Photo of a waffle slab before being poured with concrete

Beware Waffle Slabs

Let’s face it. If you even know what waffle slabs are, you’re either building one or having one built. What are waffle slabs? Are they a great idea? Are waffle slabs the best bang for your buck? I’ll answer these questions and finish up with why, after 20 years of structural engineering, I have never, ever designed one waffle slab.

What is a Waffle Slab?

Waffle slabs are a reinforced concrete footing and slab system constructed on ground. They consist of a perimeter footing (edge beam) and a series of narrow internal beams (strip footings) at one metre nominal centres running each way. The whole footing and slab system is constructed on top of the ground.

Edge formwork makes the sides of the slab and polystyrene ‘pods’ create the formed voids between the strip footings. When viewed from underneath, the system of internal strip footings looks like a waffle – hence the name.

Upon completion of the waffle slab house, the ground around the slab is built up by the builder to reduce the height of the slab above the surrounding ground.

See this blog for a great series of photos of a Waffle Slab being built.

Here’s another video of a waffle slab ready for concrete:

Waffle slabs achieve their strength by varying their height above ground. The higher the slab above ground – the deeper the beams. The deeper the beams – the more stiffness the system has.

In Australia, waffle slabs are designed to comply with AS2870 – The Australian Standard for Residential Slabs and Footings. The current version of the standard is AS2870-2011.

A Place for Waffle Slabs

There is definitely a place for waffle slabs in the construction world. Waffle slabs work really well on sites that are almost flat,  natural soils or controlled fill, that have good surface strength and where the natural ground surface falls away from the outsides of the building in all directions. They work well on non-reactive sites, slightly reactive clay sites and some moderately reactive clay sites.

Waffle slabs are not recommended on highly reactive clay sites (Class H1 and H2) because the requirements for good drainage are almost impossible to achieve.

There is no ‘deemed to comply’ design for waffle slabs for extremely reactive sites (Class E sites). In fact there is no deemed to comply raft footing design for Class E sites either – the footing and slab system for a Class E site must be designed by a relevantly qualified engineer.

Are Waffle Slabs a Great Idea?

Are waffle slabs a great idea? A definite maybe. These are the sites where waffle slabs won’t work so well:

  • Soft ground conditions. Extra bored piers or screw piers are required so that the system is supported on strong ground.
  • Sloping sites. Waffle slabs are built on flat sites. On sloping blocks, the ground has to be made level first by digging some of it out or filling some of it in. Problems arise when some of the dirt dug out is used as uncontrolled fill on the low side of the block. All houses, even waffle slabs, need firm, even support to all parts of the slab.
  • Highly reactive and extremely reactive clay sites. These sites need stiff footing systems to span over the swelling and shrinking soils. Concrete beams get stronger and stiffer when the depth of the concrete beams increases but waffle pod void formers tend to max out at 375mm deep (so providing 475mm deep beams and ribs with a 100mm slab).  Some designers try to achieve extra slab stiffness by adding more steel reinforcement. This works but the design process becomes more complicated.
  • Cyclonic areas and high wind areas. High winds generate a lot of pressure on roofs which result in some very concentrated forces in modern trussed roofs. On a regular sized 200m2 house, some truss uplift forces are as high as 5 tonnes. In a waffle slab, these forces need to be resisted only by the weight of the footing system because there is no skin friction with the ground. 5 tonnes of concrete is about 2 cubic metres of concrete. That’s a lot more concrete than is available to resist forces at the truss reaction point. The consequence? High wind forces will lift enough of the waffle slab to resist the force but this comes with deflection of the slab. In a cyclone – be prepared for your slab to lift and your walls to crack.

Are Waffle Slabs Best Bang for your Buck?

On the correct site with correct preparation and in non-cyclonic areas. Yes. They are much faster for the concreter. They are much easier for the builder. AS2870 even permits thinner slabs (85mm thick compared to 100mm thick conventional slab thickness). Therefore they should be cheaper for you the consumer. If your builder charges you more for one of these cost-cutting waffle slab systems, then you are getting cheated.

Why I don’t like Waffle Slabs

I have never, ever designed a waffle slab – but I know plenty about them. I have inspected lots of cracked houses and some of them are waffle slabs.

Conventional raft footing houses can crack too, so why don’t I like waffle slabs?

  1. The soil brought in as fill around a completed waffle slab house must not be porous. Porous soil allows water to seep below the surface and access the ground under a waffle slab. This results in slab heave. Unfortunately, builders prefer porous fill because it is easier to spread and doesn’t need to be compacted. This is my number one reason that I advocate against waffle slabs on reactive clay sites.
  2. The pipe trenches that run from under the waffle slab must be graded so that any water in the trenches runs away from the house.  This is rarely done.
  3. If the ground around your house gets eroded or washed away, you can see under a waffle slab. If you can see under the slab, so can vermin, toads, snakes and the population of wildlife seeking shelter under your floor. Yuck.
  4. The polystyrene void formers are made of polystyrene. Der. What is polystyrene? It is a is a synthetic aromatic polymer made from the monomer styrene, a liquid petrochemical. Sounds exactly like something I don’t want stockpiled under my slab.
  5. Waffle slabs don’t work in cyclonic areas. There simply isn’t enough weight in the slab and footings to resist the high forces.
  6. Ground preparation must be immaculate. It must be immaculate on day one and must be kept immaculate for ever by the homeowner. You must not overwater the ground near your house, build up the ground around your house, allow surface water to run towards your house (and under your house) or plant gardens next to your house. Does this even sound possible?

Join Me For An Inspection of A Waffle Slab

Just recently I inspected a waffle slab before it was poured. Come and check out my waffle slab inspection video.

More About Surface Drainage

The number of times I have asked the owner of a cracked house, “Where does the water go when it rains?” and they have told me. “It just goes! It just disappears!”

I have bad news for you. Water does not just ‘disappear’ into reactive clays. Reactive clays are the same clay soils used to line dams to make them impermeable. If the water around your house is “just disappearing” then it is probably “just disappearing – under your house.”

Now consider if the builder has used sand or gravel to build up the ground around your house. The same thing is happening. The water is disappearing – draining through the porous soil and potentially draining under your house. Enter slab heave.

Even if your builder has used moist clay as fill around your waffle slab house, by pouring perimeter concrete paths on a sand bed, you are providing a tunnel for water to run down your walls and sit under your waffle slab. When you build a garden against your house, the formed concrete garden edging forms a water barrier and allows rainwater to drain down against the wall.

Filling the gap between a concrete path and the walls of your house with flexible sealant is one excellent way of stopping surface water draining under your house, but the sand layer under the slab needs to be managed too.

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More Reading

It’s not just me that has an opinion on waffle waffle slabs. Check these links out for some more information on waffle slabs.

Waffle Slab Problems

HomeOne Posts about Waffle Slabs

Slater & Gordon calls for taskforce to act on faulty house slabs

HomeOne Forum – House Problems

Builder Forced to Fix Cracked House

Softley v Metricon Homes Pty Ltd

Home Owners Compensated for Faulty Slab

42 thoughts on “Beware Waffle Slabs”

  1. Hi Matt,

    Unfortunately I have just come across your post on waffle slabs as we are just nearing completion of our build using a waffle slab on a H1 site, so there is no point getting into whether that was the right option for us, I will just have to hope for the best there.
    We have been made aware that we should aim to have a 900mm concrete apron around the house perimeter to avoid issues with drainage/movement.
    Of course this restricts our options in terms of a garden, so we are considering placing a raised garden bed on top of this concrete apron.
    As long as we ensure there is proper sealing between the apron and wall of the house, and sufficient drainage from the raised garden bed away from the house, do you forsee any issues with this?

    1. Hi Heather
      I understand that you are trying to soften the look of all that concrete around your house. A raised garden bed might be the answer but these are the things I think you should be careful of:
      1. The garden bed should be positioned should be away from the walls of the house so that you do not compromise the termite barrier. I think you should maintain a visual barrier even if you have a chemical termite barrier installed.
      2. Having a garden bed near the house will make you want to water it; and you should be aiming to keep water away from the house as much as possible. Even if water drains from the garden bed onto the apron slab there is still the chance that water can run through to the foundation soils between the apron and the house or even under the apron slab.

      These are your alternatives:
      1. A wall garden. http://www.countryliving.com/gardening/garden-ideas/how-to/g1274/how-to-plant-a-vertical-garden/ Preferably on the fence – never on the wall of your house.
      2. Having the garden off and away from the house slab.
      3. Make the most of your back yard. http://www.thesmallgarden.com.au/

      Hope this helps.

      Matt Cornell

      1. Not only a apron or paving slab but the natural ground around which would be the clay should fall away from the waffle slab, on a sloping site towards the house the apron wont do anything water will still pond at the edge of slab or worse travel under, thats why really they should install AGI pipes along the edge to help get rid of water.

  2. Hello, Matt.
    Very interesting opinion. But could you name few examples, where waffle slabs were used successfully or not?
    Looking forward to your reply.

  3. After reading all the material presented simply would not take a chance on a waffle pod slab. To potentially save a few $’s the risk is not worth the unknown reward.

    Thanks for posting this valuable information for us own builders to use.

    Thks & Reg
    Frank

  4. Hi Mat, we are building a house on class H2 soil our builder wants to use a engineered waffle pod design & informs us waffle pods were original designed for highly reactive soil this is completely opposite of what you have written. Look forward to your comment, Thank you Gary Phillips

    1. Hi Gary
      Your builder and soil tester and footing engineer are all equally keen to provide you with a house that will comply with current codes and perform within expectation. It is my opinion that there is an increased risk of movement and damage in a waffle slab house built on highly reactive clays because you, as a homeowner, play such a significant role in maintaining the soil moisture conditions under and around your house for the life of the building. It’s not that any homeowner deliberately sets out to damage their home; however the limitations placed on tree planting, garden watering, site drainage and stormwater dissipation that are clearly set out in the soil tester’s and engineer’s documents are soon forgotten either by the first homeowner or not passed on to subsequent homeowners. These requirements aren’t actually any different for waffle slabs compared to raft slabs – it’s just that is harder to keep water out from under a waffle slab compared to a raft slab. The fact that your are investigating these issues now means your house, while you occupy it, will have a better chance of performing. Best of luck with your new home. Enjoy the process.
      Matt Cornell

      1. HI Matt im a structural Drafter in Melbourne, where i work we rarely use waffle unless its really wanted by the client, mostly we use it as a void former. You are right, this information actually is NEver passed on to the home owner, by the builder, most builder dont even know that a waffle site has to be cut and drained properly in the first place, Matt the best way i explain to people is that the whole slab except the edge beams is sitting on top of the ground or clay, so water can pond underneath on the clay with nowhere to go.

  5. Thank you, a very straightforward explanation as to why waffle slabs are not appropriate in areas of highly reactive soils. I live in Brunswick Victoria and am researching the type of foundations I need for a stable, long lasting construction that can be enjoyed for years and years. Thank you for putting up post.

    1. Clare, Brunswick would be a Class H site, meaning Highly Reactive, also existing trees play a role in the footing design if they are quite large. Go for a Raft slab.

  6. hi mat my name is brian like nic also english and building at cowes also on a waffle slab with approx 800mm of fall and seem to be buildig with the same company and have similar concerns i would be interested to see the piering detail of nics slab and how the building is progressing if possible would you pass my contact details to nic thanks

  7. Hi,

    Does this waffle slab much applicable to tropical countries? or are they better suited to cold countries.. i’m an architectural drafter and my boss asked me this cause some of our clients requires waffle pods.. and i don’t know the answer.

    Cheers.
    Fat

  8. I’m certainly no fan of waffle pod slabs.
    Not convinced on the benefits.

    the insulation values seem well over rated.

    Concrete amount I see little saving as most seem to do 100mm top.

    However your point on Expanded polystyrene foam (EPS) is toxic and should be avoided under a slab?

    EPS or XPS is the most common material used in North America and Europe for under slab insulation.

    I would be way more concern about the chemicals used for pre slab treatment of termites.

    I’m in hydronic heating and waffle pods are just so average for inslab heating.

    The main reason builders use it in the ACT regional area,saves excavations, and no builder here build the soil up.

    So 400mm from the ground to the inside floor height, is never a good look, most clients are not happy.

    Yep a raft and beam slab i prefer.

    Cheers

    Dave

    1. Hi Dave
      You make some interesting points about the comparison between waffle slabs and raft slabs particularly in relation to insulation and in-slab heating. In-slab heating is not so common in Queensland and one I really hadn’t thought about it very much. I guess it’s another point for cold climate builders (and owners) to consider when choosing between waffle slabs and raft slabs. Do you find your in-slab systems work better with a thicker (than 100mm) concrete slab instead?
      You might be interested in this post about EPS:
      https://www2.buildinggreen.com/blogs/avoid-polystyrene-insulation
      Regards
      Matt Cornell
      Cornell Engineers

      1. Hi Matt

        Its good to see an engineer who is interested in the bigger picture.

        I have read that article previously, and I think for walls its got merit.

        EPS is not good environmentally but its a lot of bang for buck; at around 30kg per m3 compared to concrete 2400kg, good R values.
        In slab heating without insulation is crazy.

        To install in slab heating (yes I know Qld not required.) although you could use it for cooling (off topic).

        The slab thickness and the effectiveness of hydronics not really the issue its more that 85mm concrete, with two layers of mesh and pipes not enough cover. However even without in slab heating most builders seem to do 100mm cover for waffles down this way.

        I have emailed all major makers of pods requesting further information regarding the R value for waffle pods,none responded.

        I get told waffle save concrete; I’m not so sure, standard waffle its around: times the area by 0.165.
        Add all the broken pods (many) and the concrete increases.
        However concrete is cheap looking at a build; environmentally not great.

        I saw a bloke jack his car up in the garage, slab on waffle pods and yes the whole slab cracked badly all around the jack.

        Yep no fan of waffle pods even more so now you tell me that structurally they have issues.

        Cheers

        Dave

  9. Hi Matt,

    My name is Nick and I’m about to build a house in Cowes, Phillip Island.

    My land elevates at a reasonable angle up from the front pavement at 50 degrees (approx.) and then levels off after 7-8M (Measured from the front to where it levels off). the land (A rectangular plot of 18M W x 47M Long) then falls from left to right and the fall on the RHS is debatable, but the engineers have said it’s about 1.4M to the front garage corner. We have chosen a Porter Davis design, 29 Square home through another, licenced building co. They have come back with re-engineered plans and a waffle slab. Our land is Moderate but I’m not 100% sure of what it is made up of. I don’t believe there’s any fill on our land at all.

    I’m querying waffle slabs from the minute I Googled them and saw your website / blogs!!

    I’m from the UK and have only been here recently. I’ve already heard about Metricon and their failings and they used a waffle slab. Plus, the extra cost has gone up alot with this re-engineering.

    My questions:

    1. What are my other options? Can I refuse this type of slab?
    2. It’s around $15K for a 29 Squ house to have this. Is that fair?
    3. I’d rather look at the standard way of having a slab with rio and concrete poured in. Is this possible?
    4. As you’re completely independent from all of this, could you have a quick squizz at our plans just to double check everything?

    I’m very concerned as this will be our new, long term family house.

    Please can you advise?

    Let me know if you need any other information?

    Cheers

    Nick

    1. Hi Nick
      Thanks for your questions.
      The waffle slab option will rely on the builder’s ability to generate a flat building pad on your land. This isn’t out of the question, even with the topography you’re describing however perhaps some of the cost you have quoted includes creating the building pad? A flat pad is certainly more usable later when it comes to lawn mowing and maintenance but will result in some steeper sections and/or retaining walls.
      Your other options are a timber floor on steel or timber posts or a concrete slab with (concrete masonry) retaining walls on 1 or 2 sides of the house.
      As fas as pricing goes, we rarely see cost breakdowns for house slabs so I really can’t comment.
      A structural engineer with local knowledge of Phillip Island will be a better choice for an independent review of your house design. There are too many local issues that we wouldn’t be aware of.
      Good luck with your house build. It’s a beautiful part of the world there.
      Regards
      Matt Cornell
      Cornell Engineers

      1. Hi Matt, thanks very much for your reply.
        I’ve since spoken t the SE who’s local and has been doing soil tests and ER’s since the late 70’s, so very experienced and she’s also done over 70% of the ER’s in the estate I’ve chosen.
        So it was good to know this.
        I didn’t prior to writing to you but I did also voice my concerns based on your reply and she said the most important thing to do is drainage all around the property to get rid of SW properly.
        This is where Metricon came undone and originally freaked me out when reading about waffle slabs, but I’ve since related this all to the building co. and they can’t take the risk of not supplying anything the ER says. So, all’s good there. The soil is moderate and is “rocky” underneath so the SE said that this is good as water can’t get stuck underneath as the rocks act like sand and it passes through. Plus the footings / foundations are good and solid.

        So, all in all, I’m much happier than earlier this week and thanks again for your reply. I’m a plumber / hydronic heating engineer so I’m very lucky to be able to take my job anywhere, especially Phillip Island!!

        Cheers

        1. Hi nick, intersted in hydronic slab heating through the wood heater in a 40sq house we designing at moment just near phillp island. Can u please guid ed us to what would suit our requirement
          Cheers shane

    1. Hi Aya Elizz

      There are different types of waffle slabs. The main difference between them is the material used to create the voids (gaps) under the slab. The most common void former is boxes of white polystyrene. You might have seen photos of these on our website. The alternatives to polystyrene are cardboard boxes and polypropylene (http://www.cupolex.com.au/).

      Is this what you meant?

      Regards
      Matt Cornell

  10. I live in Cairnlea in the western suburbs of Melbourne and have a serious ceiling crack with cornices separating from the wall. I have not got any concrete around the majority of the house. I have had someone check the framework in the cieling and it is fine. What sould I do? Put aggi pipe around the house? Concrete around the house?

  11. Check out Wafflemat at texaswafflemat.com – thousands of houses in expansive soils without a single failure in the USA.

    1. Hi Peter

      I don’t understand the difference between the waffle mat and polystyrene void formers. Surely you don’t think polystyrene wouldn’t compress if soil tried to expand up into the ‘void’? Could you explain please?

      Matt Cornell

      1. Matt,
        Visit the American Foundation Performance site at:
        http://www.foundationperformance.org/projects/FPA-SC-11-1.pdf
        and check out Page 9 under ‘Styrofoam Void Forms’ where ‘solid blocks of Styrofoam does not form a void space. Void Space is purposely designed void volume used to create a buffer zone or clearance between Expansive Soil and a Foundation, that allows Heave to occur without imposing detrimental uplift pressures on the Foundation.
        Various types of Void Form materials are described in the article including Styrofoam Void Forms.
        The Waffle pod System in Australia has various designs to save on the amount of EPS used in manufacture of the blocks but the cavities are generally small and close together and expanding soil bridges the gap and little enters the cavities.
        Compression of polystyrene ranges from 200 – 700 kPa according to the Doe Chemical web site so the weight of a house is unlikely to compress a solid or cavity constructed block of EPS used in waffle slab; slab heave will occur as soil expands and the EPS will work with the concrete footings to support the center of a slab as drying soil shrinks (unevenly) at the perimeter of a house potentially leaving gaps between the concrete and soil. Cracking and twisting occurs in the slab as rebar takes up the tension caused by heave.
        In the USA, they have gone down the post tensioning path where the tendons are stressed a day or so after the concrete is poured thereby preventing open cracks in uniform thickness slabs or void formed slabs.
        Wafflemat is a recycled polypropylene void former system used in conjunction with post-tensioning for expansive soil slab construction as explained in the web site above.
        You posted”I don’t understand the difference between the waffle mat and polystyrene void formers.”
        I hope this clarifies the difference for you…a totally different ‘waffle slab’ system that is being introduced into Australia with an engineering team currently working on a revision of a software program to comply with AS2870-2011.

        Regards,

        Peter Munt

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