Everyone has seen moss, it grows on the surface in damp areas. It uses sports to reproduce and because it has no roots, it can spread on many surfaces, including your roof. Algae on the other had is typically an aquatic plant that grows in damp environments. Because moss and algae thrive and reproduce in damp and shady environments, they can reproduce and spread on a roof. It specifically appears on the north side of a roof, as the north side receives the least amount of sun, or under tree branches that overhang a roof. Over time, moss and algae can damage a roof and can cause it to degrade.