I am relocating my washer and dryer to the 2nd floor where the builder originally intended them to go. The original owner had them moved before the house was built so there are no rough-ins. However, the closet I want to put them in backs up directly to a bathroom and has easy access to the bathroom sink base. So I want to just punch through the wall from my new laundry room into the sink base and use that drain and supply lines for my washer. The laundry room would have a regular WB. I have seen this type of setup used before where the washer is plumbed into the same drain as the laundry sink. The sink drain is 1 1/2".

Can I do this? Can I just get a Y adapter for the drain (which comes out of the wall) and use one side for the sink and one side for the washer, each with its own trap? The washer is an HE unit if that makes any difference.

Your bathroom sink hasn't the depth or volume of a laundry tray so compairing the two is useless. I'm afraid that you'll find your washer discharge all over your bathroom floor. Your washer pump will discharge more volume then the pipes can carry away, At this time it will back up into the lavatory and from there to the floor. I'd look for another way if I were you. Good luck, Tom

A plumber told me that the min you could use for a washer was 1 1/2" waste pipe. But if you are saying that is not enough, I will find another way. There is a 4" waste pipe under the floor somewhere in that area, I will try to locate it. Thanks!

Btw: When I say sink base, I mean the sink vanity and the plumbing underneath that. I don't mean to disharge the washer into the sink itself.

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How high will the stand pipe be?
I had thought you were going to connect above the lavatory trap.
But you'll have much better drainage by cutting in a wye in the wall. That makes it more doable.

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A plumber told me that the min you could use for a washer was 1 1/2" waste pipe.

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And that's true, however, with today's more powerful pumps we recommend using 2". Good luck, Tom

Is the stand pipe the waste pipe for the washer in the WB? If so, it will be standard height about at the level of the top of the washer. Also I was told to make sure I vent the washer waste pipe. I have easy access to the attic and will be in there anyway for the dryer vent so this is not an issue. I will just run it up into the attic and then t it into the existing vent.

What if I remove the vanity, cut into the wall behind the vanity, and wye off the sink waste pipe in the wall?

What is the worst case if the washer flow exceeds what the pipe can handle? Will it back up into the sink or overflow?

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the standpipe will be standard height about at the level of the top of the washer.

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The standpioe should be about 36" inches and na few inches over the flood rim of the washer.

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I was told to make sure I vent the washer waste pipe

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If you connect to the lavatory arm it will be vented by the lavatory roof vent. No extra vent's needed.

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What if I remove the vanity, cut into the wall behind the vanity, and wye off the sink waste pipe in the wall?

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short of installing a washer station complete with its own vent that's the way I would go.

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What is the worst case if the washer flow exceeds what the pipe can handle? Will it back up into the sink or overflow?

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You must realize that there's a lot of hair and grease in that 1 1/2" lavatory drain. You will be putting more fiber and grease from the washer.
Worst case!
The lavatory arm clogs up solid and your washer discharges.
It will hit the clog and back up into the lavatory. Since the discharge from the washer has more volume then the bowl can hold the excess will go all over your bathroom floor.
I would install a cleanout between the standpipe trap and the wall to keep the pipe clear. Good luck, Tom