When bricks are produced, all organic components are incinerated.
However the bricks are exposed to a large amount of particles outdoors
which combined with the damp bound in the bricks gives the moss spores
nutrition.
Moss grows as a rule in gaps, joints and fissures. Thereby they get
a good hold and only stop recurring when all of the surface's pores
have been sanitized and all remains of old moss –and its spores-
have been totally removed.