Special roof vent for bathroom fan exhaust?

The previous owners of my home routed the flex duct from the bathroom fan right to the underside of a normal roof vent and sort of wired it in place. I'm about to re-roof the house, and although I'm tending away from this idea, an acquaintance comes highly recommended for this kind of work. Three person crew, all permanent people. But, while looking at the roof with him today, he gave me a blank stare when I mentioned a vent specifically for fans. Can anyone describe what these things look like?
Yes, I have two estimates from "real", insured companies. :-)
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Check out your Home Center, in the heating and ventilating department, you will find a number of different types of roof vents. Or check the web site of the company that makes the fans, e.g.: Broan, and you will see what they make in different sizes, shapes and types.

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They are kind of hard to describe, but basically what you want is a galvanized roof cap for a bathroom fan. There is a picture of one here
http://media2.askthebuilder.com/askthebuilder.com/artman/uploads/546-1.jpg . In my experience, go to a small hardware store, home depot only has one or two kinds, and the small hardware may have several different kinds that willl suit your application. Another option is to use a pvc pipe and plumbing vent stack flashing, but the pvc pipe will need a trap, or a J bend at the top to prevent water from coming back into the fan.
JoeSpareBedroom wrote:

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The others have told you want you want.
Give some thought as to where on the roof to place it. Generally speaking, the HIGHER on the roof you make the holes the less likely water is to leak in. And if you have it high on the roof if often doesn't make much difference which side of the ridge it's on. If you don't care which side then put it opposite the side the BIG WINDS come.
Once you decide what you want, I suggest you buy it yourself and make it a part of the contract for the roofer to install it. It usually isn't a big deal to install it after the fact but you have guys on the roof anyway so ...
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"JoeSpareBedroom" wrote

There is a "special" type vent for kitchen/baths, it has a flap to open/close. The part I'm not sold on is, I feel a bath exhaust does not put enough pressure to push open all the time. Another thought crossed my mind about them is, hornets/wasps love to build nests where it is warm (any roofer will tell you about the nests found in vents torn off). Anyways, if wasps or whatever builds a nest at the flap, it would be possible the flap will never open. Here is a link some what similar to what I installed (once). Although I know I didn't pay anywhere near what this place wants. http://tinyurl.com/g86rm
A long time ago I seen a set up which I thought was pretty slick, and used it when I used to be in contracting. I had run across the set up numerous times throughout the years during tear-offs.
A normal roofing vent is used, pick your choice for sloped back/square etc. http://tinyurl.com/z8pdf A 3" hole is cut in the sheathing, before any felt paper/shingles are laid (can be done afterwards, I just found it easier to do if had power to replace sheathing etc), a 18-24" piece of galvanized duct work is installed, the roofer cuts 3 slots in one end of the duct, approx 3" down the length, then fold these outwards, drop the remaining of duct into the hole, then fasten the fins to sheathing. From there, run flexible duct to the exhaust fan. You _MUST_ have the flex pipe form a trap, especially if you live in cold weather areas. Otherwise you will get condensation run back into your exhaust fan. Do _not_ use all galvanized rigid duct back to your fan, you will get a fan that drips condensation.
*NOTE* Make sure if the roofers cut a hole before they felt/shingle, they don't cover up the hole up with roofing material, otherwise as you know, you'll have an exhaust that leads to sheathing failure.
Sorry for being so winded.
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I have to cut two roof openings for each bathroom but the distance from the fan to the roof is less than 4'. Will flexible vent work and if so does it need to be insulated to prevent moisture? thanks
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Can't run it to the gable for some reason?
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I can run to the gable with one but not the other. Is the gable the preferred location for the exhaust.
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