Hendrex Tile Roof

Has anyone cleaned a Hendrex tile roof? They are concrete tiles that look like ceder shake. I need to give a quote on one that is about 5500 square feet. We will need a boom lift to reach the top. He does not want me to use anything that would whiten the tile. They have aged to a gray. The house has copper gutters on it also.
Does anyone have a suggestion on what to use? I need to do a test for him before the whole roof. I am going to test Oxy cleaner.
 
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