building a deck floor on top of existing tin sheeting

Hello, I'm new to the board and looking for some guidance. Im working on a house in uptown New Orleans. second floor balcony has tin sheeting on top of old cypress flooring. i wanted to add a layer of plywood and then salvaged tongue and groove flooring with exterior seal. my question is , is it kosher to frame/level up the existing balcony with wood, then set plywood on top? on the lowest dipping part of the deck there is maybe two inches of gap where the ply would sit. can i caulk the hell out of all the seams, maybe add spray foam insulation, etc? and build on top of that? thanks!!