I am installing a full master bath away and inaccessible to the vent stack. I am using a 3" drain(PVC) for toilet, sink and shower. What is the minimum size vent stack I should use?

Hi sparky...

The minimum size vent for full bathroom is 2" (as long as there is at least 1-3" vent stack already out the roof of your house).

If you do not have a vent penetrating your roof yet, then 3" is minimum requirement for a vent.

Venting fixtures can also be a bit tricky... for example, toilet requires 2" minimum vent... and that vent can usually act as a waste line for the lavatory or the tub... making it a wetted vent... DEPENDING ON CERTAIN CONDITIONS... so let us know if you need help with that.. OK?