Yet Another Horizontal Vent

[seaneys]

I really hate horizontal vents...

Our main floor bathtub is oriented such that the drain pipe exits directly under the side of the shower. As a result, there is not a 'textbook' wall to use for a vent. In an ideal world the drain pipe would exit under the side or rear wall so that I could quickly extend a vent up.

The drain pipe going to the bathtub is a 3" main pipe that terminates at the trap (2"). What I would really like to do is to extend the 3" drain pipe about 20" past the combo wye to the drain so that I can vent up through the rear wall of the tub.

The trap arm from the shower to the combo wye is very short (about 6").

Would this technically be against code (a horizontal vent)?

I have very little clearance. Alternative solutions would force me to use a ejection pump / pit. I actually have the pit in place for the floor drain, I would just like avoid using it for commonly used items.

I have a picture, but I can not figure out how to attach it...

DWV really makes me scratch my head.

Steve