If the roof is just pressure washed with no biocide or sealer protection applied, then it can take as little as one year before it starts to accumulate grime/moss/algae again.
If cleaned and biocide treated, easily three years, and possibly much longer (it really depends on the location of the property), before it starts to look dirty again.
If cleaned, biocide treated, and sealed, it could be anything up to ten or more years before it requires attention.
Although roofs that are heavily shaded, surrounded by tree’s or with an open country aspect will always deteriorate faster, unless subject to ongoing maintenance procedures.
Posted in: Roof cleaning