lichens on roof

hey group, I have moss and lichens growing on my asphalt roof shingles, how do I get rid of it?
KDW
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This old house had a segment on that recently. Check this link...
http://www.thisoldhouse.com/toh/tvprograms/asktoh/showresources/episode/0,16663,1110925-1127488,00.html
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