What I did for the upper deck was to torch down a rubber roof over an Advantech 3/4" subfloor. I then used a minimal amt of screws through PT'd 5/4 x 6 sleepers ( I put Karnak flashing cement between the sleepers and rubber roof where I used the screws) which I then nailed with SS flooring cleats into my mahogany TXG decking. Zero leaks and it's been a good cpl of years now here in NY.
Notice how I laid my sleepers. On an angle in case any water gets through it will roll down and out the vented sides where I installed a continuous soffet vent (on the upper edges of the roof sides.)