Power washing a roof is usually not a very good idea. High pressure cleaning will often result in the loss of shingle granules, which are crucial to the health and effectiveness of your roof. Aggressive power washing can even result in water intrusion below the shingles, and places like skylight edges are especially susceptible. Power washing will usually remove most of the algae, but is it worth it if it removes part of your shingles in the process?
There are some roof cleaning chemicals on the market now that require the use of medium-level pressure. This method involves the use of a power washer, but instead of a high-powered wand at the tip, it has a device which helps to disperse the water pressure through three separate nozzles. While this method is better than using high pressure, you should be aware that each nozzle is probably still delivering somewhere between 800 and 1300 psi of water (you garden hose psi is under 100). It is quite possible that this method will still result in the loss of some surface granules, and thus could contribute to compromising the integrity of your roof over time. Why would you want to lose any shingle granules if you don't have to?
My advice? Skip the fancy roof cleaning products that require pressure, if possible. Your best bet for safe, effective roof cleaning is to utilize chemicals that will kill the algae on your roof without the need for any pressure whatsoever. This method simply involves spraying on the chemical solution, allowing it several minutes to work, and then rinsing off the dead algae with water from your garden hose. Think it's not possible? Think again! Our next post will detail exactly how this is accomplished.