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Waffle Slab versus Raft Slab

12 Jul
Waffle Slab versus Raft Slab

If you’ve decided to build your home on a slab rather stumps then which slab should you choose?

The tradiaitonal way to construct a slab for your new home or extension is a raft slab. This involves creating concrete beams excavated into the ground then pouring the slab across these beams forming a raft like structure which is very strong and resistant to cracking, especially in ground that may shift or swell with moisture in the soil underneath. This slab structure has proved itself over many years.

The main issue with raft slabs is that they more expensive and more labour intensive.

More recently, the waffle slab has come into the market which requires a flat surface, for polystyrene forms to be arranged in a grid layout usually laid on top of the ground and not laid in an excavation in the ground. Reinforcing mesh is then applied to the ribs and on top of the pods to reinforce the concrete. Concrete is poured to form a slab over the pods, so that the underside looks like a waffle.

There are two main benefits of waffle slabs:

  1. They provide good insulation by trapping air underneath in the waffle voids which is particularly beneficial for hydronic heating if it is applied in a screed over the top of the raft slab. The insulating properties of the trapped air will reduce the heat loss to the ground, which can be up to 40% of the heat load if the ground is wet.
  2. Waffle slabs are quicker and cheaper to install with lower labour costs.

However, the main issue with waffle slabs is that cracks may appear on the slab and/or house structure over time if your soil is likely to shift or swell with moisture, or if water can run underneath the slab. It is most important to understand the water flow from rain, taps, water heaters and air conditioning equipment. If this can run under the slab, swelling can occur and in extreme cases cause significant cracking and movement of the house structure.

Therefore, waffle slabs are best used on stable, sandy soil that is more resistent to movement and moisture swelling, and also prepared to allow rain and other sources of moisture to run away from the slab and not underneath.

A structural engineer can advise you on your soil type, whether it will support a waffle slab and how to ensure water will not run underneath the slab and cause swelling and cracking.

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