Lay out three treated 48-inch boards on their long, thin sides, parallel to each other. Space them about 2 feet apart since these are the platform support boards. Set down another treated 48-inch board perpendicular to the platform support boards, resting on its long, skinny side. Slide it up against one end of the boards, so that it comes in contact with all of the platform support boards. Consider this board the rear support board.
Drill three holes through the rear support board and into the end of each platform support board, for a total of 9 holes. Use a drill bit that is smaller in diameter than the galvanized wood screws, which will be installed within the drilled hole. Set a galvanized 3-inch wood screw into each hole and tighten it completely to attach the boards. Tighten the screw until the top of the head of the screw is even with the outer surface of the rear support board.
Lay 12 treated 60-inch boards across the support frame, so that they are perpendicular to the platform support boards and parallel to the rear support board. Space these boards, which are the platform boards, evenly across the support frame so that they cover the entire frame with equal spacing between them. Have them extend slightly beyond the frame in all directions, covering it somewhat from view.
Drill two holes down through each platform board and into each platform support board underneath it, for a total of 72 holes. Place galvanized 3-inch screws in each hole and drill them into the boards beneath.