The foundation of your shed is the most important feature in determining how long your shed will last. A poorly constructed foundation will lead to rotting and sagging lumber. Careful planning and proper materials will give you a shed that will last for a long time. There are three basic concepts in building a shed foundation, which include your location, constructing your frame and installing your floor.
Building the Frame
Use four of your 2 by 4s to construct a square. The 2-inch edge of each board must face up. Place the 2-inch end of one board at the very end of the 4-inch side of another board. Nail the boards together with your nail gun. You should end up with the shape of an L. Close your square in by nailing your other two boards into place.
Measure 17 inches from the end on one side of your square and make a pencil mark. Measure 17 inches from your pencil mark and make another mark. Continue this process until you have seven pencil marks spaced 17 inches apart. Complete this same step on the board directly across from you.
Place one of your 2 by 4s inside your square with each end lined up with your first pencil mark. Use your nail gun to secure the boards in place. Nail from the outside of your square into the ends of the board. Continue placing the rest of your 2 by 4s into the square on each consecutive pencil mark, and nail them into place. These boards are your floor joists.
Measure across the two opposite sides of your square 2 feet from each end. Lay your 4 by 4s across these marks. These larger boards are your skids. Your skids will be nailed to your frame going in the opposite direction of your floor joists. These boards should be set in 2 feet from each side of your square. Nail your skids to your frame and floor joists with your nail gun. You will have to angle nail your skids to your frame and joists.
Place the tip of your gun onto the skid approximately 2 inches above your frame. Raise your gun to a sharp angle with the tip still compressed against the skid. Pull the trigger. A nail should go through your skid into the frame. Continue this process across both skids until you have shot a nail through the skids and into each floor joist.
Frame Foundation and Floor
Set a concrete block 6 inches in from the ends of your skids. You should have four blocks set in place. Set the remaining two blocks directly in the center. You should now have two rows of blocks, with three blocks in each row. Your skids will set on these blocks.
Lay your remaining 2 by 4s across one of your rows of blocks. Place your level on your board. If your board is not level, you will need to adjust your blocks. You can level your blocks by removing dirt, or adding gravel underneath your blocks to adjust the height. Complete this same step on your second row of blocks.
Ask a helper to assist you with lifting your floor frame and setting it on your blocks. The skids of your frame should be resting on your blocks. If not, you need to move your frame.
Lay two of your ¾-inch plywood sections side by side on your frame going in the same direction as your floor joists. Secure your flooring in place with your nail gun by nailing through your plywood into your floor joists.
Measure and cut your remaining sheet of plywood into 2-feet by 8-feet sections. You will need these sections to fill in the rest of your floor. You should place these sections at the back of your shed and along one side. There will be less foot traffic in these areas.