Transition from 2" to 3" Vent

[Jb9]

Hello,

I need to run a 3" vent out of my small workshop (as per the AHJ) which has a toilet, lav, kitchen sink and floor drain. I don't have a lot of space and I was wondering if running a 2" vent and then transitioning to a 3" as the vent exits the roof would be a bad idea. The inspector says I can do it instead of running a 3" vent. Basically, I have a small service cavity that I would like to frame with 2x4's instead of 2x6's. A 2" DWV pipe will keep me from changing my service cavity to 2x6. As long as the vent eventually changes to 3", does the cross sectional area still allow the same airflow as if the vent was 3" the entire length?

Thanks in advance.

 

[Terry]

The home requires a 3" cross sectional, normally by using two 2" vents and one 1.5" vent.
If you only have one vent for the entire home, then they want the 3".