It is probably difficult to see, but rust may be forming on your roof right now. Rusted asphalt shingles are usually the result of corroding flashing, chimney caps, vents or fasteners. All these materials are made of some sort of metal and as we all know, metal can rust when exposed to moisture.
Rust on an asphalt roof is typically confined to the area immediately surrounding these metal fasteners and vents. You typically see elongated, orange, streaky rust stains stretching downward from the metal source. That’s because as rain falls off your pitched roof, it draws water and therefore rust, downward in streaks.
And that’s just for asphalt shingles. Asphalt in and of itself is not susceptible to rust because there are no metallic components in it. Just the nails that were used to install them which, should be treated to resist rust anyway.