I had an old prefab fireplace in and I ripped it out. The prefab fireplace had a tripple wall pipe that has about 1.5 inches of air space btween each pipe. This runs upto my cathedral ceiling. I am planning on using excel pipe (6")with my new wood stove purchase w/ a cathedral ceiling box. My roof is a rubber membrane roof and the old flashing has the membrane on it. I do not want to make ANY holes in this rubber roof membrane because I don't know much about it and don't want to screw it up. Right now there is an existing chimney that is 8 inches in diameter and the whole flashing body is about 16" in diameter. Pictures below should tell more of the story v 8" chimney pipe What is the best practice here for install?