The best way to clean anything is by following manufacturers suggestions. That is the method we use. If you go to the "Roof Cleaning" headline on the front page of
http://www.thompsonpowerwashing.com/ you will see a link to the Asphault Roofing Manufacturers Association's bulletin stating the method to clean roofing shingles. In a nutshell they suggest using sodium hypochlorite, trisodium phosphate, and water. We take it a step beyond and add our own foaming agent that helps the cleaning agent cling to steeper roofs. They also state at the bottom NOT to pressure wash a roof. Pressure washing a roof can lead to long term damage. If you've ever cleaned out gutters you may have noticed large amounts of roofing granules. These granules become dislodged naturally by rain and weather. So imagine what even low pressure can do. Once the granules are removed the underlying membrane of the shingle becomes exposed to weather, and more importantly, UV. At this point the shingle will start to dry out, crack and split. Then its time to find a roofer. Chiching$$$
Thompson uses only NON-Pressure method of cleaning roofs. We were trained in seminars by veteran roof cleaners in Florida where roof cleaning is BIG business because of long hot and wet seasons.
