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My wall drain pipe clears the vanity, but there is not enough depth for trap. That's why I am thinking running a 90 degree bend out of the wall upward. My concern is losing the trap seal.

Since wall drain pipe clears the vanity, I am thinking running 90 degree going up and then attaching the rest to the p-trap.

enter image description here

Not sure if this matters. Within the wall, drain pipe doesn't go straight down. It turns left and goes towards toilet.

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No you can't do that.

The P-trap depends on the weir which requires critical trap arm length and a slope that does not siphon the trap dry. Your drawing siphons the trap. You could add an aav but opening the wall might be easier.

https://www.jlconline.com/how-to/plumbing/maximum-length-for-fixture-drains_o

The other thing I've done is to make the p-trap assembly in a straight line to the back of the cabinet and then make a cut out in the shelf to accommodate the trap. I've done this with both drawers and with shelves.

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  • The shelf cutout suggestion is solid. I'd use a hole saw at each end of the slot to keep it fitting closely. If you happen to have a router, a bullnose bit run around the cutout will clean it up nicely. Give the raw edge a coat of paint or varnish to seal it up a bit. – isherwood Mar 24 at 12:50
  • Is the idea inherently unworkable, or was the problem with the proportions given in the drawing? I'm not familiar with plumbing but are there some code stipulations with the right-angle bends in a drainage line? – Aww_Geez Mar 24 at 13:33
  • I'm a bit amazed - when I go to look for a vanity shelf they all have a ceiling board WITH a U-cut out to allow / support the trap to "dive" into. You can cover the hole and plumbing with images-na.ssl-images-amazon.com/images/I/… or similar products – eagle275 Mar 24 at 15:14
  • Why would opening the wall be easier than adding an AAV at the end of the trap arm, if there's space? – alexw Mar 24 at 16:39
  • Would a loop of pipe which connects the two ends of the horizontal section and rises above the sink's overflow height be another way of meeting code, if one can figure out how to avoid it being too unsightly? I think that's how island sinks are accommodated. Alternatively, could the trap be moved next to the wall, and the sink pipe made to slope down toward it? – supercat Mar 24 at 17:11
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No need for that, there tail pipe extensions that add length to get to a lower P trap like you have., text Picture courtesy Home Depot

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    He can't lower the trap because of the vanity shelf, like in his picture. – Jimmy Fix-it Mar 24 at 4:56
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    @JimmyFix-it sacrifice the shelf or at least a hole for the p-trap... – Solar Mike Mar 24 at 6:32
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    I'm sure Jimmy knows this. His point is that this answer doesn't address the specific question posed, which is how to create a suitable trap in the space available. – isherwood Mar 24 at 12:49
  • Yup I missed the shelf somehow, Duh. – Jack Mar 24 at 16:12
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Use a t wye and a studor vent .right now you created an S trap.so the first 90 elbow from the sink needs to be a t w wye with a studor vent on top a min of 4 inches over trap weir

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If it comes to it, sacrifice the vanity shelf and get your waste pipes as close to perfect as you can. Future you will thank you, you can always add more storage/shelving elsewhere. You have one decent opportunity to get your sink drainage as good as you possibly can, function over form.

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Instead of 90's use 45 degree fittings. This fixture cannot be further than 7ft from its vent. And That particular offset can not be greater than 2ft. Same goes for pop up assembly to p-trap weir. The maximum is 2 ft. Or you might create a dry trap not stopping sewer gas.

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