While examining the plumbing in my upstairs bathroom I noticed there are no air pipes. It is a straight run of about five feet of 4" PVC pipe . It starts with the toilet followed by sink drain before it turns downward into the stack. The bath drain comes into this straight pipe perpendicular. I would want to add a an air pipe that vents through the roof. My question is does it matter where the air pipe is added? Some parts of this run are more accessible than others. I am aware that new building code says an air vent should be at every drain but that is not very doable. So where would be the most effective place to add a single pipe ?