High pressure roof cleaning should ideally be followed up with an Algae treatment. Algae establish it in the nooks and crannies of a roof and given the correct temperature and rain can grow again from a very tiny piece.
We have found it effective to follow up a good high pressure roof cleaning operation within a few days with Algae treatment. One eco friendly product on the market in South Africa is Algae Kleen. It is totally harmless to the environment and is applied via a knap sack sprayer. The best