If you’ve noticed dark brown or black streaks on your roof, especially after our humid Texas summers, you’re not alone. Many San Antonio homeowners mistake these stains for dirt, mold, or tree debris. In most cases, however, these streaks are caused by roof algae, specifically, a type called Gloeocapsa magma. While common and mostly cosmetic, algae growth can impact your home’s appearance, energy efficiency, and potentially mask early signs of roof deterioration. This article explains what roof algae is, why it is so common in San Antonio, and what homeowners can do to manage and prevent it, with the help of a professional roofer when needed.