We performed a series of studies to designate the algae which is the main factor of these soiling, and to establish methods which can evaluate materials in regard to soiling of external walls. The following results became clear after this investigation. Soiling of external walls is usually caused by the fresh-water algae named Protococcus Viridis Agardh. We can evaluate the degree of growth of algae on the external materials by applying two independent tests, one is outdoor ex-porsure test and the other one is indoor accelerated test in which we diviced techniques for supplying moisture, nutriment and algae to samples.
