Waffle Pod Slabs Vs Raft Slabs

There are two common ways to build a slab for a house: the raft slab and the waffle pod slab.

The Raft Slab:

With an interesting name, it can make you think that the slab floats like a raft. In fact, a lot of builders refer to this type of slab as a floating slab. This slab is usually 100mm thick with thickened edges varying between 300mm and up to 700mm deep. Throughout the slab there are also stiffening beams which are 300mm wide and vary in depth up to 700mm. These beams are spaced at a maximum of 6000mm to a minimum of 4000mm.

The depth and spacing of the edge and stiffening beams is dictated by the type of soil the slab will be sitting on. Sandy soils require no stiffening beams or shallow edge beams. Clay soils require deeper edge beams and stiffening beams. The more reactive the clay (that is the more it shrinks when it’s dry and swells when it’s wet), the deeper the edge beams and the spacing of the stiffening beams.

raft slabPros:

  • Most concreters are experienced with this type of slab
  • Uses slightly less concrete than a waffle pod slab, sometimes more labour and reinforcement. 

Cons:

  • Can end up with too many stiffening beams on homes with many corners, increasing cost
  • Low energy rating
  • Very important to have the whole site drained appropriately.

 

The Waffle Pod Slab

The name really describes the shape of the finished slab if you could look at it from underneath. The slab is set up on a grid of cardboard or polystyrene boxes that are 1200mm x 1200mm. They are set out in such a way that the slab has a grid of beams at 1200mm centres in both directions under the flat floor surface. Just like the raft slab, the edges are thickened with an edge beam, and the depth of the edge beam and boxes varies depending on the type of soil.

waffle slab drawing

Pros:

  • These slabs provide a higher energy rating and due to this are becoming more popular
  • Very simple to set up and build
  • Great for using as a piered suspended slab on filled sites. 

Cons:

  • Care must be taken when detailing, an experienced engineer can help with this
  • Misconception that they can be set up on minimal scraping of the site
  • Very important to have the whole site drained appropriately.

 

So in summary, appropriately designed and detailed raft or waffle slabs will provide a sound foundation for your home within the Australian Standards.

As experienced structural engineers, we can help with selecting the best type of slab for your home, based on your specific requirements.

 

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