Laying a Slab Foundation for a House

Set the plumbing rough-in before pouring and concrete. In many cases, the rough plumbing is accomplished first, then the digging begins.

Order a soil analysis. In many places, local laws and codes require this. The different foundation types are better suited for different types of soil. The slab foundation requires a fairly stable soil, roughly 2,000 pounds per square foot at a minimum. Even so, digging a deeper foundation, more than one foot, might be the answer to dealing with weaker soils. The deeper you go, the more solid the soil.

Excavate the foundation according to the precise shape of the house. Make sure to measure properly, staking each corner.

Dig out the foundation with a backhoe. Digging by hand would only be feasible in very small projects such as trailers, sheds or garages. Furthermore, using professionals, or at least professional equipment, will ensure a more precise job. The central parts of the foundation, as well as the deeper perimeter, need to be consistent throughout.

Dig the perimeter about 18 inches deep, though this depends on weather and soil conditions. The perimeter will be the poured footings, or the deepest parts of the foundation dug below the frost line, under which the soil does not freeze. Pouring a single slab that serves both as the foundation and the footings is called a “monolithic” slab, and is stronger and cheaper than other options.

Lay several inches of sand and gravel. This also depends on soil types. Looser soil requires a deeper layer of gravel, up to four inches.

Apply a thin coat of polyethylene, about 5 mm roughly, to act as a moisture and vapor barrier. Double the amount of chemical at your footers.

Lay 2-inch-by-4-inch boards across the entire perimeter. You can also make smaller squares to create cells within the structure. This might add strength and make reinforcement easier. Their purpose is to provide some support for the rebar or steel mesh reinforcement. Most local codes require steel reinforcement. Make sure these are laid suspended over the newly dug foundation. Using rocks is another simple method to keep these rods suspended. You will pour the concrete over them. Most projects require steel rebars of 5/8-inch spaced every five or six feet. Using a wire or steel mesh grate suspended about one-third of the way up from the gravel is also acceptable.

Pour the concrete. Level it with the trowel once it begins to dry. Use a level to ensure an even distribution of the concrete. If you are unsure about your abilities here, hire a professional concrete firm to pour the concrete. They will make sure all is totally level. The concrete should be at least rated at 3,000 pounds per square inch.