Check local building codes or homeowners association guidelines for requirements and information about:
You're reading: How to Build a Shed Foundation
- Required permits and inspections
- Foundation type
- Frost line
- Shed size and type
Follow the manufacturer’s instructions and safety precautions for your shed.
Before beginning any excavation, call 811 to check for underground utilities.
Building the Shed Foundation
You can build your foundation several ways. On-grade foundations are for areas that don’t freeze:
- One option is a concrete slab with sill plates on top.
- Another option is masonry blocks set on 4 inches of gravel.
The next methods are for frost-proof foundations. The footers are set below the frost line to prevent shifting during freezing temperatures. You’ll need additional string and batter boards to line up post holes and runners.
- The first method uses concrete tube forms on gravel with post base brackets on top.
- For our shed we’re using posts set in concrete footers, as illustrated to the right.
Good to Know
Talk with a local building inspector about the foundation requirements for your area and shed.
Lay Out the Area, Set Posts and Assemble the Foundation
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 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.
Good to Know
Enlist a helper when measuring and adjusting for square.
The post holes should be about 4 feet apart. Set up batter boards and mason line 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 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.
Good to Know
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 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.
Good to Know
Use treated lumber rated for ground contact for the posts and runners. Use fasteners and hardware labeled for treated lumber when assembling the foundation and attaching the shed floor frame.
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.
Always wear a dust mask and safety glasses when cutting treated lumber.
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.
Good to Know
Some shed floor frames use 2-by-6s.
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.