What Are the Black Streaks on Your Shingle Roof? If you're a homeowner, you may have noticed unsightly black streaks running down your roof. While they can make your home look dirty and unkempt, these streaks are more than just a cosmetic issue. They’re usually caused by certain types of algae, mold, or mildew that thrive on rooftops. Understanding what causes these streaks and how to deal with them can help protect your roof and keep your home looking its best. What Causes the Black Streaks? The main culprit behind black streaks on shingle roofs is a type of algae called Gloeocapsa magma. This airborne algae spreads through spores and can latch onto your roof, especially in humid or damp climates. Once it takes hold, it feeds on the limestone filler commonly used in asphalt shingles. Over time, this algae grows and leaves behind the dark streaks you see. In addition to algae, mold and mildew can also contribute to discoloration on your roof. Mold and mildew thrive in areas with high