A slab foundation is a simple concept. Basically, it's a concrete pad poured on the ground so in theory, it's well within the skill set of a competent DIY'er. However, because you don't get any 'do overs' when pouring a foundation most people wisely opt to contract the job out to a professional. However, to give you an understanding of what is involved in the process here's an overview of how to pour a slab foundation.
Pouring a slab foundation
Start by laying out the outline of your foundation. Figure out where the corners are located and drive stakes into the ground at each corner, then run string between the stakes. These strings will be the outline of your foundation.
Inside the strings dig a trench 18” wide and at least 2' deep (in colder climates you need to go down deeper to get below the frost line – usually 3 to 4 feet). This trench will form the footings for your foundation and provide the support for the walls of your building.
Around the trench build wooden concrete forms using 2” x 10” boards. Brace the forms every 2” using wooden stakes behind them.
Install rebar into the perimeter trench in a continuous line at least 2 inches from any outside surface. Overlap the rebar and join the pieces together using rebar wire to keep the rebar line continuous. Lift the rebar from bottom of the trench by setting it on 'rebar chairs' every 12 inches.
Spread a layer of sand or gravel covering the inside of the slab form 4 to 6 inches deep. Level the gravel then cover the interior of the surface of the pad with 6 mil plastic to help control moisture infiltration after the concrete pad is poured.
Install rebar in a grid pattern again being sure to keep it elevated using 'rebar chairs'.
Conduit for electrical wiring, water and drain pipes is installed above the plastic sheet making sure they are up from the bottom of the slab before pouring the concrete slab is poured.
Fill the footing forms with concrete and use a piece of 2” X 4” to level it. Install anchor bolts every 12”.
Continue pouring concrete into the center of the form creating a concrete pad 4” to 6” deep and level with the top of the exterior footings. Use long handled trowels to smooth the interior pad.
Allow the poured concrete to set up for at least 48 hours before removing the wooden forms.
Local building codes will actually dictate the specific details of how your slab foundation is poured in your area. Details such as the thickness of the slab, depth and width of the footing, size and location of the rebar as well as the type of sand or gravel are commonly specified in local codes.