DWV Help Needed

[dwvpuzzled]

I really appreciate this forum, and I usually find the answers I'm looking for by searching the forums so I never have had to post before now.

I'm remodeling a bathroom, and replacing the old leaking shower stall with a freestanding tub and a rainfall shower combo. The existing drain has to be moved a few inches to become the new tub drain, so it will be replaced with a 2" ABS P-Trap/Line, but will be fairly close to how the existing drain is situated (see photos).

I want to add a linear drain that will be situated approximately as shown in the photos to drain water that falls outside the freestanding tub when it is used with the ceiling mounted rainfall shower head, and I need to tie into the DWV line (floor will have appropriate slope to linear drain).

Long story short - a licensed plumber was working at my neighbors, was quite friendly, and I asked him if I could get his advice on my project and how to add the linear drain. He was nice enough to come over and take a look, and said to do it as shown in the first marked up photo. To run both drains into a single P-Trap, and tie into the existing DWV line just before the Wye. Sounded easy...

However, though this arrangement would likely work fine for the shower application, the physics for a tub usage worry me. I would be concerned that if a full tub was drained, that it could potentially backflow out of the linear drain. It seems safer to have separate P-Traps for the tub and linear drain, and tie them into the line as shown in final marked up photo.

Does this work? I thought I read that two P-Traps can't share a vent because one might get sucked dry by the other, but I've seen similar DWV layouts so I'm a little confused. Help and advice is very much appreciated. Thanks!

 

[wwhitney]

While I think having the shower and tub share a trap would work, I believe it violates UPC 1001.2:

https://up.codes/viewer/california/ca-plumbing-code-2019/chapter/10/traps-and-interceptors#1001.2

The simplest solution is to let the shower wet vent the tub, as in your last drawing. However, you currently have a flat vent, which is not allowed for a dry vent. (If the pipe labeled vent is receiving drainage from a lav, it's a wet vent, and the geometry you drew is fine. (*)) The shower vent takeoff needs to be vertical and the shower vent needs to stay vertical until 6" above the shower flood rim. (45 degrees off plumb counts as vertical.)

So you need to route your shower drain to get close to the wall with the vent stack and arrange for the vertical vent to enter the wall before it emerges from the floor. If you have the height for it, you could use a san-tee with the barrel tilted 45 degrees off plumb. The shower trap arm would enter the side entry, the vent comes out the top with a 45 under the wall plate, and the bottom gets a street 45 to turn horizontal as high as possible.

If that doesn't work, then I think the easiest thing is to use an upright wye under the wall plate, where the left side gets the shower trap arm (ideally just by pointing the u-bend towards the vent wall, and using the 90 degree outlet elbow to point to the right), the top branch entry is the vent and gets a 45 to go plumb, and the right side gets a LT90 to turn down the joist bay. It is also possible to put the wye adjacent to (in plan) but not under the wall plate, roll the wye 45 degrees off upright towards the wall, and use a (possibly street) 60 degree bend to turn the vent up into the wall.

Cheers, Wayne

(*) On the off chance that the vent is a wet vent receiving a lavatory, and at the bottom of the picture the current tub drain goes on to wet vent a WC, if you now add a shower to the wet vent, you will need to upsize the wet vent to 3" starting at the wye where the tub joins the shower and lav drains. That assumes you have at most 2 showerheads.

 

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[dwvpuzzled]

Thanks very much, Wayne.

To clarify, the pipe marked 'vent' is a dedicated vent for the old shower drain. It goes directly up the wall and through to the roof, and doesn't vent anything else. The WC, and the lavs are on the wall opposite and have their own dedicated vent.

Given that, is there still an issue?

Or should I plumb a vent dedicated to the linear drain P-Trap as shown below?

Appreciate the help, and the link to the code.

 

[wwhitney]

To clarify, the pipe marked 'vent' is a dedicated vent for the old shower drain. It goes directly up the wall and through to the roof, and doesn't vent anything else. The WC, and the lavs are on the wall opposite and have their own dedicated vent.

Given that, is there still an issue?

Yes, your existing tub dry vent is not compliant, as it is taken off horizontally and is horizontal below the floor. A dry vent needs to be taken off vertically and stay vertical as it rises out of the floor.

But as you are adding a shower drain near the wall with the vent, you should be to arrange for the shower dry vent takeoff to be vertical and to connect with your existing dry vent, as I described earlier. The shower drain then serves as the tub vent, a wet vent, which is allowed to be horizontal below the floor. Problem solved.

Cheers, Wayne

 

[dwvpuzzled]

Much appreciated.

Apologies, but I'm a little confused. The existing drain vent is taken off horizontally, but is horizontal below the floor not above.

But if I understand you correctly, the way I illustrated it in the last marked up photo is how I should do it. Vertical vent for the linear drain P-Trap tied into the existing vent line. Correct?

 

[wwhitney]

Apologies, but I'm a little confused. The existing drain vent is taken off horizontally, but is horizontal below the floor not above.

Yes, that was a typo I fixed as you were posting.

But if I understand you correctly, the way I illustrated it in the last marked up photo is how I should do it. Vertical vent for the linear drain P-Trap tied into the existing vent line. Correct?

Not tied into, instead of. And your green vent take off looks like its before the trap, not after.

So something like the drawing below, where green is all horizontal (except part of the trap) and the red is vertical. The trap won't actually be oriented as shown, you'll want to get it as close to under the wall plate as you can. You can jog the red vent over to the right to join the existing vent using a pair of 45s, at any elevation.

Oh, and while you have it open, you'll want to replace all the accessible galvanized DWV with plastic or cast iron. The galvanized vent may be OK to reuse.

Cheers, Wayne

 

[dwvpuzzled]

Not tied into, instead of. And your green vent take off looks like its before the trap, not after.

So something like the drawing below, where green is all horizontal (except part of the trap) and the red is vertical. The trap won't actually be oriented as shown, you'll want to get it as close to under the wall plate as you can. You can jog the red vent over to the right to join the existing vent using a pair of 45s, at any elevation.

View attachment 78332

So I eliminate the section you have erased? I think I'm following you, but having a hard time putting the pieces together in the space between the joists. I might have to go down to 1 1/2" ABS to make it work, but even then, I'm struggling a bit to imagine it.

 

[dwvpuzzled]

I'm guessing I will have to do this version of a P-Trap and Trap Arm:

With the minimum 3" trap arm before the Sanitary Tee. Can the Sanitary Tee be tilted toward the wall plate at 45°?

Thanks!

 

[wwhitney]

The UPC doesn't permit a sanitary tee on its back like that for a vent takeoff, that's an IPC practice. You need to use a wye as I mentioned. And yes, the wye can be rolled 45 degrees off upright towards the wall plate, which would require a 60 degree elbow to get to plumb. [Which obviously means the vent takeoff isn't actually vertical, as it's more than 45 degrees off plumb, but this is an accepted practice.]

You should be able to point the u-bend towards the wall, point the trap outlet elbow parallel to the wall, hit the wye (see this post for the minimum distance between the trap outlet and the wye https://terrylove.com/forums/index....-machine-plumbing-question.96912/#post-697328 although it's the same as the diagram you posted), roll the wye 45 degrees if necessary, and then hit a (possibly street) LT90. Hopefully that will fit in the width of the joist bay.

If not, you could try turning the trap outlet 45 degree away from the wall (or sticking a 45 degree elbow in between the trap outlet and the wye), that changes the wye orientation, so it should fit in the joist bay width. The question is whether the vent still makes it to under the wall plate before it rises through the subfloor.

If nothing else works, and your joists are 2x10s or larger, you could get a linear drain with a non-centered outlet and drill the joist for the 2" shower trap arm to go through it, assuming the trap arm would be 2" clear from the top of the joist.

Cheers, Wayne

 

[dwvpuzzled]

Thank you! The link to that thread explaining the minimum distance is really helpful.

 

[dwvpuzzled]

Before I glue this up, I wanted to post photos and see if there is any comment.

I was able to do it by turning the long sweep wye at 22.5° and using 1/16 bend fitting to vertical and to meet the existing vent.

Thanks for your help!

 

[wwhitney]

The vent takeoff looks good to me, I assume that's a LT 90 after the combo?

On the shower (edit: I mean the downstream fixture, maybe that's the tub?), you could use a wye instead of a combo if you rotate the trap outlet 45 degrees towards the bottom of the picture (and move the trap outlet by rotating the u-bend, and possibly moving the wye). But that's a minor optimization, not a big deal.

Be aware that if you have dry fit everything, when you do the glue up, the pipe will seat further into the hub than during the dry fit. The fit is an interference fit that only becomes possible as the solvent cement softens the material. So for critical runs where you don't want the shortening, you need to recut pieces slightly longer than your dry fit. I can explain further if it's not clear how to determine the new lengths.

Cheers, Wayne

 

[dwvpuzzled]

I'd thought about the dry fit vs. glue fit, and there are a few straight sections that I can make adjustments as needed.

Any suggestion on gluing order so I don't get myself in trouble?

Thank you

 

[wwhitney]

Any suggestion on gluing order so I don't get myself in trouble?

That would depend somewhat on how you are tying in at each end to the existing DWV. But the obvious thing is to plan it out in advance, so you don't find yourself with a joint to make up where you no longer have clearance to separate the pieces and apply the solvent cement. I also try to preassemble the tricky parts with lots of fittings close together.

Cheers, Wayne

 

[dwvpuzzled]

Be aware that if you have dry fit everything, when you do the glue up, the pipe will seat further into the hub than during the dry fit. The fit is an interference fit that only becomes possible as the solvent cement softens the material. So for critical runs where you don't want the shortening, you need to recut pieces slightly longer than your dry fit. I can explain further if it's not clear how to determine the new lengths.

For a single joint, what is the additional pipe length needed between dry fit and cemented joint? IOW, how much longer should the pipe be for each joint versus dry fit?

Thanks

 

[wwhitney]

That's going to vary depending on how you dry fit it, how far you push the pipe into the socket on the dry fit.

So what you do is this: for each pipe diameter you are working with, do a test glue up of a known length into two hubs, one at each end. And then measure the visible pipe length between hubs after the test glue up. The difference is the take up length for that pipe size. It should just be twice the hub depth, and the hub depth should be 5/8", 3/4", 1-1/2", and 1-3/4" for 1-1/2", 2", 3" and 4" hubs, respectively. But it's good to do a test and confirm that.

So now when you have the dry fit made up with all the fittings in the places you want, you just measure the visible pipe length between any two hubs, add the take up length for that size pipe, and that's your final pipe length for that segment. If you have any weird fittings, or different brands, it's worth confirming that the hub depth is the same on the fittings as it was on your test glue up. I seem to recall running into a difference once, not sure what the details were.

Cheers, Wayne