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Roofing/asphalt shingle roof cleaning


Dear Mr. Ryan:

I am writing because I have an ongoing issue with dark matter (presumably mildew or mold) accumulating on a low sloped asphalt shingle roof of my home (over the garage).  I had it cleaned off with a chemical (J-something), NOT powerwashing, in fall 2009.

A contractor just told me this cleaning can damage the roof.  Yet the roof has blackened again (presumably with mildew and mold) because there are two downspouts that empty from gutters above it, right onto the roof.  Also, I live in a very rainy/snowy area, a small creek is on property adjacent to mine, and there is a lot of foliage on this property.  In fact, all around the house, the siding is covered with green and gray mildew.  The dark matter also accumulates on the concrete driveway in front of the garage's roof.  The contractor said the driveway accumulation is from asphalt from the roof shingles.  Is this true?

Mostly, what I need to know is whether cleaning the roof again with chemicals could damage the roof, and if so, how much.  I would appreciate your opinion on what to do because I am planning to sell my home.  The home would look better if I clean the roof, however, I do not want to risk damaging the roof with chemicals.

The contractor suggested cleaning the gutters above the roof and powerwashing the siding.  Please be so kind as to share your opinion on whether chemically cleaning the roof is potentially damaging, and answer my question about whether the driveway accumulation is asphalt shingles that have flowed down with water.

Thank you in advance for any help you give on this matter.




Go ahead and clean the roof with the chemical that worked previously.  The black stains are an algea that feeds on the filler material added to the shingles.  The chemicals should not adversely affect the shingles.  An oxygenated bleach type product can work nicely. Adding moss control strips of zinc may also work to prolong the affects of cleaning. Capture the runoff from cleaning to prevent damage to your plants and the environment.  

I would have the contractor clean all the gutters, powerwash the siding, treat the surface of the roof with a cleaner, and power wash the driveway as well.  The driveway most likely has a moss on it, but hard to say without seeing it.

Bruce Ryan II, RRO
Professional Roof Consultants, Inc.  


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Bruce A. Ryan II, RRO


I offer solid knowlege of all types of Commercial and Residential Roofing, Waterproofing, and, Building Envelope systems. Experience ranges across low slope and steep slope roof systems. I am also well versed in matters regarding condensation and ventilation. I enjoy donating some of my time and knowledge to the betterment of others.


Bruce Ryan has 20 years of roofing, waterproofing, and building envelope consulting experience with PRC, with 5 years of commercial roofing experience prior to joining the firm. He became Vice President of the company in 1998. Bruce Ryan plays a key role in the development of practical, long-term roofing and waterproofing solutions, along with implementation of on-site forensic studies. Bruce also has a high level of experience with regard to the impacts of roofing materials and construction for demanding clients with heavily occupied structures.

Oregon Construction Contractors Board Construction Specifiers Institute National Roofing Contractors Association The Institute of Roofing, Waterproofing, & Building Envelope Professionals Oregon Board of Investigators Installation Masters

University of Maryland BS Business & Administration Registered Roof Observer - RCI Private Investigator Certified EIFS inspector - Northwest Wall and Ceiling Bureau Certified InstallationMaster™ - The Installation Masters™ Training and Certification Program (developed by American Architectural Manufacturers Association)

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