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I've seen a couple conflicting answers here for whether or not a second p=trap is necessary, my understanding is 30" is the max distance between drains to share a p-trap, which my vanity is. I was able to rig it without a p-trap but could easily add one but want the extra storage space.

enter image description here I've tested it with a full sink on both sides and it drains fine, so is there any advantage to the second p-trap? The tailpieces from the sinks are 1 1/4" and the tee and p-trap and drain are 1 1/2" so I figure there is more than enough capacity for drainage from the sinks. Also is there a recommended amount of slope here for this connection? I added enough slope so that my 6" level touches the line so like 1-2 deg.

Apologies for the distorted image, it was wide angle

Edit: so everything is working now...except the drain is very loud! When I use the left vanity, the long distance to the p-trap is causing a lot of noise to travel up the open drain for the right vanity. Thinking I should now add the second p-trap just to provide a noise baffle.

Would it be best to tie in past the left p-trap with a wye or would installing a p-trap on the horizontal tied in before the left p-trap ok? I'm guessing p-traps in series may not be kosher but having a hard time finding the code.

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  • What is the actual distance between the drain holes, center to center?
    – Ecnerwal
    Feb 8, 2021 at 22:55
  • 30" exactly C2C
    – redlude97
    Feb 8, 2021 at 23:06
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    If you need more space: cut both tailpieces shorter and use a waste ell on both, pointing back. Swivel your p-trap CCW and put the tee in the top of the trap. Use additional waste ells to run both sinks to the trap tee. This will push all the plumbing up and to the back.
    – Jimmy Fix-it
    Feb 9, 2021 at 0:06
  • @JimmyFix-it that is a good idea to save space. I wanted to limit the number of fittings but that is a good idea to shorten the tail pieces so the horizontal pipe is tucked up higher
    – redlude97
    Feb 9, 2021 at 2:15

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The "point" (as is so often the case with plumbing code) is not causing a situation where the drainage from the sink with the longer arm to the trap causes the trap to siphon dry and let sewer gas into the house.

Since the drain-holes are 30" apart, that's allowable. If they are more, it's not. Welcome to code compliance.

If "more storage" is your watchword, you could run two traps towards the rear of the cabinet and put the crossing pipe to the drain closer to the back wall of the cabinet, freeing up all the space between the drains currently blocked by the crossing pipe.

Slope is 1/4" per foot, as usual for "smallish" drain pipes. IPC states that as minimum slope for 2-1/2" or smaller pipe size. UPC is probably similar. Tape a 1/8" block to one end of your 6" level and look for level to check that.

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  • Thanks, kinda what I figured. So I confirmed this by opening the trap after draining the right sink to make sure it didn't siphon the p-trap. Obviously this might not occur all the time so I'll keep an eye(nose) on it. Good idea about pushing the traps back though might consider that just to be safe and shorten the distance between the wye
    – redlude97
    Feb 8, 2021 at 23:23
  • So I think I'm going to do what you suggested, for the connection near the back wall would it be better to use a sanitary tee with a vertical to horizontal connection or a wye with a horizonation connection of the two grains after the p-traps
    – redlude97
    Feb 11, 2021 at 19:48
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    I think you'll need to stick with a Wye so as not to turn your nice compliant P straps into (effectively) an S trap (non-compliant, barring some special trickery with pipe sizes buried deep in IPC's many options in the traps & vents chapter.)
    – Ecnerwal
    Feb 11, 2021 at 23:40

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