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I have my bathroom sink rough plumbing coming from the 3" stack vent, running about 2 feet. My question is: can I use that same drain line for another sink?

Reasoning: about 2 feet from that sink drain I have a wet bar sink I want to put in. Using the same drain line sounds easier, only because of where the stack is, not because of putting in the second trap.

  • I did the same thing but did have a trap under the bar sink. It would work without a trap as long as you tie in above the bathroom trap. – Ed Beal Apr 22 '16 at 13:17
  • Perfect. If I did a trap on the bar sink would it still have to be above the trap in the bathroom? – Orion Apr 22 '16 at 20:21
  • If they both have traps that would be the best. From your question I thought you did not have space for a trap. – Ed Beal Apr 22 '16 at 20:41
  • Thanks for the input, I have tried to clarify. I think that both having traps would not be a problem. – Orion Apr 22 '16 at 20:44
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Consider all the dual-basin kitchen sinks... "If it happens, it must be possible."

enter image description here

  • I am curious, however, if there are codes related to shared traps on sinks in different rooms. – isherwood Apr 22 '16 at 13:16
  • You might be able to hear someone using the toilet...(at the bar) – Ed Beal Apr 22 '16 at 13:18
  • Why shared traps? One per works just fine and cost difference is negligible. – keshlam Apr 22 '16 at 13:19
  • if they don't have enough access for the drop I could see the need for a single trap. – Ed Beal Apr 22 '16 at 13:20
  • Adding in the trap in the second sink would be fine. The problem is that where the first sink ties into the stack makes it difficult for the second sink to be put in at the height I would desire. So instead of going around it, I just want to use the line that already goes that direction. – Orion Apr 22 '16 at 19:23
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Speaking from personal experience, only use one trap in a dual sink bathroom. With two traps, you can get air pockets blocking one or the other's drain. You also promote stinky slime growth inside the pipe between the two traps. I don't know the biology with that.

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As far as I know, having two separate sinks served by a single trap is not code compliant. You will need to replace the existing sanitary tee on the stack (inside of the wall) with a double sanitary tee.

Normally, these double tees are used for adjoining bathrooms, so you can have a sink on each side of the wall, and share the same stack. In this case, you would be installing the tee parallel with the wall, instead of perpendicular. You will need to extend each side of the tee to reach where you want the trap to be for each respective sink. In order to do this, you will need to cut a short piece of 3" PVC, and then put on a 90 degree elbow and attach another piece of PVC so it will come out of the wall. Then you can attach the trap adapter for each sink.

Double Sanitary Tee

  • Thanks for the thorough answer. I don't think this solves my problem though because the 2 sinks are on the same side of the stack and the second sink is not on the same wall as the stack. – Orion Apr 22 '16 at 19:26
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My double kitchen sink was installed (licensed plumber) as the picture, and seems to work fine (10+ years). I see no problems.

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What the code says: If the drain in the two sinks is more than 30" apart, code states that they each need their own trap.

What reality says: Sometimes having two separate traps sharing the same tie in can be an issue venting wise. Negative pressure issues can be solved by adding AAVs or AAV traps, but positive pressure issues will be a problem with two traps.

  • 1
    So to fill in what really happened: my wife installed a cabinet with drawers that blocked the plan of going straight through the wall with the wet bar drain, so I ended up cutting through the studs for the drain line and tying it in back a way. So my issue was solved in a way 😊 – Orion Aug 4 '18 at 19:01
  • @Orion -- post that as an answer and I'll give you a +1 for it – ThreePhaseEel Aug 4 '18 at 22:26

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