There are several products on the market specifically designed to remove algae stains from roofs, such as Wet & Forget Outdoor. Make sure that the company knows which chemicals to use and is licensed with the department of agriculture for the treatment as some chemicals can shorten the structures life and can be toxic. 

Working on a roof can be dangerous, and it becomes even more hazardous when wet, so be sure to let the experts do it, I have heard too many horror stories of homeowners trying to do their own roof work. 

Bleach can damage or kill other plants in addition to algae. WE have an organic option called Safer, works amazing! Safe to use around gardens and plants. 

To keep algae from coming back once it is clean, have regular treatments done at least once per year from a good company who knows how to navigate the building and minimize foot traffic. Zinc strips do not work.