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Thanks Charles.
I think I may have scared them a little with the price.  Their other concern was the fact that the cedar would look new once cleaned, rather than the weathered gray.  I gave them the option of several products that could be applied after the cleaning to return the cedar to the weathered look, and actually help to prevent the roof from being covered in moss and algae again.  I'm not sure what will come of it, but thanks again for the lead.