Remodel Replace double san tee between two sinks with double fixture

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New Member
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Columbia MD
DIY renovating 1970s townhouse and I was putting in a new vanity, P trap etc in one bathroom and wife was using sink in adjacent bathroom (back to back toilets and sinks) and I got water on me from her using the sink while I was doing the P trap installation. After a brief Homer Simpson moment, I realized I had a shared drain and vent system with a likely double san tee for waste water between the drywall for the sinks. As I now have access to the 2nd bathroom on the remodel before I put in the second vanity, my question is do I tear out part of the drywall and replace the likely double san tee with a double fixture fitting for the sinks or am I just creating unnecessary work?

If I do not replace the double san tee should I be concerned about waste water moving between the P traps of the two sinks or did I only get water on me because additional air was in the system with one side of the bathrooms P trap removed?

I read UPC 704.2 and your column from 2008 on double-fixture fittings and the rationale for the double fixture fitting to allow proper snaking; however in this case if I had to do snaking I would unscrew the P trap and have direct access to the double san tee to guide the snake through and not come out in the other bathroom with the snake (I enjoyed the funny story of snaking through to the other condo).

UPC 704.2 Two fixtures set back-to back, or side-by-side within distance allowed between a trap and its vent may be served by a single vertical drainage pipe provided that each fixture wastes separately into an approved double-fixture fitting having inlet opening at the same level.

So is the real rationale for UPC 704.2 requiring double-fixture over double san tee in sinks for proper snaking or are there other benefits to a double-fixture fitting to make it worth me replacing?

Also I noticed the double fixture fittings appear to be much wider than the double san tee and does not look as friendly to go between the studs.

Thanks in advance for any advice


The Plumbing Wizard
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Bothell, Washington

With a 2" double fixture fitting bushed down to 1.5" on the trap arms.


With a 2x1.5x1.5x1.5 double fixture fitting.

I have never used a double san cross for back to back fixtures. I have used them for vents above the flood level of the fixtures.
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