I am remodeling a 100-year-old house, adding plumbing for a washing machine and condensing dryer. I'm installing the washing machine and dryer in the basement. In the location where I want it to go, there is a 1 1/2" brass waste pipe coming up vertically from the basement floor, about 3 inches or so. The main waste stack is about 2.75 feet away.

I believe I need to install a tee fitting, connect a waste arm to it, connect a p trap to the waste arm and a standpipe to the p trap ... similar to the below picture (except with a standpipe connected to the p trap and not a sink).


How do I connect the tee fitting to the brass drain pipe? Is a rubber reducing fitting OK?
Is my methodology correct?
The p trap needs to be 2" as per code, is it OK that the brass pipe is only 1 1/2" ?

enter image description here

  • 1
    Why is there a 1 1/2" brass waste pipe already there? Have you traced the path of the 1 1/2" brass waste pipe to see how it is connected to the house plumbing? If the brass 1 1/2" brass waste pipe was originally installed as a washing machine drain then it would have already had a trap as part of its original install. ( it could have been cut out ) Typically the trap is down line of the open end of the stand pipe and then it tee's into the drain/vent line.
    – Alaska Man
    Jan 24, 2020 at 18:55
  • @AlaskaMan I can't trace it as it protrudes from the basement's concrete floor. There was previously a sink's p trap connected to it and not very securely either. Jan 24, 2020 at 20:44
  • 1
    if there's no airflow in the pipe there's a trap downstream somewhere.
    – Jasen
    Jan 24, 2020 at 21:56

1 Answer 1


Based on you answer in the comments my answer would be.

Presumably if it was a sink then its vent was provided by the nearby main stack so you should be able to put a san-tee on it and p-trap into the san-tee. BUT it could have had a vent coming of the original san-tee and connecting elsewhere to the stack ?

You want the san-tee low so that you have room enough to put a tall vertical pipe on top of the P-trap so the washing machine discharge does not splash back.

I would, since you do not know for sure if it is vented to the main, stack run pipe up off of the top of the san-tee as high as you can ( at least a foot or more if you can above the top of the top of the washing machine drain pipe ) and put an air admittance valve on top of that.

enter image description here

  • Thanks @AlaskaMan. To clarify, when you say "put a tall vertical pipe on top of the P-trap", you are referring to a standpipe right? Jan 24, 2020 at 23:15
  • reading your answer a few times over, I wanted to clarify that I'm placing this washer and dryer in the basement. Given that, am I correct in assuming that it cannot use the main stack as a vent because other fixtures higher up drain into it? Jan 25, 2020 at 0:22
  • I added the photo for clarification
    – Alaska Man
    Jan 25, 2020 at 2:42

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