We build slabs and foundations for high-volume builders. High-volume builders need consistency in cost and delivery. For this reason, in most cases, we recommend a Waffle Slab or (as it is commonly referred to in NZ) a Ribraft Slab as our foundation of choice. Over the years, we’ve learnt ribraft foundations are the most cost-effective slab system in most situations.

However, when it comes to slab building there is not one size or type to fit all scenarios. There are plenty of situations where the humble Waffle Slab or Ribraft Foundation are not the right solutions for your project. Tidy Slabs may not be the right contractor for you either.

We want to help you anyway, even if we are not building your slab. Here’s some useful information for you to ensure you’re making the right decisions…

This article is going to answer the following questions

• What is a Waffle-Slab/Ribraft-Foundation? And what other names does it go by?
• What are the benefits of a Waffle Slab?
• When don’t waffle slabs work?
• What are my other options?

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Spoiler alert - The Waffle Slab does not win every time.

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What is a Waffle Slab/Ribraft Foundation? And what other names does it go by?

A Waffle Slab is a term used to describe several different proprietary slab systems used in residential and light commercial construction in New Zealand and Australia. 

One of the reasons it is called a Waffle Slab is because if you turn the slab upside down after it is poured and remove the pods - what you would see would resemble a concrete waffle - fresh out of the waffle press.

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Another term for this type of slab is a Raft Slab this means that it “floats” on top of the ground like a raft and is not keyed into it.

To keep us all confused, there are other terms for Waffle and Raft slabs e.g. Raft slab, Raft foundation, Ribraft Foundations, Ribraft Slabs, Superslab, Pod Floor, Poly pod floor, Qpod, Cupolex or plastic pod floor, to name a few. They all mean the same thing…

We will call them all Waffle Slabs for this article’s purposes.

The Waffle slab is the most common slab system specified by volume builders because they are the simplest to build due to a handful of key features

1. They are standard uniformed depth, usually 305mm thick and specific formwork is available at this dimension.
2. They have void formers at defined centres within the slab that help create a perimeter edge beam and reduce the volume of concrete used.
3. The ground can easily be prepared by the earthworks team 
4. A reusable bracing and boxing system can be used.
5. The “ribs” created between the pods give the slab additional stiffness which comes in handy for softer ground.
6. In most situations, they are faster and cheaper.

The slab systems have two clear categories but ultimately are used for the same reasons.

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Category #1 - Polystyrene pod system

• Firth’s Ribraft
• Allied’s Super Slab
• Maxraft Foundation

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Category #2 - Plastic Pod system

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• Firth’s Ribraft X-Pod
• Biax500
• Cupolex Ecodome
• Qpod

Most of the systems above have received their own MBIE Codemark.This means that if the ground conditions permit (see article) and the structure above is within the 3604 building code, then the proprietary slab system can be used without the need for a Structural engineer and a PS1.

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However if the ground conditions or the structure above do not permit, then it will be over to your Structural engineer to design the slab. This is called a Specific Engineered Design Foundation (or a “SED foundation”). In most cases, a Waffle slab will still be specified as the most efficient. The structural engineer will likely be following the design guidelines of one of the systems above, but they’ll use their calculations and adjustments to suit the specific situations.

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What are the benefits of a Waffle Slab?

Waffle Slab foundations are the most popular for a few reasons, below are the benefits:

• Simple Earthworks - These are simple because all that needs to happen is the topsoil is removed and a minimum 100mm layer of AP40 base course is compacted to the underside of the slab.  There is no footing excavation needed and the slab. The contractor and earthworks contractor don’t need to be on-site at the same time so it’s easier for all.

• A simple system to construct - Regardless of the actual Waffle Slab system, most of the tasks are identical and repeatable across all systems. The setout is the same, the formwork system is the same, the blinding sand and polythene are the same, and the reinforcing and the mesh are the same. These are simple and repeatable steps and so a standardised construction sequence is created that results in faster (and cheaper) build times.

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• You can build it in any weather condition (if you want) - Because there are no trenches to excavate and the ground is prepared with a nice compacted basecourse, it is possible to work in the rain because you don’t have footings that could fill with water and your boots heavy with mud. Depending on whether your slab contractor is made of sugar or not, you can build these slabs in the rain (our guys have great Swazi rain jackets). 
• Easy take-offs and pricing - Because the system does not have any footings, all of the material and labour constants have a high level of accuracy. This means it is easy to do a take-off and provide a quote. Standard rate sheets can be created and material contingency can be kept to a minimum. Quicker quotes and cheaper prices.
• A Raft Slab foundation can span soft and liquefiable grounds - The ribs within the void formers create a stiffer slab. This allows the slab to be built on softer ground conditions than a 3604 foundation without requiring any piles. A Codemarked Ribraft slab can be built on 200kpa and even lower with an SED design. When the ground is soft or has liquefaction risk, the Ribraft Xpod and Biax500 foundations start to come into their own. They save you on earthworks and piles and are easy to build.

When don’t Waffle Slabs work?

Although Waffle Slabs are our bread and butter and we swear by them all day long, there will be times when they are the wrong system for your project. Here are a few examples of when to consider something else.

• Small Slabs less than 60m2 - If your slab is less than 60m2 and the ground conditions are at the 3604 standard. We often find that there is no point specifying a Waffle Slab because it will not end up any cheaper for you. For these smaller slabs, it will just add in extra material (the void formers) that won’t give your small foundation any further value. In this case, we suggest sticking with a conventional 3604 foundation with a 300mmx300mm footing excavated around the perimeter and a 100mm topping slab. It will be cheaper 9 times out of 10.
• Slabs with floor levels raised greater than 500mm from the current ground level - If your slab has a floor level that must be greater than 500mm off the ground, due to it being in a flood zone, a waffle slab may not be right for you. It all depends on your liquefaction risk whether you can have a waffle slab or not. We have seen slabs designed that have a footing and block ring foundation for the entire perimeter of the slab and then a Waffle slab specified within but this is a costly approach.  In this situation, the best thing you can do is go for a 100mm topping slab within the block ring foundation. It will be cheaper and faster. Or even better - go for a Timber Subfloor.
• Slabs that have multiple floor levels stepping up or down a hill - When this happens you will have several slabs to pour across several levels,  blockwalls form most of the perimeter of the slab acting as the boxing. When this happens the best thing you can do (in most cases) is backfill behind the blockwalls to 100mm below the FFL and just specify a 100mm topping slab with Se62 mesh. It’s better to keep the pods out of it and keep it simple. Your slab will go down faster and be cheaper. Most of the benefit of a waffle slab is actually in the formwork step.
• Slabs with deepened edge beams - We love 305mm deep Waffle Slabs, but when a structural engineer specifies a waffle slab and then also puts a deepened edge beam in, we end up losing a lot of the benefits of the system! Remember, the benefits are in the standard formwork and consistent material/labour constants. In these slabs, we can’t benefit from either in this situation. More labour, materials and contingency get built into the price and ultimately more cost for the homeowner. There are many reasons why an engineer might do this, but we always believe that if we can make the perimeter footing wider instead of deeper, then this is a win.
• When the ground conditions are TC2/TC3 and you are in a flood zone - If you have highly liquefiable ground and you are in a flood zone, you are required to meet a certain floor level above the current ground. A standard Waffle Slab may not be right as the cost will likely be enormous. You could look at the larger Biax500 and stick with the Waffle Slab, or you may just go with a Timber Subfloor instead.
• Timber-driven piles are required…sometimes - It is pretty common for engineers to specify timber piles under a Waffle Slab when the ground bearing is not good enough for a standard waffle slab design.  However, many times this will add more complexity to the slab-building process. It may be worth considering other systems, like a timer subfloor. It is another subtrade to bring in and depending on the detail of how the pile connects to the slab it could add a lot of extra work. There are Waffle slabs that can be designed on lower bearing capacities, as well as the option to build a gravel raft under the slab, both will remove the need for piles. Sometimes the cheapest thing to do is consider something other than a Waffle Slab. This is one of those times.

What are my other foundation options?

A 3604 “conventional” foundation.

• Consider using when:
  • 3604 “Good Ground”
  • Small Slabs ( under 60m2)
  • You have your own digger

Ring foundation

• Consider using when:
  • 3604 “Good Ground” is found at footing depth.
  • Or when a SED foundation requires a Floor level greater than 500mm above the ground level.
  • Or when there is a multi-level stepped foundation on a hill site.

Timber Subfloor

• Consider using when:
  • The floor level is required to be greater than 500mm off the ground and flood planes require a flow path under the foundation.
  • You are not in a Moderate / Highly Liquefiable zone.
  • There is a slope on the site
  • You have to install piles into good bearings anyway.
  • Caveat: It will be more expensive than a Waffle slab in most instances and it will not perform as well thermally - but it may be your only option.
    

SED conventional slab

• Consider using when:
  • The structure above is a more than 3-storey timber-clad residential dwelling and requires significant reinforcing for the point loads.
  • The site requires deepened edge beams for landscaping or additional point loads.
  • TC3 ground conditions.

Fibre-reinforced concrete slab

• Consider using when:
  • When the foundation requires additional reinforcing steel in the slab due to the load above or TC2 ground you can look at substituting the stirrups and mesh for fibre-reinforced concrete.
  • There are times that a 200mm thick fibre-reinforced concrete slab may work out more cost-effective than a 305mm thick Waffle Slab.
  • Check with your slab contractor for options.

Whatever we call them, we swear by Waffle Slabs, Ribrafts, Super Slabs, Pod floors, poly pods floors and plastic pod floors. They are easiest to quantify during the planning stage, the easiest to build, and conform to a standardised build process - we can do them faster and cheaper.

It is likely in “most cases” to be the best solution but not “all cases”. When it's time for a client to consider a better solution for their project, we won’t stand in the way. Our goal is to get our clients the most efficient solution because too often we see people building inefficient foundations. We are agnostic to the actual system itself, but we are hell-bent on getting the best result for you!

Good luck with your project!! 

While we hope this best slab for your site is a waffle slab, we are happy to provide advice either way. 

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