Pick a spot for your shed that's close to level and doesn't collect water. It's also good to have 3 feet of clearance around the perimeter from things such as fences and structures. Larger sheds — 160 square feet and up — need 4 feet of clearance. This area gives you room to build the shed.

To ensure you have enough space at your location, mark the area with mason's string and 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's string 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.

Tip: Enlist a helper when measuring and adjusting for square

The post holes should be about 4 feet. apart. Set up batter boards and mason's string to mark these points. The intersections of the string indicate the locations of a post corner. Use the 3-4-5 method described above to check for square at each intersection. Adjust the string along the batter boards as necessary and mark the location of the string on the batter boards.

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

Tip: Consult with local building code officials for the frost line depth in your area.

Once the concrete has cured, set a post on top of the footer. Use the intersection of the mason's string 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.

Note: Always wear a dust mask and safety glasses when cutting treated lumber.

Tip: Use treated lumber rated for ground contact for the posts and runners. Use fasteners and hardware labelled for treated lumber when assembling the foundation and attaching the shed floor frame.

After 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 4x4 runners.

Build the floor frame with treated 2x4s and nails according to your shed's directions.

Tip: Some shed floor frames use 2x6s.

Set the frame on the 4x4s, leaving an overhang at the ends, and attach one side to each 4x4 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 4x4s. Then use screws to secure the frame at each point that contacts the 4x4s.

Next, set a 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.

With the foundation set, you're ready to build the shed. Kits come with detailed instructions to do it yourself, or you can have it professionally installed.