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riray wrote:

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I have a roof with bad black stains on it from a power vented chimney. The stains seem to be oil from the boiler for the building. 

Does anyone have a solution that would clean this?  I would assume that multiple applications would be necessary for full removal. Looking for your expert

any ideas?

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 Sure we do   This stuff will do the trick http://www.newagain.net/psjs_faqs/06089406.shtml

The standard SH and Water roof cleaning mix with some soap is not effective on these types of discolorations. 

Normally, I do not suggest S Hydroxide based roof cleaners, but they are effective in removing old grease and oil type stains, to a point! 

It is doubtful if you can ever perfectly remove all this stain,  but you can get it to look much better.

Follow the instructions, and keep rinse off pressure to a bare minimum, or you can harm the roof.