Black streaks are caused by an algae known as gloeocapsa magma. Airborne spores deposit it on your roof, where it soaks up plenty of sunlight for photosynthesis and munches on the limestone filler in your shingles. Having a professional roof cleaning completed every one to two years can help extend the life of your roof by 10 to 20 years. A pro can clean your roof in conjunction with other exterior projects, such as gutter cleaning or whole-house power washing. Black algae and similar types of issues are also very common on asphalt shingle roofs that are not properly maintained, all of which mean that cleaning your roof on a regular basis should always be a top priority in the life of any homeowner.