Most homeowners are unaware, that the roofing manufacturers specify exact cleaning methods and chemicals, that may be used on their products, without voiding their warranties.They all say "DO NOT PRESSURE WASH"
Seeing as how the roofing manufacturers not only make the product, but also stand behind it with a warranty, common sense dictates we listen to them.
"We" being you the homeowner and myself and others, who are in the business of cleaning roofs as contractors.
Here in Florida, for instance, the black stains that appear on a roof are the living organism ( Gloeocapsa Magma algae ) that will gradually reduce the lifespan of the investment you made in your home.
A roofs life expectancy, left infested with algae, can be reduced by as much as 50%. The appearance of a heavy infestation of this black algae can lead the homeowner to believe roof replacement is necessary.unfortunately, many [not all, many roofers are very honest] roofers called by a concerned homeowner, will readily agree that you do indeed need a roof replacement, knowing full well that it just needs to be cleaned. The difference is in spending thousands or hundreds of dollars.
To read quotes from a number of roofing manufacturers and have the ability to actually click on the roofing manufacturers links and read those same words on their websites and technical bulletins, please visit my website and click on "Manufacturer Specifics" First, you may choose to watch the slideshow on the main page? http://www.bergmanpressurewashing.com/