Assuming it is rubber material. I wash mine 2 or 3 times a year. Prior to washing the roof I take my plastic covers off my air conditioners and clean my air fins.
Then I inspect the entire roof, including the edge trim. I inspect all cut outs where vents, antenna, or anything else comes up through the membrane. If I find any places where the sealant looks weak or worn I re-seal it.
IMHO the roof doesn't need to stay perfectly clean, but you have to pay attention to it and keep stuff from growing up there.
2014 Montana 3725RL (Goodyear G614 Tires, Flow Thru TPMS)
SPENDING THE WINTERS AT OUR HOME IN SW FLORIDA. THE REST OF THE YEAR SEEING THE U.S. FROM OUR LIVING ROOM WINDOW!