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I have a negative pitch in the drain pipe coming out of my sink drain that attaches directly to the vent stack.

The vent stack is mostly inaccessible at the height of the p-trap though inside the sink cabinet.

My thought is that I can cap off the pipe near the vent, and run a new drain line next to the p-trap, add a Studor AAV, and run the new drain line down through the floor into the crawlspace which will be reattached either the horizontal or vertical section of the vent stack pipe.

Am I overcomplicating things, or is this the best plan for solving the issue that my drain pipe has a negative slope? If so, can I attach to either the horizontal OR the vertical part of the vent pipe?

Couple notes: The disposal and/or the p-trap can't be raised up any higher. Also, the vent pipe feels immobile (I suppose it's affixed at the boot at the roof line).

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You will be good by going what you propose. If at all possible do reconnect the new drain in the vertical section of pipe. The gluing of the joints should be made easier by doing so. You will need to temporarily remove any straps supporting the horizontal pipe so it can be lowered the 1" to 1 1/2" to add the new sanitary tee.

Yes, you will gain room when you cut out the section of pipe needed to fit in the new tee, but if there are restraints that keep the pipe from moving up and down freely, like other drains tied in closely, you will need to judge carefully how much you can take out, or, how much you need to leave in. If you can cut out what is needed to fit the whole tee in and when it is glued back up and it gives more pitch to the pipe, great. Do be aware that changing the pitch in one place may lessen the pitch in another place.

You will not need an AAV if the P trap is no farther away from the tie in to the main line by 3ft. I think code is 5ft., but I am not sure. So closer the better. It will stop the water from gurgling while draining. If you still choose to add an AAV, it will not be an issue either way. The AAV serves the same purpose as tying in the drain at a close distance.

Another tip, instead of capping off the old drain, add a cleanout to the old drain. It is a new code that is now being used.

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