How to Pour a Concrete Slab Against an Existing Wall

Drill a series of 3/8-inch holes 2 inches from the base of the existing wall with a hammer drill, at 4-foot intervals. Clean the debris out of the holes by blowing into them with compressed air.

Build a framework of 2-inch-by-6-inch form boards around the proposed slab area. Secure the ends of the boards together with double-headed nails. Prevent the framework from moving by hammering wooden stakes into the ground on the outside of the form boards at 4-foot intervals. Secure the wooden stakes to the form boards, making sure the nails do not go all the way through the form boards. There must be no form boards on the side of the concrete that will abut the existing wall.

Slide the ends of rebar pieces into the pre-drilled holes in the existing wall. Place small blocks or pieces of wood under each of these pieces to keep them stable. Begin laying your grid work from this point, as described in the next step.

Lay a grid work of 3/8-inch rebar inside the form boards, creating 4-foot squares. Where the rebar intersects, use special rebar ties to secure the adjacent pieces together. Place small rocks or blocks under the intersections of the rebar to keep it approximately 2 inches above the ground. This must be about midway of the depth of the concrete.

Pour the concrete into the form boards, over the grid work of rebar. Use a hoe to push the concrete around inside the form boards, ensuring that the entire area is filled. Do not push the rebar off of its stabilizing blocks; if you do, you must pick the rebar up and put it back into place.

Screed the concrete once the form boards have been filled, using a long piece of lumber and pulling the board across the top of the form boards to push high spots of concrete down and to fill in low areas. Allow the concrete to harden slightly, for about an hour. Smooth the concrete further with a bull float by dragging the float across the concrete in an arc fashion. Allow the concrete to cure for three to seven days before allowing traffic in the area.