Determine if your unit has ceiling joists or if it is truss-framed. If the unit has ceiling beams that run across the 10-foot span of the unit, check that they are 2-by-6 inch joists, then skip to Step 8. If they are 2-by-4 inch joists, nail another 2-by-4 alongside each one, then skip to Step 8. If the unit was built with a truss-framed roof, meaning you can see to the roof's peak, you will need to build in ceiling joists.
Cut two 2-by-6 boards 16 feet in length, for ledger boards. This will leave you with a 4-foot clearance for entry to the loft area.
Mark off every 16 inches on the center of the ledger boards for the joist hangers.
Bolt these ledger boards to the 20-foot sides of the shed, where the sides of the shed meet the roof line, taking care not to put a bolt where you need to place a joist hanger.
Nail up joist hangers, 12 on each ledger board, using joist hanger nails.
Cut 12 2-by-6 inch boards 10 feet in length and drop them into the joist hangers.
Nail the ceiling joists into place, using 3-inch galvanized nails.
Lay 3/4-inch plywood on top of the ceiling joists. Plywood usually comes in 4-by-8 foot pieces, so cut to fit the loft area, which is 10-by-14 feet, and stagger the joints. Be sure to cut the pieces of plywood so the joint ends over a joist, rather than in between, which would create a weak spot in the floor. Nail in place.