Is a Slab Cheaper than a Raised Home?

This is a very common question asked, and the answer is Yes... and No.

On flat ground, with an S or M type soil, a slab will be the cheaper option. However, where soil quality is poor or affected by trees or ground movement, a raised floor will often be cheaper.

On sloping sites, where fill is over 500-600mm, a raised home will often be the most cost-effective option. Where fill is needed, concrete piers need to run through the fill into natural ground. The amount of concrete required can be very expensive. Similarly, where sloping sites have slope stability issues, these piers can be extremely large and expensive.

Another question arises whether garages should be raised or built on an in-filled slab. This is where blockwork is laid and in-filled with compressed soil, in which a slab is laid on top. Personally, I saw the in-filled slab method used for years when I started working with builders. Then I realised that:

a) it was much cheaper to keep the garage floor raised and only block the driveway level with the garage,

b) in-filled slabs often sank as the soil further compressed over time. This resulted in cracking in the slab and walls pulling away from the main home.

Demographics also play a part in deciding between slab and raised options. In rural areas, or areas where labour and/or concrete is very expensive, a raised floor option will be cheaper.

If owner-building, a raised floor generally works out comparable to concrete.

Generally, anything on a flat residential block is built on a slab. Most homes on semi-rural, rural or sloping sites are build with a raised floor.