How to Build a Shed Foundation

To ensure you have enough space at your location, mark the area with mason line and batter boards. Read Making and Using Batter Boards to see steps for building batter boards.

For some installations — such as the one described here with runners — the shed overhangs the runners several inches. The runners are set back from the shed frame the distance of the overhang. Set batter boards and mason line to mark the perimeter of the posts that support the runners.

To square the area, measure 3 feet along one string and 4 feet along the adjacent string. The distance between the two points should be 5 feet. Adjust the string as needed along the batter boards, and mark the location of the string on the batter boards. Check the other corners.

Dig the post holes 12 inches in diameter and 12 inches below the frost line. Pour about 4 to 6 inches of gravel in the hole, compact it and then add concrete following the manufacturer's mixing directions. If you move the mason line, use the marks on the batter boards to return it to the correct location.

Once the concrete has cured, set a post on top of the footer. Use the intersection of the mason line to set the post square. Making sure the post is plumb — and holding it straight — add concrete around the sides and cover with soil. Brace each post to keep it in position while the concrete sets. Watch How Do I Set a Post in Concrete? to see an illustration of bracing.

After the concrete has cured and all the posts are set, determine the height you want for your shed floor and mark one post. Use this as a guide to mark the other posts and cut the posts with a saw.

Attach post base brackets and treated 4-inch-by-4-inch runners.

Build the floor frame with treated 2-by-4s and nails according to your shed's directions.

Set the frame on the 4-by-4s, leaving an overhang at the ends, and attach one side to each 4-by-4 with one screw.

Check for square by measuring the diagonals of the frame. Measure between two opposite corners and then measure between the remaining corners. The measurements should be the same. Make any adjustments and secure the other side of the frame to the 4-by-4s. Then use screws to secure the frame at each point that contacts the 4-by-4s.

Set a plywood floor panel at the corner of the frame, flush to the edges. Nail down the short edge, and check the frame for square one more time. Make any final adjustments. Nail down the rest of the plywood. Attach the other floor panels according to the directions and check for level. Your foundation is ready for the shed.