>>13355738A few ideas off the top of my head that may all play a part:
>The rotation of the earth in relation with the sun causes differing atmospheric conditions (humidity and temperature) over the course of a day. The angle of light (or shadows) along with the varying atmospheric conditions may make one side more favorable for that species of moss given the roof geometry.>Wind causes more potential nutrients to accumulate on one side versus the other.>Santa always pees down the same fold of your roof every Christmas 
