DIY Concrete Garage Floor

Define the garage site with landscaping paint.

Excavate 8 inches of soil from the defined site with an excavator machine. Run a vibrating compactor machine over the soil to force out air pockets, stabilizing the ground.

Drive 1-by-4-inch wood stakes around the inner walls of the frame about every 3 feet. Against the stakes running parallel on the sides of the garage perimeter, pull a string from the back of where the garage will be to the front. Lower the front end 1 inch for every 8 feet of length from the back. Hammer the front stakes down until the top is level with the string. Work from the front back, hammering the side stakes to be level with the grade string.

Nail layers of 2-by-4-inch boards against the stakes with a hammer to frame the expanse. The top of the boards must align with the top of the stakes so that the frame slopes away from the house for drainage.

Roll out a sheet of vapor barrier over the floor of the expanse to block moisture from traveling through the foundation and damaging the floor.

Pour 4 inches of 3/4-inch aggregate over the barrier and compact it until the rocks form a hard surface.

Place rebar over the compacted base to reinforce the floor. Refer to local building guidelines to see where you are required to lay the rebar.

Divide the floor into sections with strips of expansion joints. Use ¼-inch thick joints that are the same height as the thickness of the floor slab. Space them about 2 feet apart for every inch of thickness. For a 4-inch-thick slab, lay the joints every 8 square feet.

Pour concrete from a mixer over the framed surface. Begin at the back and work your way down. Spread concrete over the first section with a hoe. Press the expansion joint vertically against the inner concrete wall and begin pouring the adjoining section.

Move a 2-by-4 board over the surface of the first section to level it off. Continue to pour the garage in sections, standing up the expansion joints and leveling the surface before the concrete sets. Keep the top of the joints clear of concrete.

Go back and forth over the sections with a bull float to smooth the floor. Level and smooth the sections in the order they are poured.

Wait four days for the concrete to set, and then cover the joints with expansion joint compound to stop water from running down the sides of the joints.