A solid shed floor uses a combination of treated 2-inch-by-6-inch beams for joists and an exterior-grade plywood floor. The assembled floor sits directly on top of levelled concrete blocks with the shed sitting on top of the floor. Building the floor follows the same principles for framing a wall or floor for a house. The project outlines a basic box frame with 16-inch spaced joists for stability. When possible, build the shed first to obtain proper dimensions to accurately fit the floor.
Measure the width and length of the shed along the bottom edge. Cut two 2-inch-by-6-inch beams to the length measurement with a circular saw. Cut two additional beams to the width measurement minus 3 inches.
Assemble the four pieces into a basic box shape. Connect the ends of the width pieces to the inside edges of the length pieces with all 6-inch edges standing vertically. Drive 3-inch wood screws through the outside of the length pieces into the ends of the width beams with a drill and screwdriver drill bit to create the basic box shape.
Measure and place a vertical mark along each inside edge of the length beams every 16 inches creating several pairs of marks. The end pair will be closer than 16 inches from the ends.
Measure and cut additional wood beams to the width measurement minus 3 inches. Attach one beam to each pair of vertical marks with wood screws driven through the length beams into each end of the width beams creating the actual frame with joists.
Attach plywood sheets to the top of the floor with 3-inch wood screws drilled through the plywood into each vertical and horizontal beam spaced every eight inches. Cut excess plywood off flush with the exterior edge of the frame using a circular saw.
Place the floor frame onto levelled concrete blocks spaced every 3 to 4 feet for stability. Set the shed on top of the floor and anchor the shed to the floor with 3-inch wood screws or an anchoring kit.