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Roofing/Hot Tar vs Torchdown


We have a very low sloped roof (18" in 20 ft) which is hot tar roofed. It's getting close to 30 years old and should be replaced. It's also in the shade and constantly grows moss which requires pressure washing once a year. Can I hit the moss with a bleach solution then pressure wash without damaging the hot tar? Would hot tar or torchdown be a better under these circumstances? What minimum slope must torchdown be excluded? Thanks in advance.


You should not pressure wash the surface of the roof membrane unless you are using a wide fan at a good distance, otherwise you are doing more damage to the roof than the moss.  It the moss is not visible, you can leave it run for 3-5 years or until it starts to obstruct drainage.  Stiff bristled broom and a good commercial leaf blower should do the trick.  No bleach.

If you have received 30 years of service from your current roof, I would replace it in kind.  That is excellent service, and with no ill effects, you should expect the same from the new one.  Don't experiment.  Torch down and hot tar can all be applied down to flat, equally performing in a watertight manner.

Best of luck, but you have already had it with 30 Years of service from your old roof!!

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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