
I would not suggest putting zinc or copper on the roof. There are products like RoofWash and Wet N Forget which can be used to help remove any accumulated biological growth.
We're in Washington state where moss is a problem, even on a metal roof. I know a sacrificial zinc rod can be used on asphalt shingle roofs to create an unfriendly environment for moss, but I suspect the dissimilar metals would cause galvanic problems on a metal roof. Are their any safe moss/lichen deterrents for metal roofs?