I want to thank everyone in advance for their insights. Here is the situation. I would like to renovate the kitchen of my two family home built in the 1920s to include, among other things, a dishwasher. The kitchens are stacked on top of one another and the sinks are served by the same drain and vent system. (The main waste stack is not involved here and is located in another part of the house.)

Currently, I cannot install the dishwasher because of some clearance issues caused by the drain which is connected to the drain pipe on the right as depicted in the attached photo. I would like to move the sink drain to the left pipe because it is not located behind the area proposed for the new dishwasher. In short, both drain pipes will remain, I will just be tapping into a different one. Can this be done — is there any problem with having two sinks on top of one another?enter image description here

  • The proposal results in a wet vent. This might not be permitted due to the pipe size.
    – Dan D.
    Commented Jun 21, 2019 at 16:23

1 Answer 1


I can’t speak to a specific code or have an alternative plan in mind, but I think you would have three problems with your proposed drawing.

First, if there is a blockage between the first and second floors, there is no longer venting for the first floor sink.

Second, if there is a blockage just below the first floor sink and the second floor is not aware of it, they’re water will back up into the first floor sink.

Third, first floor could end up sending some of their vent gas into the second floor sink depending on the path of least resistance.

  • Thanks, those are all very valid points which suggest avoiding the change even if somehow permitted by applicable code.
    – Mikejones
    Commented Jun 21, 2019 at 14:13

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