If a roof is strongly dirtied, a roof cleaning should be carried out to protect the roof in the long term against damages.
Contaminations on a house roof look not only neglected, but can also destroy the protective coating of the roof tiles. For example, this damage can happen by the mosses which have settled on the house roof.

The mosses store water which dissolves bit by bit the surface layer of the roof tiles. If the surface layer of the roof tiles is cracked or is porous, the humidity can penetrate into the roof tiles which can cause blowing offs or tears in the roof tile in frost. As a result the roof tile must be substituted mostly, because a cracked or jumped roof tile offers no more sufficient protection for the lying underneath roof layers, as for example the isolation of the roof framework any more.