Angie's List puts out a great article on Roof Cleaning!
Your homes curb appeal is being directly affected by the "black streaks" all over your roof. Experts are saying that although these black streaks cannot be stopped permanently a proper roof cleaning can eliminate the mold causing them, for as much as five years. According to highly rated roof cleaners on Angie's List, a roof cleaning could save you thousands of dollars versus the cost of a new roof replacement. They are also very clear about the negative affects of pressure washing your roof. These professionals also suggests asking what kind of chemicals will be used, how long will it last and for proof of Insurance.
What's causing those black streaks on your roof | Angie Hicks