I'm not sure what you mean.

NYC code for example requires 1- 4 " main vent stack to penetrate the roof and we can connect all the fixture vents horizontally to it.

Pitch the vent Away from the fixture so if there is any condensation it will flow back to the fixture drain being served and the vent has to be above floor level rim of the fixture NOT the over flow rim and the reason NY requires a 4" vent is to prevent hoarfrost

Hi, Thanks for the reply! It's somewhat difficult for me to explain exactly what I am asking. From what I have observed and seen in plumbing books, all branch fixtures such as tubs, toilets, sinks, etc. drain into the main VERTICAL vent line, the pipe that exits through the roof. The waste material then enters the horiontal drain line then goes to the septic system. My question: Is it acceptable to connect fixtures such as the tub, toilet, sink, etc. into the HORIZONTAL drain line instead of the VERTICAL line? In my particular case the septic tank is located at the opposite side of the bathroom from the 4" vertical vent stack (that goes through the roof). The 4" horizontal drain line (sigthly sloped toward the septic tank) is positioned directly under the bathroom floor and runs its entire length to the septic tank. It would be easier for me if I could tap into the horizontal line than the vertical line. I hope this helps some. Thank again. Pete