Why the difference between fart-fans and DWV vent roof flashing options?

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diy-goonery

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I apologize if this is obvious to the pros, but I've spent a lot of time searching this the past couple days, and haven't found any answers. I'm plumbing a bathroom, and have already put a DWV vent through the (metal) roof. I used one of those rubberized/epdm-type boots, combined with some butyl tape, sealant, a bunch of screws, and felt the connection was totally bomber, super water-tight, not worried about it for 10 years (it doesn't get much UV exposure).

However, now I'm working on installing the vent for the fart-fan through the same metal roof. Unfortunately, due to geometric constraints, going through the wall is not an option. I have been unable to find any caps for fart-fans that have a rubberized-type flange that is secured the same way. It seems all the goose-neck style fart-fan caps have a simple metal flange that seem like wouldn't be too complicated to integrate with an asphalt shingle roof. I've watched a few youtube videos of DIY'ers installing them on metal roofs, and it seems kind of hacky, using angle-grinders to cut a slit in the metal roof, trying to slide the assembly up into the slit, etc. It seems for new construction, there are some strategies for having the cap in place during the roofing process, allowing proper shingling of metal roofing panels around the cap, but thats not possible in my situation.

Is there a reason I'm not understanding for why they don't make the same type of rubberized-flange for fart-fan vents for metal roofs? Am I not searching for the right type of product? Apologize if this is more of a roofing question than a plumbing question. I do live in a location with snow, so it would need to be a solution that has a proper "shelter" for the flapper to operate with periodic snow load.

Edited to say the fart fan i'm stalling has a 6" outlet in case thats relevant. I could downsize it to 4" if needed, but would prefer having the extra flow-capacity.
 

Breplum

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from a no-snow experience plumber: I would choose a fan with a backdraft on the fan., and use any good flex RJ
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, then, use a above the snowline stub out, plus two 90s so snow cannot block it.
 

diy-goonery

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from a no-snow experience plumber: I would choose a fan with a backdraft on the fan., and use any good flex RJ
shopping
, then, use a above the snowline stub out, plus two 90s so snow cannot block it.
Gotcha, so you would transition the metal ductwork to PVC just below the roof or something?
 
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