Stability and longevity for a shed is determined by the quality of the base. Throwing a piece of plywood on the ground is asking for the shed to fall into ruin. The best floors are either concrete pads or elevated wood structures. Wood floors allow the shed to be relocated as needed without having to demolish the ground. A 10-foot-by-12-foot shed benefits from a floor built out of exterior-grade plywood and a treated-lumber subframe.
Measure and cut two 2-inch-by-6-inch beams with a circular saw so they are 12 feet long. Cut 10 2-inch-by-6-inch beams so they are 9 feet and 9 inches long for the joists.
Mark one side of each 12-foot beam with a vertical mark every 16 inches. The first and last of the 10 marks will be flush with the ends.
Assemble the basic frame with the 12-foot beams sitting lengthwise with the marks pointed toward the center, creating pairs of marks. The joists must sit at 90-degree angles to the 12-foot beams and be positioned so that one joist is centered on each pair of marks. All 6-inch edges need to be standing vertically.
Drive two 3-inch, self-tapping, exterior-grade wood screws through the 12-foot beams into each of the ends of the joists with a drill and screwdriver drill bit to create the basic subframe.
Lay 3/4-inch plywood sheets on top of the assembled frame. Drive 3-inch wood screws through the plywood into each joist beam and 12-foot beam, spacing the screws 8 inches apart along each beam. Cut excess plywood off flush with the exterior edge of the frame using a circular saw.
Place concrete blocks on the ground spaced out every 3 to 4 feet. Lay a scrap beam on top of the blocks with a level on the beam. Level the blocks for side-to-side and front-to-back. Dig out beneath the blocks to lower them to the proper height for a level surface.
Place the floor centered on the blocks. Place the shed on the floor. Anchor the shed to the floor with wood screws or an anchoring kit.