How to Lay Out & Pour a Mat Slab Foundation

Stake off the build site using the dimensions for your structure. Place stakes to indicate the outline of the building -- alternatively, drive four stakes at the corners of the future building, then spray-paint lines between each one.

Excavate the build site to a depth of about 5 inches. If you have sandy soil, you only need to remove the sod and topsoil. If you have a lot of clay in your soil, dig so you can pour 6 to 8 inches of gravel in the site before pouring the concrete.

Build concrete forms out of 2-by-12-inch lumber. If your lumber falls short on any side of the build site, join two pieces together with a cleat made of 2-by-12-inch board. Abut the two pieces, then place a 4-foot length of board over the seam and nail it down.

Make the two side walls about 3 inches longer than the front and back walls. Check that the form walls are all level. Nail through side walls to connect them to the front and back panels.

Brace the form walls with stakes placed every 2 feet. Make the stakes out of two-by-four lumber and drive them to a depth of 8 to 12 inches so the tops of the stakes are just below the tops of the form walls.

Nail the stakes to the form walls with 16d duplex nails. Place kickers made of two-by-four lumber against the stakes at a 45-degree angle, then nail them to the stakes. This will prevent the form walls from bowing out.

Fill in the forms with granular fill made of sand and gravel. Fill in 3 inches at a time, but leave 5 inches of space from the top of the fill to the top of the form. Leave a 12-inch deep by 12-inch wide trench around the inside perimeter of the form to created thickened edges.

Tamp the fill down with a tamping tool. Check that the top of the fill is completely level.

Build reinforcements out of rebar. Drive 12-inch lengths of rebar 4 inches from the perimeter of the form every 12 inches or so. The tops of the rebar stakes should be about 4 inches below the tops of the forms.

Bend and tie lengths of rebar around the outside of the stakes. Make a pair of parallel lines with the rebar about 3 inches apart and tie them to the stakes with metal wire ties.

Create a grid of 4-by-4-foot squares made with rebar in the center of the form. Tie the intersections together with wire ties. Set the grid on top of the rebar perimeter reinforcement you built earlier.

Determine how much concrete you will need. Multiply the length by the width by the depth in feet to get the volume of the slab. Figure out the trench and the center part of the slab separately, then add them. Divide the total by 27. Add 5 percent to this number, and that will give you the number of yards of concrete you will need. Use this figure to place your order for your local cement company.

Pour concrete starting at the location furthest from the concrete truck -- use concrete-grade wheelbarrows to move the concrete from the truck to the form. Work with a team of at least four other people to pour concrete and smooth it with a rake. Continue pouring and smoothing until the forms are full to the top.

Drag a length of two-by-four lumber across the top of the form to make the concrete level. Use a zigzagging motion as you drag the board across the form. This is called "screeding."

Make up to four passes with a bull float over the top of the concrete as soon as you finish screeding. This will smooth the surface of the concrete.

Watch the concrete after you float it. Notice the water start to bleed up through the surface. Wait for the water to disappear, then press your thumb into the concrete -- if it resists, you can continue finishing the slab.

Place an edging tool between the form and the slab. Move the tool along the perimeter of the slab to create a smooth, rounded edge.

Groove the slab if it is very large. Place a two-by-four over the top of the form in a straight line -- check it with a level. Move a concrete groover along the board to create a groove -- this will help prevent cracking as the concrete dries. Repeat every 10 feet across the width of the slab.

Drag a magnesium float over the top of the concrete to smooth out any imperfections. If desired, make additional passes with a steel trowel for a smoother finish. If you prefer a rougher finish, drag a push broom over the slab.

Spray the concrete with curing compound. This is available at most home improvement and hardware stores. Application varies by manufacturer, so check the label for detailed instructions. Alternatively, you may cure the concrete by laying plastic sheeting over it, but this may lead to discoloration.

Allow the concrete to cure overnight. Remove the kickers, stakes and form walls. Allow the concrete to cure for at least two more days before attempting to build on it.