Hi Patrick,
You must prepare the roof by power washing to remove loose or surface rust flakes. You may have to wirebrush any extremely rough areas. Once prepped, it should be primed as recommended by the coating manufacturer you choose.
As a basic rule of thumb, fibered coatings are used when strength and resistance against tears and abrasions are the main focus, and non-fibered coatings work best for applications that require greater flexibility and the ability to stretch the material farther over a surface area. Non-fibered may suit your situation the best since your coating over primed metal.