How to Pour Concrete Slabs in Garages

Prepare the ground for the garage floor by removing existing pieces of concrete, dirt or sand. Dig out 12 inches from the site with shovels and compact the bottom of the foundation with a compactor machine. The bottom of the expanse should have a slight slope of 1 inch for every 8 feet of distance from the house. The machine will force out air pockets to stabilize the ground.

Frame the garage floor by stacking two layers of 2-by-6 inch boards on the sides against the inner wall. Attach the boards with a hammer and nails. The top of your frame marks where the surface of the slab will be. Use a carpenter's level to check that the frame is higher in the back of the garage and lower in the front, following the drainage grade.

Fill the bottom of the expanse with 4 inches of aggregate and compact it with the machine until the surface is hard. Place wire mesh over the aggregate to add support to the base.

Place rebar near the sides of the floor and through the center. Raise it to half the thickness of the slab with rebar chairs so it will reinforce the slab from the center.

Divide the garage into sections with ¼-inch expansion joints equal in height to the thickness of the slab. Expansion joints will absorb tension and prevent the slab from cracking when the concrete expands and contracts. The joints should be set apart about 2 feet times the thickness of the slab. For a 6 inch slab, place the joints in a grid every 12 square feet.

Begin to pour the concrete from the back of the garage out. Use shovels to spread the concrete into corners and rake it with a gauge rake to remove air bubbles. Drag a screed over the surface from the back to the front to remove excess and fill in low areas.

Stand the expansion joints up and press them against the inner wall of the first section. Immediately begin to pour the adjoining section while the concrete in the first section is leveled. Pressure from the two sides will lock the expansion joints in place. Clear any concrete from the top of the expansion joints.

Use a long-handled bull float to smooth the surface of the concrete.

Secure strips of expansion joint compound over the expansion joints to seal them so water cannot run down the sides and into the base.