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https://www.metalroofing.com/spirit/comment/3634/find/
Generally, the non-porous nature of metal does not promote or support the growth of fungus which is what those stains are.
However, over a long time, some of the cheaper paint systems can break down a bit. However, one of the ways they usually break down is to start "chalking" a bit and chalking actually will "self-clean" the panels a bit.
However, if there are trees overhanging the roof, those trees will drop sap and various dirt, etc which will stick to any roof and, over time, it can support fungus growth.
I guess my point is that metal is considered pretty resistant to this but nothing is infallible.
8/5/2004
8/6/2004