The biofilm dies as the product dries on the roof even if it rains shortly afterwards. Nature then takes over : Each specie will disappear at it’s own pace : Immediately for green algae, moss remaining thallus will disintegrate in time, dark metabolites will need rain to thin down : Allow 3 weeks to notice a difference, approximately 3 or 4 months for a marked difference and a little longer for the last few lichen to fall off.
The ACP biocide works on a biofilm by “cationic activity”. A concentration of 250 ppm – part per million – will sanitise a biofilm of algae, bacteria, fungi etc. The concentration in use is at a dilution of 1 to 20 is 80 times the lethal dose of 250 ppm. The safety margin is large enough to cover some adverse site conditions. On porous substrates the salts will dry deep and release slowly giving plenty of time for the tiles to recover their clean appearance and stay that way for predictably 3 years and often more.
After this the biofilm will return at the rate it came in the first place if the environment ( proximity of trees ) has not changed.
After the works there is a certain amount of dried up biocide on the roof and the gutter. The first good rain will wash the excess and the water butt is reconnected afterwards. The cationic activity drops as the product enters in contact with biomass. A drain will sanitise over a short length. There is no long footprint to the environment. Do not drain into a pond.
There is no timed guarantee to ask for cleaning a roof. If granted it would be void anyway since it may only be given when the risk at arms length.