I need to replace 2nd floor painted T&G fir decking that is on a plywood sub-floor covered with rubber matting. Wood began rotting after 3 years. The underside has a finished ceiling. Third floor drips on second floor porch, and the drop is about 2" for the 8ft. depth. Can you recommend a way to waterproof the deck and prevent the boards from rotting. House in midwest, so have snow/ice in winter and heat/humidity in summer. I have columns and metal rails on each porch as well. Thanks for any ideas!