How to Lay a Concrete Garage Base

Mark the garage base’s location on the ground by spray painting the perimeter or driving wooden stakes into the ground.

Dig a hole for the base inside the lines you marked with a 1 1/2-ton excavator. Going beneath the frost line prevents ground freezing and thawing from damaging the base. Contact your local zoning board to learn the frost line’s depth.

Build a wooden form around the hole’s edge by driving wooden stakes into the ground every 3 feet and nailing 2-by-4-inch boards to them. This frame gives the concrete base smooth edges around the top of the slab. If you're building the garage so it is attached to the house, only use the wooden forms on the three outer edges. Place expansion strips against the hole’s side where it abuts the house. These strips provide a buffer between the garage base and the house’s foundation, as they expand and contract.

Place a plastic sheet called a vapor barrier over the dirt in the hole. The vapor barrier blocks ground moisture from disturbing the base through erosion or the freeze/thaw cycle.

Cover the vapor barrier in the hole’s bottom with sand or gravel. Follow local building codes when determining the type of material you're using and the layer’s depth.

Placing rebar or wire mesh into the hole on top of the sand or gravel provides additional reinforcement. As with the sand or gravel, follow local building codes to determine the material you use and how it is to be installed.

Fill the hole with concrete. Hire a concrete contractor for a job this big.

Dragging a 2-by-4-inch board across the top of the wooden form with a back-and-forth motion smoothes the concrete.

Smooth the concrete’s surface further with a tool called a bull float. This is a wide float attached to a long pole. Place the float on the concrete by your feet and push it across the surface, keeping the back edge raised as you do this task. Lower the float to the surface when you reach the other side, and then pull it back.

Place a plastic sheet over the base. Remove the sheet once a day and spray the concrete’s surface with water keeping it damp, and then cover it back up again. Add water to the concrete for a week after pouring it, and then keep the slab covered for an additional week as it cures.

Remove the plastic sheet and wooden form from around the concrete.