It is a pellet grill which is forced air so a lot of those "facts" don't mean squat in this situation.I cant remember the exact number for a roof vent , but I want to say 18 " . You are trying to avoid a down draft coming over the high part of the roof . I'm not sure at what point the length of the vent affects the operation of the smoker itself . I think I would try going out the back wall with a larger diameter pipe . The larger pipe causes a lower pressure route for the smoke to seek , and keeps the original length of the factory stack . Maybe . Just thinkin ,,,but some facts behind the thoughts as well ..
Chop
I'd hard pipe it through the wall and see how it goes if you're trying to keep smoke out of the shed.
If it doesn't work you could always go back to the gapped airspace that you have now.
Sweet looking set up.
