Why is algae growing on my roof. Algea growth or moss on a roof indicates poor drainage. Since a roof’s primary function is to drain water, algae growth means that water is hanging around. That is a sign that water is pooling on the roof rather than draining off creating moisture pockets. When you see algae or moss on roof shingles real roof damage can follow. A lot of algae is a signal it is time to contact a roofing contractor to replace your roof before major roof leaks occur.