the soil absorbs the moisture and the clay swells.

As the clay swells, it expands and the ground surface lifts.

For a site classified as a reactive clay site (Class M, Class H1, H2 or Class E soils) the lifting forces are large enough to lift a house noticeably.

Water Around a Slab

When the water is only around the perimeter of the house slab the ground around the perimeter of the slab swells and lifts.

The house footings and slab directly above the swollen ground lift too when the ground surface lifts.

The dry ground under the middle of the house doesn’t swell and the slab height there doesn’t change.

Water near a waffle slab
Poor site drainage can lead to slab heave

High edges and low interior slab is known as a dished slab or edge heave.

If the ground is only wet on one side of the house then only the side will swell and the slab will look like it has tipped slightly.

If the whole under-slab area on a house under construction was totally saturated during construction the whole slab would lift semi-evenly.

Water Laying Around a Waffle Pod Slab

When water sits against a waffle slab it isn’t actually just sitting there; the water can and probably is seeping in under the house – especially when it sits near your patio area (which is set lower into the ground than the rest of your house).

Water next to a slab
Remove water from around a slab to stop water from entering under a house.

Now the clayey soil under the slab can get swell and lift too.

However, the amount of lifting is still uneven. The ground where the water is soaking in lifts but the ground that stays dry does not lift.

Now Let’s Build

The slab has now deflected due to the soil swelling and lifting.

The carpenter arrives to build the walls and the bricklayer arrives to lay the bricks.

Water sitting next to a slab
Poor drainage around a slab can cause brickwork cracking

Both the carpenter and bricklayer take great care to build walls that are straight and plumb.

They notice variations in slab height that they assume were caused by concreter failing to pour a dead flat slab. They adjust their work to suit and build walls on a deflected slab are nice and straight and true.

The walls get plastered and finished and you move in.

And the Movement Starts

You know not to water around a Class H1 and H2 site (because you read the website – ps yay you!) so you take great care not to water around the house, not to plant trees against the house and to make sure the site is well-drained.

The ground around the house dries out over the next 12 months and the slab surface drops back down to its original height.

Your brickwork cracks and your plasterboard walls crack and you panic.

You’ve done everything right and yet you still have cracks in your house.

The builder sends an engineer out to check the slab levels and they look pretty good. The slab is nice and flat – but there is damage everywhere.

Why Me?

Did the builder do a good job or not?


The site drainage has to be maintained from the start of site preparation, all through construction and until the end of time.

Water sitting near a waffle slab
Control site drainage from the start until the end of construction

Do not let water sit against the slab of your newly built house slab. Speak to your builder, point out the engineer’s requirement for the builder to maintain site drainage. Ask the builder to stop work and repair the site drainage immediately.

Do not let water sit against the slab of your newly bricked up house.

Do not let water sit against the side of your fully complete, ready to live in house.

The effect of water sitting next to a house during construction may not be permanent until it is.

Ask your builder to maintain excellent site drainage right from the start of construction.

If You’re a Builder

If you’re a builder reading this (well, welcome!) some sites are hard to maintain site drainage. Yes, it’s true. But do you really want to keep going back to a house that is suffering from slab heave?

The National Construction Code 2019 Volume 2 says this:

Extract from BCC2019 Volume 2

I’m sorry if you think this is optional. It is not.

It doesn’t matter whether you are on a flat site or a sloping site with the ground falling towards your building. The external finished surface surrounding the slab must be drained to move surface water away from the building and graded to give a slope of not less than 50mm over the first 1m.

If you can, raise the pad height with compacted clayey fill so that the whole site is free-draining and water drains off and falls away from the building pad.

  • Use spoon drains on the upslope side of a house to direct stormwater around a house.
  • Keep an eye on your plumber. If the plumbing trenches that run under your building are backfilled with sand, these tranches could be a conduit for water under your slab. Get the plumber to observe the rules in AS2870 as specified (very carefully) by your engineer.
  • Keep an eye on your contractors. Don’t allow bog holes to form and not get filled. Fill bog holes with clayey fill. Turn off taps. Connect downpipes asap.
  • Find a way to drain water from under slab setdowns like verandahs and patios. Speak to your engineer about this because this is a major fault with waffle pod slabs.
  • Read the engineer’s plans with respect to site maintenance. Pass this information on to your sub-contractors and then the owners.
  • Ensure pipes are articulated if they need to be.
  • Keep subsurface drains away from buildings.

Waffle Slabs on Piers

Say your waffle slab footing is on piers.

If a house has piers that go down to bedrock things change slightly – but it depends on how the piers were built.

If the piers were poured first and the waffle slab was poured on top of the piers but not connected to them, an increase in soil moisture will cause the waffle slab footing and slab to lift off the piers.

If dirt gets between the piers and the slab then the house can’t go back down when the soil dries out again. Depending on the amount of dirt that gets in between the piers and the slab might stabilise but end up permanently out of level. Bad.

If the piers are connected to the slab and reinforced then the heaving forces will be somewhat resisted by the friction between the piers and the ground – especially if the piers go very deep. The amount of slab movement will be moderated. Good.

Water Around a Waffle Pod Slab

Waffle pod slab footings allow water to penetrate under a house. If water sits next to a waffle slab it is probably draining under a waffle slab. Don’t allow water to pond next to a waffle slab.

It’s just not swell.

13 replies on “Water Around a Waffle Pod Slab”

Hey Mat,
I have a H1 class site with waffle-pod slab (32mpa) on screw-piers (87).
My Slab also consists of a sunken lounge approx. 600mm below ground level. During the construction of the slab i am not sure if the builder had used a sand bedding material under the damp-proof membrane which is shown on my engineer plans. During the course of our building and the extensive rain last year, i argued that drainage needs to be arranged urgently which was not done until only recently. As a result we experienced hydrostatic pressure and water ingress at our sunken lounge. Now the builder has arranged (after remedial reports) a 600mm wide trench that is 500mm deeper than the sunken area on the higher side of the property next to the slab. The trench has a series of 450x450mm pits and 3 large aggylines connecting to a council easement. As part of this repair they intend to impregnate and negative membrane all the external and internal concrete of the sunken area (walls, floors etc) and provide a toping slab. They are also wanting to construct a concrete waterproofed wall between the house slab and drainage trench outside tat is 300mm wide at the same depth of the trench. Do you believe that this is an appropriate fix for this situation? i am very concerned as i have polished concrete floors and hope not to see extensive cracking.
Any advice would be greatly appreciated.
If you are happy to contact me via email that would also be great.

With thousands of homes built each yr on waffle pod slabs if they were failing surely there would have a huge number of complaints about it already since this system has been in use for around 15-20 + yrs in Australia . Surely builders are taking proper precautions during the build .
I know of only one volume builder taken to court and had to rebuild the house .
I have friends in Nz who have their house built on waffle pod slab in 1995 and they have not had a single issue with their house .

Hi Matt,
I have house at its last stage of the build.
Waffle pod slab on a P-H2 site. 60 screw piles and builder didnt use drainage as per AS2870-2011. I complained months ago with no response and water on high side pooling against the edge beam. Heavy rain for months. No slope grading was done (1:50) amongst other issues. Failed to put in any articulation joints and turn up membrane on edge beams.
Soil graded up to top of edge beams and level. No surface v drains in place. Can you contact me on email. Help!

Hi Mat,
My backyard falls towards my waffle slab, so I would like to protect it from water.
It is a new house and the water doesn’t seem to pool around the slab, but I want to ensure this doesn’t happen. My proposed solution is:
(1) outside the bedroom, concrete one metre around the slab to make the water drain away, AND
(2) outside the sliding door is an area of 8x4m where I would like to build some decking. I am not sure if decking will suffice to keep the water away. I could concrete this whole area and then deck on top of it using the plastic pedestals.

Could I please get your opinion.

Great information – thank you! Our build has just been completed and thankfully our waffle slab was poured in summer when it was as dry as a bone. However, we are wondering what is best to do now to prevent water getting in to the slab. We have heard advice that we should concrete around the perimeter of the house. To do this though we have to get a retaining wall made though because our slab is raised above ground level. We have had some quotes done and it seems to be a very costly exercise.. one that we can’t afford to do at the moment. Can you suggest any other options?

Your website is very interesting and informative 😁
Our home is nearly complete. It’s a waffle pod 400mm high on screw piles 1.4m deep. It sits high on our block which is good for drainage.
How far away should we be keeping the ponding? Water is collecting about 1.5m away where the storm water pipe trenches have not been filled and compacted.
Many thanks

Hi Justin
Thanks for your question. So long as storm water can’t drain into the plumbing trenches and backflow under your house (yes that is a thing) then having the ground fall away from the house for the first 1.5m is a good distance. To prevent storm water filling the trenches and flowing back under your house the Australian standard recommends a tamped clay ‘plug’ in the trench. I think that the trenches should also be free-draining away from the dwelling.
Enjoy your new home!
Matt Cornell
Cornell Engineers

My house is built on clay and has sunk at on side were tbe previous owner put concrete up to the footings ,I have changed the roof to tin from concrete to reduce the weight and cut away the concrete I have been told to add water slowly and it should level back out ,here’s hoping the other side of the house is garden bed so I am thinking this is very relevant

Hi Ken
I hope you’re right. If that doesn’t work try opting for a dry regime. Remove the garden, improve drainage away from the building in all sides and allow the ground to dry out all around the house. This could take up to six months so you have to be patient. Best of luck.
Matt Cornell

Thanks so much for the explanation. I am building and my builder will be using waffle pods. Can you please advise what I can do to ensure good water draingage? Thank you

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