The roof of your home is under constant exposure of storms, intense sunlight, debris, and rainfall. Moreover, the roof can fall prey to the accumulation of ice and water if you live in an area where snowfall is common. Due to standing water, algae and grime may also form on the roof.
Original Source: https://www.linklocal.com.au/articles/things-you-should-know-about-roof-cleaning-as-homeowner