A floating slab is the basic foundation structure used for buildings that do not have basements (garages, sheds, barns, and even some homes in high water table or coastal areas). Concrete floating slabs can and, in areas that experience freezing temperatures, should be insulated. Because there are no footings on this type of foundation the insulation is either built directly into the concrete by sandwiching it between layers of concrete or by laying it directly on top of the concrete. These additions are far cheaper than building a full foundation with footings and frost barrier as the excavation and extra concrete costs far outweigh the cost of insulation
Determine the area for your floating slab and mark the four corners with 3-foot metal stakes.
Determine the height of the top surface of your concrete slab. Once you have made this determination, use string to mark the height by wrapping it around the metal stakes. Use a level to make sure the string is straight and the height is consistent.
Measure out two feet from the perimeter (laterally) and mark off this area for drainage.
Measure down 2 feet and 11 inches from the perimeter strings. This is the starting point of the base for the slab. Excavate the entire interior area of the foundation to this depth. Also excavate the 2-foot drainage section.
Fill the excavated area 3 inches deep with crushed stone to form a drainage area for the foundation.
Cover the 3 inches of rock fill with 2 feet of sand.
Compact the sand with a tamp (sand compactor) until it is 8 inches below the perimeter lines. Check the area in several locations to make sure the entire area is level.
Place 2-by-10-inch lumber around the perimeter of the proposed slab to create the slab forms. Connect the corners with butt joints and secure them with framing nails. Make sure the forms are level.
Insert support stakes [2-foot metal stakes with nail holes] every four feet along the outside of the slab form. Secure these with framing nails as well.
Remove the initial guideposts and perimeter string. At this point it is time to install any drainage lines or electrical conduit that needs to run through the foundation.
Dig a 16-by-18-inch trench just inside the slab form to provide extra support on the exterior of the slab to support walls.
Install the 2-inch foam insulation over the sand. Cut pieces to fill in the sides and base of the trench as well.
Install vapor barrier over all of the foam. Overlap pieces by 2 to 4 inches and secure with tape.
Place the three-eighths-inch rebar in a criss-cross pattern over the vapor barrier. Be sure to use the 2-inch rebar supports to lift the rebar off the ground so liquid concrete can flow around it.
Add the five-eighths-inch rebar to the trench area. Make a cage formation with the bars to add extra support.
Mix and pour your cement. If you are working with a cement delivery service they will determine the amount of concrete necessary for the pour, otherwise you will have to follow the calculations on your cement bag to determine how much to pour. Remember that the size of your cement slab will be length x width x 6 inches (the depth of the concrete slab) plus the additional 16-by-18-inch length of the perimeter for the trenches.