Depending on the materials roofs can either be organic or inorganic.
New bricks are inorganic but gains organic qualities when it has been
exposed to moisture and dirty. Tarred roofing becomes organic with tar
from the firs resin and damp. Pasteboard is produced from, among other
things, wood fibres –which also has organic qualities.
Lichen on roofs is an extensive problem since they bind strongly to the roof, grows fast and is the most difficult affection to remove. There are risks involved when treating affections that are high up and it is also complicated to remove lichen from a roof.
If you have spots on your roof that look like bird droppings –especially on roof tiles- is it most likely to be lichen.
The fungus in lichen makes an extensive network of threads –the mycelia- that penetrates the roof. By then the lichen have a strong hold on the roof, can endure harsh conditions and will not stop growing until the entire roof, all spores and remainders have been completely sanitized. Sterilization. Disinfection.