Here are some pics of how my hood/exhaust is set up....
The forges, and how the hood is positioned over them..
The hood is suspended by chains from the ceiling. Going straight up/through the roof wasn't a option for me, due to the type of insulation in the ceiling, so I went out as close as I could....
In this pic you can see the gable fan to the right.... even it gets so hot sometimes that it's thermal protection trips.....but once it cools down, if fires right back up again.
This is the gable fan on the other end of the shop....just remember that if you do install a gable fan, position it on an end, and have a window or door to open on the other end.
As I said before, I tried having a "draft" fan in the hood piping..... but every one I tried burned out within a few uses. I finally just gave up on fans in the piping, and found that I really didn't need one..... it's not a "suck your face off" kind of draft, but you can watch any smoke of fumes waft there way to the hood and get taken outside.