Roofs are either organic or inorganic depending on the material used.
Brick roofs are inorganic in new and dry condition but the surface becomes
organic when it has become damp or dirty. Tarred roofing is organic
–tar is produced of the resin of fir- and the board is produced
from wood fibres.
Roof bricks soon or later –often very soon- gets affections since they lean in narrow angles and are as well exposed to the climate and vegetation. The roof gets dirty and damp which gives spores and algae nutrition.
Affection by algae furthers when the spores gets nutrition –when they have established themselves - and they grow fast. The affections do not stop until all growth, all pores and all residues have been sanitized.
Nevertheless the whole surface of the roof must be totally free of algae spores.