Some vents/ducts protruding thru roof are steel. Steel rusts, and this rust runs down on a pitched roof and discolors the shingles. If you have this problem then here's an idea in how to clean the shingles. http://www.diychatroom.com/f9/rust-stain-roof-2997/
In case in future the above link is brokened here's a review/quote from that link "THank you......thank you.......thank you.......thank you.....OXALIC acid worked wonders. My stain is GONE. What an eye sore that was, I really hated it. THe house is really nice looking, but the big stain on the roof would always catch my attention from day 1 (I bought this house in January)."
Some vents/ducts protruding thru roof are steel. Steel rusts, and this rust runs down on a pitched roof and discolors the shingles. If you have this problem then here's an idea in how to clean the shingles. http://www.diychatroom.com/f9/rust-stain-roof-2997/ In case in future the above link is brokened here's a review/quote from that link "THank you......thank you.......thank you.......thank you.....OXALIC acid worked wonders. My stain is GONE. What an eye sore that was, I really hated it. THe house is really nice looking, but the big stain on the roof would always catch my attention from day 1 (I bought this house in January)."
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