This is how you build the deck floor. Essentially you are building a deck. Start by making a 7' x 7' square with some lumber resembling 2 x 6's. Cut enough floor joists to span the distance with 16" on center joists (or something reasonably close to 16" o.c.) With the deck structure made, lay 2 x 6's across the deck to form the floor, much as you would in making a deck.