A long lasting shed will need a sturdy foundation to keep it level and protect it from moisture. Follow these steps to build your shed foundation. Before you begin, check local building codes or homeowner’s association guidelines for any requirements on foundation types, frost lines, and location specs in your yard. 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. When you have your location, mark the shed area with masons string and stakes. To square it, measure 3 feet along one string, and 4 feet along the adjacent string. The distance between the two points should be 5 feet. Adjust as needed. You can build your foundation several ways. On grade foundations are for areas that don’t freeze. One option is to pour a concrete slab, and install sill plates on top with adhesive and concrete screws. Then set the floor frame. Other options use 4×4 timbers—also known as skids or runners—with the frame on top.
It’s best to mark the runner locations on the strings for easier layout. Here are a few methods using timbers. The first is another on grade foundation. Set masonry blocks on gravel about 4 inches deep. (Don’t pile the gravel above the holes–it’ll shift.) Use different size blocks to level the 4×4 timbers, and roofing shingles for fine adjustments. The next methods are for permanent foundations in frost areas–the footers are set below the frost line to prevent shifting during freezing temperatures. (You’ll need additional strings and stakes to line up post holes.) For the concrete tube method, pour gravel into the holes, then insert the concrete tube forms. The tops should be above ground and even. Add concrete, and insert post base brackets. Make sure they’re lined up and let the concrete cure. Finally, attach the 4x4s to the brackets. The last method uses posts set in the holes. Dig holes about 4 feet apart, 12 inches in diameter, and 12 inches below the frost line.
Pour about 4-6 inches of gravel in the hole, compact it with a post, then add concrete following the manufacturer’s directions. Once the concrete has cured, set a post on top of the footer. Make sure the post is plumb, and hold it with stakes. Now add gravel around the sides to allow for drainage. 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 with a chalk line and level, and cut with a saw. Then attach post base brackets and 4x4s. Next, build the frame with 2x4s and nails according to the directions. Set the frame on the 4x4s, leaving about 1 foot over hanging the ends, and attach one side to each 4×4 with one screw. Check for square by measuring the diagonals–they 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. The floor panels go on 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 and 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 have it professionally installed. Want more great ideas and how-to’s? Go to Lowes.com/HowTo or just click to subscribe. Next, learn how to install French doors and skylights in your shed. .
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