Constructing a subfloor for your shed provides a solid foundation for years to come. If you're building on a concrete slab, a subfloor is optional: The concrete is its own subfloor. If you choose to build a subfloor over a concrete pad, anchor it to the slab with angle irons, lag screws, and anchor shields. Build a subfloor made of 3/4-inch exterior grade plywood on a frame of pressure treated 2x6s. Strengthen the corners with corner brackets and anchor them to the foundation.
There are 6 steps to completing this project:
Cut the 2x6s to make the four sides of the platform. They're called end and header joists. The headers are the ones that all the other joists nail into. Cut the end joists 3 inches shorter than the frame dimension to account for the thickness of header joists.
Nail the end and header joists together with 10d (3 inch) common nails. Set the frame in place on the foundation. Check for square by measuring across opposite corners. Adjust the frame as needed until the two diagonal distances are equal.
Nail a corner bracket at each corner of the frame to reinforce the joint. Attach the frame to the skids with hurricane/seismic ties. Recheck the corner for square.
Mark joist positions every 16 inches on the opposing header joists. Extend the lines down the inside face of the header joists with a combination square. Centre a joist hanger at each mark. Drive nails through the flange on only one side of each hanger so you can insert the joists easily.
Cut the 2x6 joists to length and set them in the hangers. Press each hanger closed, nail the other flange to the header joist, then nail the hanger to the sides of the joist.
Lay 3/4-inch exterior-grade plywood on the joists. Trim panel so joints are at the centre of joists, but stagger the joints so no joint runs into another joint. Leave a gap of 1/2-inch between edges to keep the panels from buckling. Fasten the panels to the joists with 8d (2 1/2-inch) nails or #8-2 1/2-inch deck screws. Space the fasteners every 6 inches along the edges of the panel and 12 inches else where.
