Advantages of a waffle raft slab in comparison to a conventional stiffened raft slab

What are waffle raft slabs, and how are they constructed?

Waffle rafts are a particular form of raft slab where the raft, including ribs, is constructed on a prepared flat ground surface. The regular grid of ribs is formed using void formers.

The construction is completely on-ground rather than in-ground like a conventional raft slab and this has several features:

  • The shrinkage behaviour is improved due to the lower restraint compared with a raft with embedded beams.
  • The structural performance is enhanced, as there is no concern about the down-drag of embedded beams due to clay shrinkage. Conventional raft slabs are susceptible to this.
  • The proportions of the cross-section may be achieved reliably without excess concrete being needed due to over-excavation. Quantity of materials is generally more accurate with a waffle raft.
  • There is a greater propensity for ingress of moisture under the slab.

Adequacy of Drainage

Waffle raft slabs are commonly utilised in the NSW residential market because construction is more controlled and less impacted by wet weather.

Regardless, the benefit of a waffle raft slab is that it has a flat building platform preparation level so rainwater runoff can be drained, providing that there is adequate temporary and permanent site drainage.

Alternatively, conventional raft slab tends to be more affected if an extended period of wet weather were to be experienced following the excavation of all the beam trenches. The trench walls and bases could become soft and will require to be re-excavated and dewatered, which would be troublesome and time-consuming.

Our Experience

Here at Residential Engineering, we have a team of professionals with over 30 years of experience in this industry. We have designed over 250,000 waffle raft slabs in NSW following the AS2870 compliances. Contact us today for more information.