Roofs can be organic or inorganic depending on the material used. Bricks are
inorganic when they are new but gain organic properties when they have
become damp and dirty. The tar in tarred roofs is organic, as well as
the paper's wooden fibres.
Roofs are exposed to pollutants, dirt and litter. It furthers
collection of damp and therefore affections by Cladosporium –black mould. The very small spores get a hold everywhere where it is damp and it is the most severe affection we have.
When black mould have gotten a hold, it is very hard to treat. Disinfection of the roof and sterilizing of all spores and growth is necessary. After this are all affections by Cladosporium becomes impossible for four years.