How to Install Slab-On-Grade Concrete

Dig a foundation hole for your slab with a spade, consisting of a 6-inch deep central area surrounded by a perimeter strip that’s 14 inches deep and twice the width as the wall you’re placing onto the foundation. Make certain that the foundation is level after digging, using a carpenter’s level to check for differences in slope, and correcting the differences where needed.

Tamp down the foundation hole with a vibratory plate compactor to compress the hole. Check the level after compressing the base of the hole and adjust the level of the ground to ensure that it is level.

Build a concrete form along the edges of the foundation to create a smooth side to your concrete slab. Use a 2-by-6 inch plank for the walls of the form, joined together by wood screws. Place wooden stakes along the outside form edge, and screw them to the form to provide extra support.

Pour a layer of gravel 2 inches deep in the base of the hole. Compress the gravel with the plate compactor and then level it out. The gravel will prevent moisture from rising through the ground to the slab.

Cover the gravel layer with a sheet of 10-millimter polyethylene to serve as a vapor barrier. The barrier will prevent a rising water table due to rain or snow from making its way through the gravel and onto the slab. Cut the vapor barrier at the edges of the foundation hole with a utility knife, and overlap any seams by 12 inches with a 3-inch strip of construction tape covering the seamed joint. Cover any protruding pipes or electrical stubs with a cut 4-by-4 foot square of the vapor barrier. Cut out the square from a sheet of the plastic with the utility knife, and then cut a square in the center. Slip the square over the protrusion, using the cut X to fit it smoothly into place, and then tape the plastic around the protrusion to provide the necessary seal.

Add rebar to the foundation hole by placing 2-inch concrete blocks spaced every 5-feet throughout the foundation, and then setting the rebar on top of the blocks to center the grid in the middle of the foundation. Cut the rebar with a hydraulic bolt cutter so that it’s short of the edge of the foundation 3 inches on either side. Space the rebar every 5 feet, running it along the blocks, in two layers, one crossing the other perpendicularly. Tie the layers together with a wire tie where they cross.

Mix the concrete mix in a cement mixer, using a low-shrinkage cement to avoid cracks. Pour the concrete into the foundation hole from a single corner, spreading the concrete throughout with a spade until you’ve completely filled the form.

Run a screed over the concrete surface to level the mix out and to distribute the aggregate throughout the depth of the material. Cut a control joint through the concrete every 10 feet using a trowel edge. Make the depth of the joint about 1-inch deep. Broom the concrete surface to finish it, creating a slight texture that will give you some grip on top of the concrete. Allow the slab to cure for 21 days, keeping the surface slightly damp for the first three days to keep it from drying out and cracking.