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.
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.
Best Answer
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.