Hot Topics: Connecting Laundry Sink Drains to Standpipes

laundry room sink with plumbing drain

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Original Thread: Connect laundry sink drain to washer standpipe?

NYtraveler - Member

I live in Westchester County, New York, and I want to install a utility sink in my finished laundry room on the first floor of my house. About 24 inches to the left of where I want to install the sink, there is an existing 2" PVC standpipe which the washer drains into, via a u-shaped flexible hose.

I have not opened up the wall yet, but I assume near floor level the standpipe is connected to a P-trap and then to a vent/drain. (In the basement below, I can see the 2" drain coming down through the basement ceiling and connecting to the main 4" PVC drain pipe. This 4" pipe runs directly underneath where I will install the sink.) What is the correct way to install the drain for the sink?

I have read several different versions on the web. One source states that the sink can be directly tied to the standpipe (using a wye), at a point above the washer's P-trap, and not use a second P-trap for the sink. This effectively uses the washer's P-trap and vent for both the washer and sink. Another source says the sink must have its own trap and vent, which would be a lot more work!

Thanks for any help you can offer!

Zorfdt - Forum Topic Moderator

You definitely don't want to connect to the standpipe. You'll want to plumb it correctly into the drain.

How you connect it depends on how it's set up right now. You may need to add a vent, or you may be able to use the existing one, again, depending on how it is now.

I don't think you can do too much planning until you either open up the wall or figure out what's there now. Can you take some photos?

lawrosa - Super Moderator

Yes open the wall and take a pic.

Most likely no!!!! You will need to add a pump to the sink and pipe the pump to the drain.

It's all about venting. The pump allows you to vent properly.

guy48065 - Member

As long as there's no drains above, won't the standpipe serve as a vent? This seems the most obvious plan unless there's some code or reason a standpipe can only be used to drain a washing machine and not a sink.

I'm facing a similar situation where adding a vent to the roof would be difficult and ugly (a log cabin).

NYtraveler - Thread Starter

Thanks for the feedback. This weekend I'll open up the wall and post some pictures. In the meantime, at the risk of trying your patience, could you tell me what is wrong (if anything) with each of these proposed solutions, and why? I'd like to understand this better.

In case A the sink ties into the existing drain/vent.

In case B the sink ties into the standpipe, with its own P-trap.

In case C the sink ties into the standpipe, without its own P-trap.

In case D the sink ties into the main drain pipe.

laundry plumbing diagram

Thanks a lot for your help!

lawrosa - Super Moderator

Case A: wet vent. Not allowed.

Case B: same as case A

Case C: same as case A and B

Case D: No vent for sink

laundry plumbing diagram

Here is what you need to do. Case D. Vent sink. Tie in to vent three feet above flood plane of the sink.

guy48065 - Member

I can't be the only one who doesn't understand. What's a wet vent? IF there was no sink added it would still be wrong?

toolmon - Member

I'll never understand venting either. I'm not going to argue plumbing code with a plumber, lawrosa but isn't plan C the same as having a double bowl kitchen sink except for the pipe sizes? How about using the existing drain for the new sink and hooking the washer drain hose on the edge of the slop sink? That's how mine is set up.

lawrosa - Super Moderator

A wet vent is when a fixture is using a vent. Then you tie another fixture into that vent above it. The space between the two fixtures is a wet vent. Meaning its not always dry.

So you see in case D that I modified the washer vent is dry as well as the sink.

The only other way to do it to use that single vent would be to use a double fixture fitting. But to find one with a 2" side for the washer would be near impossible because I have never seen one. It would work with two 1 1/2 discharges though because that's how they make them. Washer needs to be 2"

Shop Mueller Streamline 2" ABS DWV Double Fixture Fitting at

rosannfitz voted this post useful.

guy48065 - Member

OK, I get the wet vent part now, but I really don't see how it could be a problem with a standpipe where there is an INdirect connection to the pipe allowing air in, essentially venting the vent stack.

In any case, isn't D the norm with a sink draining directly into a waste pipe with a nearby vent stack? Surely the attachment of the standpipe doesn't mean a 2nd vent is required?

And like toolmon I don't think I'll ever quite understand all the implications of venting. Frankly, I always thought a standpipe was intended to BE the vent and an actual outside-vented stack wasn't required on that branch. I've seen just this numerous times—but of course, that doesn't make it right Or perhaps it was OK in the past to use a standpipe instead of a vent?

lawrosa - Super Moderator

The standpipe is not a vent. The standpipe and trap are always tied into a vent.

A vent is used so other fixtures do not siphon water out of the traps.

guy48065 - Member

How does D violate the code? This is the setup in my mother's condo...built in 2004. The SP is right behind the washer. The vent is in the wall right behind that and the slop sink is next to the washer with the drainpipe after the trap drops through the floor.

lawrosa - Super Moderator

In NY travelers drawing the sink has no vent. In my drawing, it has a vent. That's code as I know it.

tevil - Member

Wouldnt Case A be fine if the vent is at least 2" pipe?

That's how we used to tie stuff up here in CT in all the time doing roughs.

I see where you're saying the sink needs its own vent for siphon, especially with a w/d and the drain pump force/volume but a 2" vent pipe should suffice no?

On a side note Lawrosa I don't know if you remember/reread my other thread but I completely redid my downstairs drain/vent set up last we talked. I saw the issues you were mentioning and corrected it.

guy48065 - Member

"In NY travelers drawing the sink has no vent. In my drawing it has a vent. That's code as I know it."

In D (OP version) the sink does have a vent (shared) and as long as it's within the recommended distance from the trap it doesn't need one of its own.

If adding a slop sink to an existing approved system is this complicated I shudder to think what debates will arise when I ask how to vent a kitchen island sink. I think I'll just Google that one.

Zorfdt - Forum Topic Moderator

"In D (OP version) the sink does have a vent (shared) and as long as it's within the recommended distance from the trap it doesn't need one of its own."

D in the OP version is an "S" trap and therefore doesn't have a vent. Fixtures can sometimes share vents, but not in a vertical drain like that.

NYtraveler - Thread Starter

Thanks, lawrosa for your answers- they are very helpful. Everything you said makes sense, but there are a few details I want to clear up.

1. In your corrected version of Case D, I assume the vent for the sink should be connected to the drain of the sink, downstream of the P-trap, like in the figure I attach here? This is what I plan to do (pending opening up the wall to see what I have!).

2. I read part of the New York plumbing code, and it states that wet venting is allowed, but only if the fixtures are within two bathroom groups. So Case A would be allowed if the washer were a toilet?

3. As toolmon stated, Case C is the way a kitchen double bowl sink is plumbed. Is there a special exception in the code for a kitchen double bowl sink?

4. Continuing from point 3, I understand Case C is not up to code and I don't plan to do it, but I don't understand why it would not be safe. Is the concern that the washer discharge might come out of the sink? Or is there another problem?

Thanks a lot for all of your help. I very much appreciate it. And have a Happy Thanksgiving!

laundry plumbing diagram

lawrosa - Super Moderator

#1: yes

#2: I would have to read it. Wet venting and how its interpreted by non-plumbing professionals can mean many different things.

#3: It may be somewhat how a double basin is piped but the vent is still dry. That has nothing to do with your layout.

#4: There are many ways to plumb things. I gave you one way in regards to case D.

Here is another way to do case C, or really a modified version. I can't explain in detail about venting and rough plumbing. It's a learned trade.

laundry plumbing diagram

guy48065 - Member

Is there any limit to the number of branches & loops that tie into an existing vent above the ceiling? My attic is unfinished so I can run horizontal vent pipes all over the place and would prefer to do that than have additional roof penetrations.

lawrosa - Super Moderator

Yes, you can tie it into the vent stack in the attic.

As far as how many fixtures can attach to that vent is specific in the code. It's based on the size of pipe and units per fixture.

Example, and this is just for reference: a toilet is four units, a sink three units, tub, two units...etc. Equals nine fixture units. A three-inch vent stack and support say 15 units.

Then the vent branch, as in our drawings of 2" have a certain unit capacity. As such can usually supply a bath group of a bath, sink, and toilet. That vent can terminate through the roof or tie into the main vent.

This is usually not an issue in one or two bath homes. Large homes with multi baths will have two or three main vents stacks or have the bath groups vent terminate individually through the roof for each bath group.

That's it in a nutshell.

NYtraveler - Thread Starter

Here is an update on my project:

I opened up the wall and found three studs ganged together that I would have had to drill through to get to the vent. So I decided instead to add an AAV (which is allowed in New York). I added it under the sink, immediately downstream of the trap, and made sure it stood at least four inches above the top of the trap.

The drain then goes down through the floor and directly into the horizontal 3-inch drain pipe in the basement. I also made sure I used a wye to connect to the three-inch drain pipe, and not a sanitary tee on its side, which is what the guy at Home Depot recommended! It all came together in two days, and so far no leaks!

Thanks a lot for all of your help, especially lawrosa.