Let’s start with discussing what the black stains on the roof actually are. In most cases, the black stains are simply algae that is living off of the minerals found in different types of roof shingles. Algae flourishes wherever there is moisture. You will see more black spots on north-facing roofing slopes or shaded areas where the sun is less prone to shine.
Algae does not damage your roof but it isn’t pretty. One simple way to get rid of the algae is to simply bleach it away. Start with a pump sprayer and mix one part bleach to two parts water. Be aware that bleach will damage foundation plantings so be sure to rinse them before and after you are finished cleaning the roof. Scrub the roof shingles gently with a soft brush with an extension pole. It is best NOT to use a pressure washer because that can damage your shingles. Be sure to use a safety ladder instead of climbing on top of the roof. It is not an easy job but you can do it.
Once you have cleaned the algae from the roof shingles, place copper strips or zinc near the edge of the roof. This will cause any rain water to wash over the strips and pick up ions from the metal, inhibiting the algae from returning. When it is time for you to get a new roof, be sure to ask for algae resistant granules.