This chimney had been absolutely covered in tar and neglected for a long time. Everything within 3 feet of the rock had rotted. We replaced the bad rafters and plywood, fabricated the coated aluminum, cut a groove into the rock, then created a floating flashing that required zero upkeep. 

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The bad news is that the homeowners eventually had to have the mortar replaced between the rocks as water leeched in from the top.