Moss
Moss growing on any surface will be greater where areas are shaded and exposed to damp weather. The amount of sunlight is a determining factor of how much growth can amount on your roof. The more sun the less moss.
Moss is more than a cosmetic issue, it holds moisture on roof surfaces speeding up the wear of the shingles. Also freezing increasing frost damage as well as lifting the shingles allowing wind and water under. The mosses roots eventually penetrate and separate the roof materials causing damage.
