Simple question and probably stupid question...but this forum is generous

[JPapiPE]

What is the clearance for HT A-type chimney as it goes thru the roof and attic? I have an insulated attic and think the clearance is 2" I also have a 1/2" decking (roof sheathing) beneath my metal roof. and suspect the clearances should be 2" around the wood roof sheathing, but could come in contact with my metal roof. Is any of this correct?

 

[Highbeam]

My installer cut the roof hole (sheeting and comp roof) large enough to provide the 2" required clearance. The 2" is clearance to combustibles so you could make a case that the metal part of your roof is non-combustible but the sheathing has to be cut anyway.

 

[JPapiPE]

When you say 2" clearance you mean 2" all the around the chimney right? So the total hole would be 4" larger than the pipe diameter. Example a 6" pipe would require a 10" hole from my combustible roof sheathing and also a 10" clearance where my chimney passes through my insulation? Giving it 2" on all sides. Is this correct?
Thanks so much , I do think i understand this principle