When you encounter a customer, who asks "Why should I clean my roof", here is one of many reasons I have found to work well. Start out by asking them this question " You care about your neighbors, don't you" ? When they say "Yes, I care about my neighbors" spring this on them ! "Just as we will not allow Dandelion Weeds to take over our yards, because they will soon spread to our neighbors yards, the infection on your roof will soon spread to your neighbors home, and your own property! We can see the spores of the Dandelion blow in the wind, but we can not see the microscopic airborne spores of the roof algae, that are spread by wind, rain, and roof visiting critters like birds and squirrels, etc, etc."