I’ve got a 6″ B-type doublewall gas vent running up the outside of the house.  This is the exhasut vent for the new boiler that was just installed.  Pipe is 10 feet long, and the part that runs up the wall is OK. The issue I have is where it penetrates the flat roof overhang.  The overhang isn’t very wide, and has some extra framing inside, so I don’t quite have 1″ of clearance between the house and the fascia, more like 5/8″ to 3/4″ on the house and fascia sides

Since the doublewall is good for 1″ of clearance to “combustibles” I was wondering if putting something against the wood like sheet metal or 1/4″ hardiboard would be a good idea, and if so, what would be best.