There are four reasons: The longer the cleaning solution is is contact with the the roof, the more thorough and longer lasting the clean will be. Waiting for rain to wash away the cleaning solution and the organic contaminants on you roof minimises the impact to the environment. Rinsing concrete tiled roofs, especially older roofs where the tile coating is oxidised, will result in the loss of colour on the roof. Increasing the potential for water ingress into the roof similar to that with pressure cleaning.