fungus and algea are living organisms. The require several things to live: moisture and some shade and some sun.
You need to remove what ever is causing this to occur. Do you have allot of tree branches hanging over and around your roof? Do your shingles stay wet along time after a rain due to tree canopies?
You need to get some circulation going around and on your roof. You need more sunlight to hit on the roof area. you need to remove the things that these guys live on.
Jay is right on, the bleach will do the trick, so will vinegar, white kind. vinegar will not kill vegetation, but is not as strong as bleach.
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