Instead of having your roof blasted clean with a pressure washing machine [ also called power washer, pressure cleaner ]
Have it cleaned, using the method described by many roofing manufacturers and also by the Asphalt Roofing Manufacturers Association.
It is a mild 60 psi [pressure washing machines run 3000 to 4000 psi] application of the cleaning agents described by those manufacturers and by the ARMA.
Watch me in a couple of Youtube videos, as I clean roofs with this far superior method! This method can not get your roofing warranty voided, like power washing it surely could!
Please visit my website to see more videos and a slideshow.
Also at my website you can go directly to the mentioned manufacturers websites and to the ARMA and read for yourself-they say "DO NOT PRESSURE WASH" http://www.bergmanpressurewashing.com/