Depending on the type of project you’re currently working on, either raft slabs or waffle slabs will be a better choice of footing for your new build. You might be able to use them interchangeably, but researching them properly first will prevent you from making a mistake and choosing the wrong option. Keep reading to learn the benefits and drawbacks, as well as the main differences, between raft and waffle slabs.
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Raft slab vs waffle slabs – what’s the difference?
Firstly, let’s take a look into what each option is and how they differ from one another.
Raft slabs
Raft slabs, otherwise known as engineered slabs, are still widely used across Australia and are favoured over other alternatives. They use thick concrete slabs reinforced with steel beams dug into the ground for extra support and strength.
Key features and characteristics of raft slabs
- Around 100 mm thick with thickened edges that vary from 300 to 700 mm
- The overall depth will depend on site conditions
- Suitable for where an existing dwelling has been removed for a new one
Benefits of raft slabs
- The raft foundation can be used under variegated site conditions where ordinary footings aren’t otherwise suitable, including soft soil
- Can be used where the earth is prone to movement
- These slabs distribute weight over a wider area to reduce the pressure put on the ground underneath
Potential downsides of raft slabs
- More expensive as extra materials are needed
- Can take longer to build
Waffle slabs
Waffle slabs, made from a reinforced concrete footing and slab system, are considered the luxurious alternative to raft slabs. When viewed from underneath, they resemble waffles, which is where they get their name. They use 30% less concrete and 20% less steel than raft slabs, so they’re a great option for many.
Key features and characteristics of waffle slabs
- Around 80 mm thick
- Constructed directly on the ground
- Made from edge beams and selectively spaced internal beams that run each way before the concrete is poured
Benefits of waffle slabs
- Waffle slabs tend to be easier and quicker to construct
- They’re cheaper to build
- Offers enhanced stiffness with reduced weight
- Can be used to build large, unobstructed floor spaces
Potential downsides of waffle slabs
- Not suitable for use on soft ground conditions
- When built on sloping sites the only thing stopping it from sliding is its own weight
Raft slabs vs waffle slabs: key factors in deciding your choice of raft slabs or waffle slabs
There are a few important considerations you should take before choosing which slab choice is best for you, including:
Site conditions
The condition of your site is one of the most important considerations to make. While raft slabs are suitable for almost all ground types because of how much they’re reinforced into the ground, letting them withstand much more movement and weight.
However, waffle slabs are much more finicky and can cause serious problems if used on a site that’s too soft or prone to movement and erosion. This movement can put pressure on the thinner slabs and create space beneath them for wildlife to live. So, waffle slabs are only considered suitable for hard soil conditions with as minimal movement as possible.
Sloped ground
Raft slabs are built into the ground with steel bars reinforcing the concrete into the soil, meaning they’re much sturdier and less inclined to move. While building on a slope is never the optimal condition, this extra reinforcement helps keep your new build’s base where it’s supposed to be to avoid movement.
On the other hand, waffle slabs are built on top of the ground, so there’s nothing reinforcing them into the soil except their own weight. While this doesn’t pose an issue on flat ground, as the house’s weight will keep it where you’ve positioned it, building on a slope can become dangerous and lead to the structure sliding in extreme weather.
Check out our guide on building on a sloping block for more information.
High wind areas
Both raft and waffle slabs should be able to resist the force of high winds and cyclonic conditions, but raft slabs will generally be more effective and safer if the area you’re building in is prone to this extreme weather. The reinforced steel will help anchor the new build into the ground to prevent it from moving.
Waffle slabs should resist the force, but there’s the potential for deflection that comes along with this. As waffle slabs are built on top of the ground instead of being built into it, there’s a possibility that they can lift and move, which can, in turn, cause cracking in your beam walls.
Budget considerations
When building a new home, the budget is one of the most important things that you’ll need to keep an eye on. It can be tempting to go for the cheapest option, but make sure it’s still a viable and safe choice before blindly choosing it! In the same vein, there’s no point choosing the more expensive option when a budget-friendly alternative is right in front of you – this is why research is so important.
Waffle slabs use less materials, so they tend to be cheaper. As long as it’s safe for you to do so and your building certifier and construction team agree, there’s no reason why you can’t opt for this option over raft slabs and move the saved budget to another outstanding job.
Which slab design is right for your home?
Most builders you talk to will still use raft slabs during construction because they’re more versatile and dependable in all kinds of site conditions. They also require less excavation and have a smaller chance of contractor error while pouring, leaving you with a more dependable base for your new build.
However, if you’re building on hard ground with little movement and low wind conditions, waffle slabs could be a viable option. Depending on your individual circumstances, it’s worth talking to your contractor and getting professional advice.
Book your free, impartial consultation with Buildi today for help through your building journey
Still not 100% sure whether you need raft slabs or waffle slabs for your new build? Choosing the right option is essential but ultimately comes down to your site and building plans. Buildi’s team of experts are waiting on hand to give you free advice on anything you may be unsure of, so why not give us a call on 1300 947 132 or get in touch through our Contact Form today for free, impartial advice?
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