Q: Why are there dark streaks on my roof?
Q: Why are there dark streaks on my roof?
A: Dark streaks on your roof are caused by an algae species called gloeocapsa magma. It is not generally harmful, though it can be a sign of excessive moisture and even cause granule loss over time. You can lighten the dark streaks on your roof using a solution of bleach and water, or prevent it by choosing algae and streak-resistant shingles.