A. The discoloration is actually caused by a pigment produced by a species of blue-green algae known as Gloeocapsa. It appears throughout the U.S. in warm, humid climates, but it thrives in the Gulf states, along the Eastern Seaboard, and in the Northwest. It exists on darker roofing, too, as you guessed it might, but the stain is not as visible as it is on white and pastel roofs. Despite its unsightly appearance, algae growth will not affect the durability of the shingles.
Algae discoloration is difficult to remove from roofing, but it may be lightened with a solution of one part chlorine bleach to three parts water, with some trisodium phosphate added. This treatment will lighten the staining, but it will not completely remove or prevent the algae from returning.
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