I wanted to see if this is possible to do. I have a drain/vent stack in the wall between my garage and my bathroom on the ground floor. The stack is ~3.5" in diameter for the bottom ~2 feet, and then changes to ~2" line and runs up into the ceiling, where I believe it runs all the way to the roof.

The downstairs sink drain connects to the stack at the 3.5" section. There is no other connections to the stack (e.g. no "above the sink" vent connection). The connection from the sink drain to the stack is 24 inches at the most.

I am not positive, but I think the shower and toilet connect to the stack under the slab.

What I would like to do is add a utility sink on the other side of the wall (in my garage). Can I connect the utility sink to the same line off the stack to the internal sink? Similar to how a dual vanity shares a single connection?

If not, can I connect the drain directly to the stack? Should the sanitary tee be below or above the sanitary tee to the other sink? The stack is right against a stud, so I can't replace the single outlet sanitary tee with a dual outlet.

Lastly, is the venting going to function correctly with just the single connection, or will I need to put an AAV on the utility sink?

Thank you for any advice on this!

first, put a full size cleanout tee in the 3 inch stack. then use a double sanitary tee. one side for each sink.
that's probaly not 100% to code but will give you no problems.

Thank you for the advice, I have a follow up question if you don't mind.

The 3" stack already has a cleanout tee below where the existing sink drain connects to the stack. How can I work around the fact that the stack is located directly next to a stud? (The hub on the 3" connection actually touches the stud, that's how close it is) All of the double sanitary tees I've seen have the two 2" drain connections opposed each other by 180 degrees? Do they make one where they are opposed at only 90 degrees?

If I use a double santee the way it is right now it wouldn't fit because the other tee would extend past the stud.



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How does the elevation of the existing sink drain, line up with what you need in the garage? Usually garage floors are stepped down from the house floor elevation and this might put the starting point of your new drain too high. How much pipe exists in the stack, between the cleanout and the sanitary tee for the lav?

drill the hole for the drain pipe going thru the very close by stud and elongate it up and down to give you room.
the hole might be 3 inches tall and 2 1/2 wide. won't hurt it a bit..
if you are worried about strength, double up the stud before drilling/cutting.

I took a few pictures to show it better. I'm in CA and the wall between the garage and the interior is a shear wall, so it's covered in OSB. (This isn't just a fire break, it runs all the way up to the crest of the roof)

Because it is a structural thing, I would like to avoid notching or boring through the stud that is on the left side.

I do think the garage floor is a few inches lower than the interior floor, and I know utility sinks usually have a lower drain than normal because they are a deep sink. If I understand the question right, the drains from each sink must be level, correct? I suppose I could either elevate the utility sink or use a shallow sink.

I'm not sure if this makes a difference to the draining/venting requirements for the utility sink, but I do plan on using it to drain the brine/rinse from a water softener into. (e.g. softener discharge over the utility sink (air gapped of course) and the discharge down the utility sink drain.)


The distance between the cleanout and the drain is small, under a foot I think.

Here are the pictures:
Under vanity sink (inside bathroom)
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Opened wall cavity:
(This is not my handiwork, this is tract work)
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The water closet is centered on the stack, and I am pretty sure it connects to it under the slab. The shower is to the left of the stack by probably 5 or 6 feet. The sewer access you see on the floor doesn't have a drain, it's just a backwater valve.

Thank you all again for the assistance, I really appreciate the help!



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You should have enough room under the cleanout tee, to install a sanitary tee for the garage sink. You would need to cut the white sleeve down.