While researching the causes of roof fungus and mold I leanerd that the spores from the fungus spread from roof to roof. If one of your neighbors has a dirty roof, you are likely to get the same problems on your roof.  I always encourage my customers to talk to their neighbors about getting thier roofs cleaned also. I would be dissapointed  if a roof I just cleaned was re contaminated by the nighbors moldy roof.  I offer a discount in these situations since the Jobs are just next door to eachother..  I even sweeten the deal witha discount if you refer other services of mine to your neighbors..  Remember I offer,  Window cleaning, power washing, roof cleaning, deck spraying, concrete washing, and more..