Roof exhaust duct terminations should be avoided in the Northeast. Use metal ducts, secure the joints with sheet metal screws and if the attic is unconditioned, provide for condensation.

Why poke a hole in your roof if you don't have to? Yes you should be concerned about it being buried in snow, leaking and helping ice dam formation. Through the sidewall is much preferred. Insulated pipe will help with condensation both winter (inside the pipe) and summer (outside the pipe).

The first order of business is determining how or if your new roof will be vented. If you're going with a continuous ridge vent, the gable vents will go. If you're going with a "hot roof" there will be no ventilation.

Thanks so much to everyone, I really appreciate your time in providing input. Just to clarify my situation, the attic is vented by a continuous ridge vent and soffit vents, and the gable vents (louvered, clothes dryer style) are exclusively for venting the bathroom fans, not the attic.

Check this out.

I would go through the gable wall. Mine goes through the roof, and there are periods during cold snaps (well below 0°F) when the fan can't exhaust. The pipe is well insulated, and I haven't had problems with condensation inside the pipe or out (I do check periodically).

My neighbor's bathroom fan is vented through the gable end. It doesn't go through the attic vent, but is a separate penetration in the wall - just like a range hood exhaust would be. The attic venting is completely separate.

JDS and Joe don't laugh at my lack of knowledge but explain something to me. We have wooden hexagon shaped attic vents in three gable ends. See link below - mine are similar to this. I need to remodel a bathroom and install an exhaust fan. Can we run the duct work to the exhaust fan out the hex attic vent? I would use the one closest to the bathroom of course. If so, how does the duct attach to the attic vent? I am assuming there are some type of clips or fasteners or thing-a-ma-jiggies that does that. DH and I DO NOT want to go through the roof, but would if absolutely necessary. I had always assumed we would need to go through the roof so this discussion may help solve our problem.

Joe don't you dare get sarcastic - just tell me if I am on the right track. I would hire you to do this if you were in Kentucky :). And I wouldn't get three quotes.

Hex wooden attic vent

We have decided to go through the gable wall. Thanks again to everyone for helping with our decision!

The "termination" of a bath exhaust duct has a slim hood to prevent rain from entering and a backdraft damper to prevent air from blowing back into the bathroom. I don't know how or why you would attach a duct to the back side of an existing attic wall vent that presumably has some kind of a screen behind louvers. Perhaps it would require some kind of metal enclosure/box with flanges deep enough for the damper.