Moss and lichens can be really a huge problem for roofs, especially in wet and cool geographic areas. Neglected, they can drastically shorten the life of the roof.
1. First, prevention is the best way to fight against moss and lichens. Buy copper rolls from a roofing supply company, and cut the thin copper material in long stripes. Place these thin, long copper strips near the top of the roof. When it rains the runoff from the copper will act like a poison and will kill the moss. Instead of copper stripe, you can use a simply copper rod. Zinc works well too.
2. Washing the roof with a solution of laundry bleach will kill the moss grown on the roof.
3. Dusting the roof with zinc sulphate monohydrate powder (Corry’s Moss B Ware ™) will clean your roof in one month. Rain will dissolve the powder making a poisoning solution for moss and lichens. You should dust your roof with this powder annually. You can use a garden hose to spray the roof, if not raining. If you wish, you can make a solution from this powder (3lbs per 5gallons) and then you can water the roof.
Let them dry, and then sweep them off cleaning the roof.