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Here is my kitchen sink drain configuration:

enter image description here

(The horizontal run after the trap is actually pitched correctly, not as drawn.) The portions inside the wall are galvanized steel pipe. Under the sink, the galvanized pipe transitions to white plastic trap and tailpiece. Under the floor, the steel pipe is coupled to cellular ABS drain pipe.

While I am replacing kitchen cabinets, it would be convenient to move this drain line to the floor. Is it kosher to cap the existing line inside the wall, then join a new vertical leg to the existing horizontal run under the floor, as drawn below?

enter image description here

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    The vent line needs to take off when the trap arm drops from horizontal to vertical. Venting from below the floor will not work, you will siphon water out of the trap with that configuration.
    – bcworkz
    Apr 17, 2014 at 1:28
  • I do not know the code thoroughly but from what I have read, this can be done as long as the existing pipe going up is a vent only and does not serve as a drain for other sinks or tubs. When another sink tur tub is used above the kitchen sink, that is when the trap would get sucked out, so I thought...
    – Jack
    Apr 17, 2014 at 12:00
  • @bcworkz, I think you're right if the trap stays above the floor: 2009 IPC 906.2: "Venting of fixture drains. The total fall in a fixture drain due to pipe slope shall not exceed the diameter of the fixture drain, nor shall the vent connection to a fixture drain, except for water closets, be below the weir of the trap." But what if the trap is below the floor? Apr 17, 2014 at 13:51
  • :D That's thinking creatively! But probably not OK. Tailpieces are supposed to be "as short as possible" and in any case no longer than 2 feet. If your sink is unusually deep it may comply. Also, vent lines are not supposed to take off horizontally, they are supposed to be at least 45 deg. off horizontal. However, I see improper horizontal takeoffs quite often and they don't seem to cause any real issues. Code aside, I myself would not install a sink trap unless it is readily accessible for cleaning.
    – bcworkz
    Apr 17, 2014 at 15:40

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This is forbidden by the International Plumbing Code.

2009 IPC 906.2: "Venting of fixture drains. The total fall in a fixture drain due to pipe slope shall not exceed the diameter of the fixture drain, nor shall the vent connection to a fixture drain, except for water closets, be below the weir of the trap." (Emphasis added.)

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