I live in an older home with only one bath on the second floor. I plan to turn part of an existing room on the first floor into a bathroom. This will be a full bath with a toilet, sink and shower. The main DWV line is 4" C.I. and about 35' from the new bathroom. I plan to run 4" PVC and tie into the C.I. My question is do I need an additional vent for the new bath? If so, what is the best way to do it? What size should it be? I planned to run all the drain lines under the floor then connect to the new PVC.

My question is do I need an additional vent for the new bath? If so, what is the best way to do it? What size should it be?
Hey Joe,
Every fixture that has a trap has to be vented so the answer is yes.
The good news is that if wet vents, (seen image) are allowed in your area all you will need for your group is a single vent. You will maintain 2" on the lavatory drain and vent. Good luck, Tom

Yes you will need to vent those lines. The vent line must be at least half the size of the waste pipe but not less than 1-1/2"

Go to Google Images and type in " WASTE AND VENT DIAGRAM" you might be able to find your configeration in one of the many pics there.

Have fun!