How to Form & Pour a Garage Slab

Clear off an area where you wish to place the slab. The size of the cleared off section depends on personal preference. Measure the area off and use a chalk line to mark the outline of the slab. Using a shovel and a gravel rake, level the area off as best as possible. Remove any large rocks or roots from the ground. Pour a fine layer of masonry sand down over the cleared area.

Create a buffer zone that will keep the concrete in the area when it is poured. Use four 2-inch by 10-inch boards cut the length of the area. Place them along the edge of the slab and push them together so they form either a square or rectangle. Hammer stakes outside of the frame to hold the edge boards up and to deter any shifting that may occur when the concrete is poured.

Spray the edge boards with lubricant oil to keep the concrete from sticking to them when drying. Allow the oil to dry on the boards before pouring concrete. Dampen the masonry sand in the slab area with water from a garden hose. This will allow the sand to form a harder setting area. Rake the sand again to ensure it is lying smooth.

Place the concrete mix from the bag into a wheelbarrow. Add water to the mix until it becomes a pudding-like consistency. Refer to the cement mix bag for manufacturer's recommendations. Pour the concrete inside of the slab starting from the middle and working outward to the edges. For the amount of concrete needed to fill in the slab, use a concrete calculator.

Use a trowel to level the concrete when the slab is completely full. Make sure the concrete is filled to the top of the 2-inch by 10-inch boards. Allow the concrete to dry for two to three days. Cover the area with a plastic tarp if rain is in the forecast before this time or if debris is likely to fall in the slab. Remove the 2-inch by 10-inch boards used for the form.