The best flat roof protection is probably a continuous membrane. If you decide you want to tile it, there are several methods that also would waterproof it. The roof needs some slope so stuff doesn't pool and if it can't slope to the outside, needs a drain of sufficient size, with the area sloped to the drain (similar to a shower). www.schluter.com makes a system that is designed to tile an outside roof deck, but if you don't want to tile it, and the area is walked on, you need something else. Most membranes are not designed to be walked on, but many can support an applied, floating wear surface on top of it. Flashing is critical on any roof, but even more so on a flat one, along with proper drainage.