Best way to eliminate roof algae

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Q: We have a house that was built in 1995, in a neighborhood of other houses all built around the same time. Our house has a gray composition shingle roof that has developed large dark stains in many areas. Other homes in the neighborhood with gray or charcoal roofs also have it, but the ones with light-colored roofs don't. Is it algae? If so, would zinc strips help? --Shirley L., via e-mail A: It's very likely that the staining is indeed some form of algae growth, and I contacted the Asphalt Roofing Manufacturers Association to confirm that this is their opinion as well. As to how to deal with it, their recommendation is as follows: "Algae discolorations are difficult to remove from roofing surfaces, but may be lightened by applying a solution of chlorine bleach, trisodium phosphate (TSP) and water. Solutions for these ingredients may vary between shingle manufacturers and depend on the amount of discoloration. Solutions range from one cup TSP, one gallon bleach and five g...