I have a shower drain vent pipe that slopes to the wall and then goes up the wall along the stud, then has a 90% elbow acoss the ceiling joist about 10 ft at a slight slope. In order to tie into another vent, I would need it to slope downward at this point, not continue upward. Is this an issue?

HI Teton...

All vent pipes MUST pitch from the roof back towards the drain so rain water and condensation can drain back to the drain system via gravity... ;)

Questions? Let me know, OK?

Mark