plumbing
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.
Not sure if this matters. Within the wall, drain pipe doesn't go straight down. It turns left and goes towards toilet.
A 1 1/4" female FIP adapter worked perfectly. As soon as I saw it, I was embarassed for not having thought of it earlier.
I used some teflon tape, screwed the FIP adapter onto the adapter coming out of the wall, then just glued my 1 1/4" pipe directly into it. No reduction in pipe sizes, and 100% ABS parts.
What size is that copper? You can get plastic p-trap kits in 1-1/2" and 1-1/4", and these can be easily connected to your copper via slip ring compression fittings. Basically they consist of a nut with a cone-shaped plastic washer, which when compressed by the nut squeezes against the (copper) pipe to create a seal. Usually these are only on the sink side of things and the wall side has glued-on plastic but you can use the compression fittings on both ends.
Here is a 1-1/2" Plastic p-trap
If you want to solder, a better solution than using a compression ring on the wall side would be to sweat on a copper x female adapter, followed by a male PVC adapter and a PVC trap adapter. Better yet if you can find one, just use a brass copper trap adapter like the following:
A less desirable but acceptable solution would be to use a fernco fitting to go from the copper to PVC, and then you can avoid soldering.
Best Answer
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.