Wow, what does mopping have to do with a "rubber" roof? I think he was talking about the old timer tar roofs. I have had subcontractors do a great many rubber roofs for me in the Boston area. We would always install "floating" deck sections on top of them that could be lifted up if needed to repair punctures. Also, don't forget your venting issues. These can pose challenges but it is possible to vent these roofs correctly. Here is my suggestion and what I am going to do next time I encounter this: FIBERGLASS! I recently met a guy who has done many fiberglass covered decks and while the building inspector scratched his head, he had to pass it because it was so darned tough! Use a roller with broom handle to wet the plywood with resin, roll out fiberglass cloth from 6' wide bolt overlapping edges, roll more resin over this and repeat with another layer going the other way. Don't use any more resin that necessary or the results will be weaker. You can tint the resin any color you want. This roof will still be water tight after the house has crumbled below it and turned to dust.