A box spring mattress is a specially designed mattress that has a wood frame around the springs. This helps support the mattress, making it more suitable for heavy weights. You can build a frame, or foundation, for the mattress to provide further support. Keeping the mattress off of the floor also eliminates the effects of dust, mites, drafts and colder temperatures from the floor.
Place a 6-foot board with the wide edge vertical. Measure a distance of 18 inches on one side of the board. Set the other 6-foot board at that location. Measure a distance of 18 inches on the second board in the same direction as the first. Put the third 6-foot board at that spot. Even up the ends of each board so they all match.
Put a 3-foot board with the board face against the ends of the three longer boards. Make the ends and edges flush. Put the other 3-foot board at the opposite end of the longer boards in the same way. Square off the ends to form a large rectangle.
Attach the frame together by drilling three screws through the sides of the short boards into the ends of the longer boards. Use a horizontal pattern for the screws.
Place the plywood sheet onto the frame with the side lengths of the plywood matching the lengths of the frame. Square the corners. Drill screws every 12 inches around the edge of the plywood into the frame. Drill another series of screws every 12 inches through the plywood into the middle board.