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After cutting open my drywall for a medicine cabinet (to replace what had been a large plain mirror glued to the sheetrock), I finally realized that there was more in my way than just studs: as shown in the first attached photo, there is also a 2 inch vent pipe leading straight up to the roof of this one-story home, and a 3/4 inch vertical pipe which is for A/C condensation to drain into my bathroom sink.1

I think the condensation line is an easy fix: I'm planning to cut the stud that it is strapped to anyway, so I can just relocate its output to the other side of the stud at the bottom. Its output would still be within the footprint of the sink cabinet base. I'll just have to go up into the attic and drill it a new hole through the plate, about 4 inches over.

For the sink vent: as shown in the second photo, there is a toilet very close by and I think only one more stud between it and my recess cutout. I am wondering if it is advisable to solve my problem by:

  1. cutting one more stud to gain access to the toilet vent (total of 4 consecutive studs cut, since there will be 2 side-by-side medicine cabinets)
  2. redirect this sink vent leftward (I suppose at an upward angle), and tee into the toilet vent
  3. cap off the now unused portion of the original sink vent leading up to the roof, so my wall cavity doesn't fill up with condensation/rain.

Other possibly relevant facts:

  • this wall doesn't have just 2x4 studs, there are also 2x2s nailed to the studs to make the wall cavity 6 inches deep
  • I don't think this is a load-bearing wall. It is an interior wall that butts up against an exterior wall (the wall on the other side of the toilet, shown in the 2nd picture.) There is probably a joist (roof truss) running directly above along the top of the wall I'd be cutting these studs from.

1 Please ignore for now the mystery piece of Romex that is cut loose at both ends, and was shoddily sandwiched between a stud and the drywall. I want to guess this was (A) a thoughtful thing by electrician in case someone wanted a lighted mirror later, and (B) a lazy thing by the drywaller who didn't care if it bowed out the wall.)

Image showing pipes to be relocated (ignore mystery romex Image showing how close the toilet is, suggesting possibility of just using its vent

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This is a scenario where you could consider an Air Admittance Valve (AAV), also known as a Studor valve. It would install under the fixture within the vanity cabinet, and eliminate the need for a connection to the roof vent. They are not allowed in all areas so please check with your AHJ before doing this, as it's no fun to have to redo work when you hit inspection. I believe you will also need to remove the entire rest of the vent rather than just capping it off.

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