Laying Foundations for a Kit Home

Kit homes can either be built with bearer and joist construction or with a concrete slab foundation, making them suitable to erect on most building sites. If you choose to have a slab and you opt to do it yourself, this article covers the basics of laying a concrete foundation for your kit home.

What to consider when laying foundation for a kit home

Soil types

The type of soil on your block will determine the type and strength of slab required. Soil types include:

A – sand and rock with little movement due to moisture
S – slightly reactive clay with slight movement
M – moderately reactive clay with moderate movement
H – Highly reactive clay with high movement
E – Extremely reactive clay with extreme movement
P – soft, unstable, collapsing soil due to moisture and structural changes.

Slab types

There are three main types of slab construction used, depending on soil type and the kind of house being built:

  • Stiffened raft – poured onto the ground with a concrete edge and internal beams (suitable for stable sites).
  • Waffle raft – made up of stiffening ribs formed using polystyrene moulds laid out in a grid pattern and locked together (suitable for highly reactive clay sites).
  • Pier & slab – holes are drilled into the soil and concrete poured in to form piers, which the slab is then poured on top of (helps anchor the site in extremely reactive clay soils).

Laying the foundation: Laying the slab

These are the basic steps involved in preparing and pouring a concrete slab foundation:

  1. Excavate the site down to twice the thickness of the slab and remove any rocks, roots or debris from the soil
  2. Compact the soil with a mechanical compactor and level the site with a spirit level
  3. Lay down a layer of sand or gravel to provide drainage
  4. Put in the form work (a wooden frame that encases the slab
  5. Put in the plumbing and drainage pipes (make sure they are in exactly the right positions, as removing them is a costly and time-consuming job)
  6. Lay down a vapour barrier (a damp-proof membrane to prevent moisture from seeping up out of the ground)
  7. Put the steel mesh reinforcing in place (either supported on rocks or held in place while pouring)
  8. Pour the concrete (make sure beforehand that trucks have access and everything is ready prior to the pour, as time is of the essence once the cement arrives)
  9. Smooth and level the poured concrete with a straight-edge and trowels
  10. Set anchor bolts into the wet concrete at intervals along the perimeters (these will be used for attaching the walls)
  11. Remove the form work
  12. Allow the concrete to sit and cure (can take up to 28 days to fully cure).

The slab plays an important role in supporting walls and sealing out damp, so if you’re at all unsure about laying it yourself, have a talk with your kit home provider. They may be able to assist you with any building permits required, or recommend a professional to do the job for you.