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FYI: No inspections or permits, I live in the middle of mostly nowhere. But, of course want to do this "right".

Details: While face on the right is 9/12, face on chimney side is 11/12. This is a hip roof, all framing sheeting is done (except Chimney). This chimney needs to be approximately 12ft taller than the eave to pass code ("10-3-2 rule" for chimneys). It will get a 6", double-wall insulated pipe with 2" clearance requirements. Thus, i plan the chase to be 1'x1'. I am NOT installing the pipe, because i'd feel more comfortable if a pro does that. I am an experienced framer, but have never framed a chase which extrudes this far atop the roof. I DO get strong winds on occasion and this house is already VERY tall (42' to peak). Needs to withstand wind gusts of 100mph (we never see more than that in 30 years)

My question:

I need a 12ft tall, free standing 1'x1' structure. Is this too tall to just do standard framing with post corners, double top and bottom, then screw on 3/4" sheeting all 4 sides? I could run a 16' 2x4s and fasten to the exterior wall + inside porch truss network. Am I in the metal / angle iron or even engineered territory considering height and wind shear?

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    You will be fine with what you are suggesting, though you might want to make the chase more rectangular than square, for aesthetics, like Michael Karas said. Pre-fab metal fireplace flues are framed in that way all the time.
    – SteveSh
    Commented Jan 25, 2021 at 1:50

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I did something similar to replace a chimney that was damaged in an earthquake and had to be replaced. Instead of building a new chimney I replace the old removed flue with 10 inch inside diameter stainless steel triple wall insulated stack piping inside of a tall chase. I built the chase much larger than your proposed 12"x12" size. My chase was, as near as I recall as it was about 26 years ago, about 30 inches parallel to the wall and 16 inches out from the wall. My structure was 16 feet in height so I framed it in two ~8 foot sections with conventional double plate between the vertical sections of studs.

I see no need to sheet out the sides with 3/4 inch material. Whatever sheathing is currently on your house should be a guide whether that be 1/2" or 5/8" plywood or OSB. In my case I used 1/2" plywood because it was a good base for the stucco exterior that was applied afterward.

So the crux of my suggestion is that you go a bit larger than your 12x12 size. It will be stronger. It will also be better looking in my opinion than the tall skinny box.

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With 3/4" material screwed to it it will be crazy strong. stronger than those engineered beams that only have 1/4" material for the web.

Talk to the flue installer to see if they want it fully enclosed or only 3 sides enclosed for the install.

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