First published at Ecohome.net, which has a guide and videos for slab-on-grade construction.
A slab-on-grade is a great alternative to a basement if you’ve got the space. Building with prefab kits make them easy and affordable.
Rather than building wooden forms and taking them apart afterwards, a prefab slab-on-grade form kit uses shaped foam as the form itself. Once the top-rail metal fasteners are in place, no further bracing is required. This makes the installation much quicker by eliminating the steps of building forms and bracing them, then taking all that apart afterwards. Since prefab kits rest unsecured on the surface, they are also very easy to adjust for level and square.
With a typical slab-on-grade [either kit or hand-built forms], the load of exterior walls is carried by the footing alone. An alternative option is a ‘raft-slab’ which has a uniform thickness.
Don’t be put off by the higher cost of a kit over raw materials; the speed at which you can put these together offsets that to the point where it can easily be cheaper than building from scratch. With some soil conditions the raft-slab is likely the cheaper option of the two designs due to the uniform distribution of weight.
Sites with high water tables or soil with poor bearing capacity may incur significant additional costs in drainage, soil replacement and compaction before an engineer will approve a project where the load rests only on a footing. By distributing the weight evenly, a raft-slab acts a bit like a snowshoe and can effectively ‘float’ on terrain that would otherwise be unsuitable for building.