Why do I have black streaks on my roof?
Black streaks are usually algae that is spreading down the roof. In time it can cover the entire roof. Algae can easily be removed by professional washing at low pressure (soft washing). This is complicated and dangerous for someone without proper training. The roof mix is much stronger than that used for vertical walls.
The roof cleaning process can extend the life of your roof if done correctly. Be careful not to use a pressure washer as that can void the warranty on shingles. Weak roof mix can remove mildew and mold. For black streaks, be sure the roof mix is strong enough to remove algae.