What’s the Difference Between Waffle and Raft Slabs?

Depending on your property’s conditions, both waffle and raft slabs can provide a suitable footing for your new home. However more often than not, one type of slab will suit your project better.

Before you make the important decision of whether to build on a waffle or a raft slab, you should understand a little about each style, including their advantages, disadvantages and major points of difference. After all, the future of your home rests on this decision!

WAFFLE SLABS

If you’re not a builder, you’ve probably never heard of a waffle slab until you began thinking about creating your dream home. So, what exactly is a waffle slab? 

A waffle slab is a reinforced concrete footing and slab system, usually around 80mm thick, that is constructed directly on the ground. Waffle slabs consist of an edge beam (perimeter footing) and selectively spaced internal beams that run each way. Polystyrene is used in between the beams to form voids where concrete is poured. This process creates a ‘waffle’ like aesthetic when viewed from underneath, hence the name.

While waffle slabs tend to be faster, easier to construct and cheaper, there are a few things you need to keep in mind before you choose this slab style:

Site Conditions

If the earth beneath your slab is soft, it can cause serious problems for your home – especially if the ground is prone to movement or erosion. This movement can put pressure underneath the slab where it’s no longer touching the ground and also become a haven for unwanted reptiles and wildlife seeking shelter.

This means you’ll need to pay careful attention to your property during and after the construction process. For example, when watering the garden, you need to take care not to overwater and ensure proper drainage is available so excess water doesn’t filter in below the slab and wash away the earth.

Sloping Sites

This is fairly straightforward, a waffle slab is built on the ground, meaning its own weight is the only thing stopping it from sliding.

Cyclonic/High Wind Areas

A waffle slab should resist the force but not without potential deflection. The nature of the slab’s construction means there is potential for lift and movement, which can cause cracking in the beam walls.

RAFT SLABS

A raft slab (or engineered slab) tends to be the favoured option for most builders including us here at Ramsay Builders. Raft slabs consist of a thick steel reinforced slab that is integrated with steel reinforced beams, which are dug into the ground for added strength and support. 

The depth and spacing of the edge and stiffening beams is dictated by the ground it is being constructed on. For more info on this see our blog about site surveys and soil reports.

Raft slabs are approximately 100mm thick with thickened edges that can vary from 300-700mm. Things like water can cause these slabs to shrink when dry and swell when wet, meaning they require much deeper stiffening and edge beams to counter less predictable site conditions. This means sandy soils don’t require as much stiffening or shallow edge beams, whilst clay soils are much more reactive.

This slab is more suitable where an existing dwelling and vegetation has been removed and to allow for the new dwelling. It’s also more suited to sloping blocks as the edge beams are dug into the ground and working the slab into the soil and slope.

WHICH IS BETTER? 

In a nutshell, your choice of slab will come down to your unique site conditions, your goals for your home and, of course, your builder. In saying this, raft slabs are generally the preferred method as they require less excavation and mean a lower chance of contractor error whilst pouring. 

It is essential to the get the foundation of your new home right, which is why Ramsay Builders includes extended warranties on our new homes, to make sure we get this crucial stage correct.

To find about more about raft and waffle slabs or to discover what’s next from here, get in touch with the team at Ramsay Builders today.