How to Make a Concrete Slab for a Home Addition

Mark the boundaries of the new slab with wooden stakes at the corners and string stretched between the stakes. Excavate the area with a shovel to a depth of 4 inches, removing all grass, roots and dirt.

Place 2-by-6-inch form boards around the perimeter of the excavated area, using the existing concrete slab as one of the sides. Do not install form boards on this side.

Drill a series of 3/8-inch holes with a hammer drill at 4-foot intervals along the side of the existing concrete slab. Carefully insert rebar rods into the holes. Using these connecting pieces of rebar as a starting point, create a grid of 4-foot squares in the remainder of the excavated area. Place small rocks or broken bricks under the intersections of the grid to keep the rebar from sinking all the way to the bottom of the concrete. This step is critical to ensure that the two slabs are secured to one another.

Mix the concrete according to the manufacturer's directions in a wheelbarrow, using a hoe to ensure that the ingredients are thoroughly mixed. Pour the concrete into the form boards, filling them to the top edge.

Pull a long piece of lumber across the top of the form boards to screed the concrete surface. This knocks down the high spots in the concrete and helps fill in the low areas.

Smooth the surface of the concrete with a rubber float. Slide the float across the surface of the concrete in an arc pattern, being careful not to press hard enough to leave marks in the concrete. Allow the concrete to cure for seven to ten days before allowing traffic on the new slab.