Can I reduce 3 inch vent to 2 inches?

[Doodle1979]

I’m having a heck of a time getting a straight answer to this question….maybe someone here can help.

I have a small house with a toilet, shower, lav, washer, and kitchen sink.

All of these fixtures tie into a three inch drain that currently vents straight up through the roof with a three inch vent.

I have a gable side wall about 12 feet away however and will be installing a metal roof and would like to exit the side wall to avoid those nasty boots on the vent stack that never seem to quite work right.

I’d like to cut the cast iron at about 5 feet from floor and put a reducing coupling on it and run a 2” pvc vent up into the attic and then two 90 degree turns and out the gable sidewall. I’d slope the vent pipe a few degrees to allow for proper drainage if any water enters system.

From what I can tell in the IPC which is what the Florida building code is based on, a 3 inch drain only requires a minimum 1.5 inch vent. Based on the number of fixtures I have though and the 12 foot horizontal run through the attic I’d upsize to 2”.

Everything I can find in the code says this is good to go. Am I missing something?

 

[James Henry]

If you're in Florida a 2" is probably fine you don't have to worry about the vent opening freezing over but you need to check the international residential code for where the vent is aloud to exit an exterior wall.