You will have no problems if you water tight the flat roof, but, when you build the deck, make sure you don't nail it to the deck. Put 2x4 runners on the flat deck then nail your deck to the runners so there are no penetrations in your flat deck. The weight of the deck will hold it in place. If you have any questions call me
Thanks,
Lenny
Hudgins Contractors 817-718-3127