The way our house is constructed the only option to vent the bathroom outside is through a modified bitumen flat roof. Is there a specific type of vent that can be used? Does something like this work, or is there something better?

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Can you go through the side wall? If not then you can cut through the roof for a 3" single wall pipe with 2 elbows on the end or a rain cap. Add a pipe boot to the pipe just like was used for your plumbing vent stack. You can buy flashing lits for 3" pipes at most home centers.

Thanks!

Is 3" enough, I thought it was 4"?

Can I use PVC or just single wall pipe?

Thanks!

PVC should be fine unless the manufacturer directions states other wise. Don't know why you need 4" unless the fan discharge is 4"

Agree, use whatever size the discharge is out of the box, usually 3". PVC is fine but do not use 90 elbows, if you must make a turn use 45s or 22.5, each elbow decreases the air flow.