Why asphalt shingles turn black?

It's algae! Not from a swimming pool, a specialized algae that loves limestone (your shingles are full of limestone) Algae needs three criteria to grow. Heat, Nutrients, and moisture. Gloeocapsa magma thrives in warm humid environments and normally appears on the North slope where additional shade and moisture support its growth. Gloeocapsa produces its own energy source, and feeds on inorganic materials like calcium carbonate, an asphalt shingle filler material. Feeding on inorganic material distinguishes algae from fungus and mildew, which require organic material to survive.

Thus, elimination of algae may reduce fungus and mildew accumulation; so will removing loose debris from your roof deck and gutters. (Leaves, twigs, and tree bark.) These roof stains are seen in nearly eighty percent of the United States in various degrees. In Florida your roof may be completely covered with algae in as little as four years. Also, this particular algae strain has become hardier and is able to migrate to less humid environments than it has in the past. In areas where it traditionally has been found, staining is manifesting earlier and more severely on a greater number of roofs. Treatment early is essential, otherwise your 25 year roof may only last 12...