I am doing a remodel of the first and second floors of my house – moving fixtures and adding a half bath. I will tie into the basement apartment plumbing, like it is done currently – there is a 4” soil stack and a 2” waste stack that tie into a 4” sewer line under the concrete floor.

Can anyone comment on this layout:



The basement has a bath with WC, sink and tub. The first floor also had a standard bathroom which I am shifting a few feet and adding an adjoining half bath. The upstairs also had a standard bath which I am also moving a few feet and changing the layout.

The original plumbing had a 4” soil stack in the basement wall, but it transitioned to 3” before it got to the first floor and the other two toilets (first and second floor) tied into it. Then it went to 2” to vent to the roof. I didn’t draw any of the vent lines, but each fixture will individually be vented, and I will bring the two 3” vertical waste pipes thru the roof as 3”. All other drains (and vents) will be 2” or 1 1/2”, as appropriate.

I came up with this diagram as the pipes will line up with walls, and most horizontal drains will run parallel to the joists. I have 10” joists under the first floor and 8” under the second, so I wont be able to cut holes for anything but the 1 ½” pipe. I’ve tried to minimize the amount of 3” and 2” horizontal runs, as they will be in living space, covered by a valence, and in the basement I don’t have too much extra headroom.

I am in Washington State that uses the UPC.

These are a couple of things I am unsure of…
1. The UPC says that although a horizontal 3” drain is OK for 21 DFU, it is limited to 2 WCs. I will now have two WCs on the first floor branch, but does the drain from second floor WC count as a third?
2. The UPC also says a 2” horizontal pipe can accommodate 6 DFU, which I have between the first and second floor. But they all drain into the 2” vertical drain into the basement, which services the basement tub. The 2” pipe is in the concrete, so I don’t know where it enters the sewer drain. Is this against code?
3. There are NO cleanouts anywhere, so I want to add some. I was thinking about one low in the soil stack, and another after the Y for the first floor toilets. Do I need others?
4. And one question about venting the two toilets on the first floor – I will add it within a few feet downstream of where the Y enters the waste branch – can this common vent be 2”?

you may be close or you may be a mile away depending on how things are vented and sized.
need pipes sized and need vents drawn to even venture a guess as to what is going on.

I hope this isn’t too confusing...

basement toilet is not vented.

second floor single toilet does not need a vent. it is wet vented thru the sink if the sink drain and vent are 2 inch.

keep your main vent stack 3 inch and reduce the double toilet vent to 2 inch

the vent for tub 'B' needs to move to the downstream side of the tub trap

the shower vent can be 1 1/2 inch OR you can eliminate it and wet vent the shower thru one of the sinks. increase the sink drain and vent to 2 inch.

I am a little confused where the tubs and showers and sinks come together into a tee ?

all in all, you are on the right track. not there yet but not that far away.

Thanks for your advice. You've been very helpful.

Tub B is vented downstream – it just wasn’t shown well in the picture.

Tub 2 essentially drops down to basement 2” drain, but has to make a horizontal turn. The two sink1 are the master and ½ bath sinks, both draining into the 2” drain to the basement – so the T is not appropriately drawn – it should be a Y with 45s to the horizontal – both ends sloping appropriately toward the vertircal drain. At the close end of the run, shower 1 and sink1 T, that should also be a Y with 45s.

Yikes about the unvented toilet in the basement. That’s the way it is now (when I bought the house), with the 1st and 2nd floor toilets draining into the 4” vertical pipe. Perhaps it is wet vented by a pipe under the concrete that ties into the sink vent – I’ll have to open up that side of the wall to find out for sure. And I’ll measure the sink vent that is there now – maybe its 2” But if its not, I guess I’ll have to open the concrete and add a vent to the closet elbow, right? Maybe the sewer exits the other side of the house (at the Tub B end) and it is wet vented by the basement sink? There are no outside cleanouts, and the city diagrams are ambiguous as to which side of the house is connected to the sewer line.