I presently have a post based (no concrete footings/walls) 3 season room off of our walk out basement. Above it I have a deck. The room size and deck above is 14' wide by 20' long.

I want to tear this down, and rebuilt it with a concrete footings/foundation. That's not the problem design I have.

For the joists on the ceiling of the 3 season room (which is the floor of the deck above), what can I do to make sure no water or snow leaks thru? Presently I have a one piece rubber composition sheet over it, which has held up for 15 years, but now is leaking some.

I had thought of tapering the roofline, for drainage, and then building up the end of the deck for a flat surface. But still, what can I use to keep the 3 season room dry as I would like to convert it to a 4 season area, with built in Vermont Castings wood fireplace, etc. I've looked but found now solutions and local building supplies stores have offered no options.

I had thought of making galzanized metal sheetings overlapping the ceiling joists at an incline to move moisture off, but......... stumped.