I would first build my roof or deck on a slight fall. If you want the deck level, you still can, just have the below platform on a downward fall.
You are going to have to have some hefty posts and beams for this structure, because my plan is going to be very heavy.
Do your joists and framing and support plan, then install 1 inch or 3/4 inch marine ply wood to the roof. Carefully silicon caulk in the seams and even run down some flashing over the cracks.
Now coat the entire surface with Silver Dollar Mobile Metal Roofing Covering. You can apply this with a mop. It goes on very thick. But, you must have another person there just to stir. the metal flakes will settle to the bottom and you want them stirred up and on the top.
Now, build your deck on this using angle cut joists to re gain the levelness you want.
Build the deck as usual but leave an extra wide gap between each decking board for drainage and make sure all joists are also running down hill as to not create a dam effect with water running down the roof.
Normally decking is secured with a 16D nail set between each board. You want this much wider.