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The main instances I have seen of this have involved trees overhanging the roof which lost sap that then adhered to the roof and ended up holding dirt and even some biological growth. In most cases, this can be power washed.
Many of the coatings used today on metal roofing are inherently quite resistant to supporing biological growth.
However, in areas subject to high heat and humidity and much airborne fungus, that stuff can sometimes stick to anything.
This will usually, if it does occur, have a "mildew" appearance and will not be actual "moss" as you have asked about.
I hope this helps. In all, metal roofs are far more resistant to this than other more porous roofing materials such as standard shingles and tile.
4/28/2004
Isaiah Industries, Inc.
4/29/2004