“Which type of slab is the best for my new house?”

This is a question we get asked quite often.

There are a number of different options available when it comes to choosing a slab and footing system for your new home.  There are the deemed to comply or “standard” designs from AS-2870 Residential Slabs and Footings and then there is “engineered” type designs. For the purpose of this discussion we’ll stick with the standard type designs as this covers the vast majority of slabs being constructed on a daily basis. Before we continue I should point out that there is no such thing as a standard design, it is the design philosophy that has been standardised to provide a simplified method for designing residential slabs and footings.

Generally four common footing systems are used:

  • Stiffened raft slabs (raft slab)
  • Waffle raft slabs (waffle slab)
  • Strip footing systems
  • Stumps with pier type footings.

The question is, which one is most suited to your new house? This will depend on:

  • Personal preference
  • Builder / Concretor preference
  • Soil type
  • The style of house and the materials you are constructing the house from.

The choice of a strip footing or pier footing system is largely dictated by the style of house being constructed. If you need one of these systems there are really not too many other options available. That being the case, we’ll now concentrate on the advantages and disadvantages of raft and waffle slabs.

Raft Slabs

Raft slabs are the conventional slab and stiffening beam type construction. The stiffening beams are dug into the ground the slab and beams are generally poured in one go.

Advantages include:

  • Minimal concrete use on slightly reactive (S) sites
  • Easily modified to meet any required footing depths
  • Most concretors are experienced in raft slab construction.

Disadvantages include:

  • High concrete volumes on highly reactive (H2) sites
  • Can have high numbers of piers on uncontrolled fill sites.

Raft Slab ready for concrete pour

Waffle Slabs 

Waffle slabs are constructed using styrofoam void formers (pods). The gaps between the pods create a higher number of thinner beams when compared to a raft slab (refer photos).

Advantages include:

  • Less concrete volumes when compared to a raft on a highly reactive (H2) site
  • Less excavation which can save time and money on sites with shallow rock / boulders
  • Easier to locate piers on sites with uncontrolled fill
  • Potentially less shrinkage cracking due to a more even spread of concrete throughout the slab and footings.

Disadvantages include:

  • Higher concrete volumes on slightly reactive (S) sites
  • More susceptible to moisture problems if drainage around house is not adequate.

Waffle Slab ready for concrete pour

 

This has been a very quick run through of the options available for slabs and footings when constructing a new house. The different slab and footing types outlined above should provide satisfactory performance providing the site classification has been conducted correctly, the slab has been designed in accordance with AS-2870 and good maintenance has been maintained (slab maintenance will be the subject of a future post).  Your engineer should be able to explain the options to you in more detail with their advantages and disadvantages.