The Roof Cleaning Institute Of America Training & Certification Forum
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I am Chuck Bergman Roof Cleaning and Pressure Washing 941-698-1959 -- WATCH THIS VIDEO OF ME PROPERLY CLEANING A ROOF. https://www.youtube.com/edit?video_id=rQjszicmWWA

Never allow a pressure washing machine to be used to clean your roof! They run 3000 to 4500 psi. Even 100 psi [ pounds per square inch ] will blow the granules off your shingles and flip the tabs loose! Roofs should be chemically cleaned, as the manufacturers and the ARMA roofing manufacturers association says.

Chemical cleaning, called "No-Pressure" "Non-Pressure" or "Soft Wash" does not involve the use of any pressure for cleaning. I have cleaned roofs using only the Soft Wash method since the mid 90's. Pumps to deliver the manufacturer specified chemical mixture range from 45 to 100 psi - but again, that pressure is just what is needed to bring the fairly thick roof cleaning agent mix, from my trailer, in the street, up to the top of a roof. By the time it travels through my 230' of 1/2" ID hose, through a gun and through a spray tip, the pressure is around 25 psi and even that has no part in the cleaning-just delivery.

So, why have your roof worn out, by having it blasted with hundreds or more often thousands of pounds of pressure, when it's just not necessary? 

Spare your roof the wear and tear and just follow the manufacturers instructions here - http://www.bergmanpressurewashing.com/Manufacturer_Specifics.html