I'll probably get yelled at for this, but I just put in a bathroom vent
fan, and in my attic I have 2 roof vents plus an electric whole house
attic fan that vents through the top (about 18" in diameter). Rather
than cut another hole in my roof, I ran rigid 4" duct from the
bathroom fan to just under where the attic fan is ( about 8 feet total
duct). So far its working great. I know this is still not the right
way, but at least I routed the vent close to where the opening in the
roof is. Some people just let the fan vent in the attic, and letting it
gradually find its way to the vent in the roof.