Trying to figure out this drain design for my basement project... thought this would be easy but the more I’m looking at it, the more difficult it seems.

My situation is I have a kitchen upstairs which has a sink with garbage disposer, and dishwasher. I plan on following your previous advice and using 1.5” from trap to the vented sani-t, where 2” will then come down into the basement. I want this 2” to drop down inside a new interior wall, under the concrete floor. (Will place a cleanout at the bottom of this stack). Underground, I plan on transitioning to 3” for the horizontal run, and picking up a basement bathroom group on this new branch pipe.

I also want to relocate my clothes washer to the area near this new stack. Few questions:

1. Can the new 3” branch handle a bathroom group, kitchen, and washing machine?
2. Can the washer simply tie in to the stack using a sani-t? (Wet vented under kitchen upstairs) or do I need a dry vent tied into the washer arm and then wye it down into the stack?
3. If I put a floor drain in the basement nearby the washer, does this need venting? Or can it simply connect to the branch underground?
4. How must I calculate the dry vent pipe sizes above all this? Or is several 1.5 fine?

1) 3 inch if fine.
2) it can but if you are increasing to 3 inch under the floor, why not put the washer drain into the same 3 inch? you can do this under the floor with a 3 x 2 x 2 wye or above the floor with a 3 x 2 x 2 san tee. the washer will need to be separately vented into the kitchen vent 42 inches above the kitchen finish floor. 1 1/2 is fine for both vents. washer will look like this.

3) every trap need to be protected with a vent. the vent can be further away from the floor drain itself and tie into the washer vent.
4) 1 1/2 is fine

In Massachusetts any drain or vent underground can t be smaller than 2".

We've been piping all laundry drains (including trap) 2" for a while now.New machines are just dumping to much water to fast.I think it should become code.

Did you mean 3", because 2" has been the norm, and code, for many years.

You can use 1 1/2" on residential laundry in mass.

Yes please clarify. I can do 2” from the laundry “wall box” as Packy instructed, down to p-trap then horizontal to top-vented sani-t, then down at 2” to a 3x2x3 wye for the 3” horizontal underground towards the main drain.

If I was to run 3” straight to the laundry, that won’t fit into one of those wall boxes so I would need an exposed standpipe. Which I can do since I’m making all this fresh. I would need a 3x3x1.5 sani tee however, as I’ll only have a 1.5” dry vent above that tee...

Ok just saw your reply.. in va a standpipe requires 2” but if you have a laundry sink you are still allowed 1.5” under the theory that the sink kind of slows down the drainage rate.

It acts as a reservoir to hold the excess water and lets it drain over a longer period of time.

don't run 3 inch to the laundry. that is overkill.
do run 2 inch under the floor to the 3 inch.
I suggested using a 3 x 2 x 2 wye under the floor because you also have a kitchen coming down from above.
3 inch is the outlet of the wye. 0ne 2 inch inlet for the kitchen and one 2 inch inlet for the laundry.
kitchen will have a 2 x 1 1/2 san tee and the laundry will have a 2 x 1 1/2 x 2 san tee. ( a 2 inch san tee with a reducing bushing in the top is fine)

Thank you so much guys. Will try to draw this and post in a new thread for approval/comment.



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