Let’s face it. If you even know what a waffle slab is, 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 28 years of full-time 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 the 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.
Because the waffle slab is constructed on the ground, when the waffle slab is finished it sits about 400mm higher than the surrounding ground.
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.
Here’s another video of a waffle slab ready for concrete:
Waffle slabs achieve their strength by varying the height of the slab above ground. The higher the slab above ground – the deeper the beams. The deeper the beams – the more stiffness the system has. The more stiffness the slab has, the more it can resist ground movement.
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.
In most states, footing and slab systems can only be designed and specified by professional engineers. In Queensland, your engineer has to be an RPEQ (registered professional engineer in Queensland).
A Place for Waffle Slabs
There is a place for waffle slabs in the construction world.
Waffle slabs work well on almost flat sites, 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 (ie when the building pad has been built up with solid, water-shedding dirt).
Waffle pods 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). 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. Not just around the perimeter though – all through the middle of the house too.
Waffle slabs are meant to be 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 (fill that isn;t properly compacted) 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.
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. Experienced structural engineers know that steel reinforcement plays a very minor role in achieving increased concrete beam stiffness. The best way to increase the stiffness of a concrete beam or slab is to increase the depth of the concrete beams or slab.
Cyclonic areas and high wind areas
High winds generate a lot of pressure on roofs.
This causes some very concentrated uplift forces in modern trussed roofs.
On a regular-sized 200m2 house, some truss uplift forces are as high as 5 tonnes (50kN). Some are even higher!
In a waffle slab, these forces need to be resisted only by the weight of the footing system because the waffle pod is sitting on top of the soil.
5 tonnes of concrete is about 2 cubic metres of concrete.
That’s a lot more concrete than is available to resist the large concentrated uplift forces from some heavily loaded trusses.
High wind forces will lift and deflect enough of the waffle slab to resist the force.
In a cyclone – be prepared for your slab to lift and your walls to crack. Will you be covered for this damage?
But your insurer may well want to have a little talk to you, your builder and your footing and slab engineer about sharing some of the repair costs!
Some waffle pod engineers will tell you they are just designing the waffle pod and thy haven’t taken into account wind loads!
Are you kidding??
Make sure you ask your engineer if they have designed your waffle pod slab for the roof truss uplift reactions.
Are Waffle Slabs Best Bang for your Buck?
On the correct site with correct preparation and in non-cyclonic areas. Yes.
Waffle slabs are much faster for the concreter. They are much easier for the builder to build.
AS2870 even permits thinner slabs (85mm thick compared to 100mm thick conventional slab thickness).
Therefore waffle slabs 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?
- 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.
- 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.
- The pipe trenches that run under the waffle slab should have a clay plug to prevent stormwater from running under the house. Plumbers just don’t do this often enough.
- 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.
- The polystyrene void formers are made of polystyrene. Der. What is polystyrene? It is a synthetic aromatic polymer made from the monomer styrene, a liquid petrochemical. Sounds exactly like something I don’t want stockpiling under my slab.
- Waffle slabs don’t work in cyclonic areas. There simply isn’t enough weight in the slab and footings to resist the high forces. Furthermore, waffle pod engineers try to design the waffle pod slab without considering the uplift loads from the roof framing. These engineers become easy targets for litigation after a cyclone (note: designing a waffle pod slab with no consideration for cyclone loads is like designing a bicycle tyre and putting it on an aeroplane – it will work right up until you put some load on it).
- Ground preparation must be immaculate. It must be immaculate on day one and must be kept immaculate forever 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
Houses often crack because of uneven soil moisture conditions.
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 clay.
Reactive clays are the same clayey 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 after the house is finished.
The same thing is happening. The water is disappearing – draining through the porous sand and gravel 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 directs rainwater towards your house and down under your slab.
When the plumber backfills the trenches that run under your house with sand or pea gravel – that trench is also a conduit for water.
Do you see how many ways there are for water to enter the ground under your waffle slab??
And just try asking your plumber to backfill trenches with moist, tamped clay! If you’re lucky the plumber will just laugh at you.
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 and sand-filled trenches under your slab need to be managed too.
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It’s not just me that has an opinion on waffle slabs. Check these links out for some more information on waffle slabs.