Wetvent in Bathroom Group with downstream laundry. Experts is this ok?

[RedBird52]

Hello Experts! First of thank you so much for all the the knowledge you have shared with me. I am embarking on my next bathroom renovation. It has a laundry, and will have a bathroom group above. I drew the images below with craft amigo.

Does the wetvent look ok? I am just not sure if it is ok to have a toilet upstream of the shower on a wet vent system such as in my layout below.

 

[RedBird52]

In case it is relevant, here is what the entire thing looks like which includes a bathroom and laundry ABOVE the bathroom in question that we are discussing. I am comfortable with the wet-vent in the bathroom above.

 

[Sponsor]

 

[wwhitney]

MA has its own plumbing code, 248 CMR 10. Section 10.16(5) covers wet venting, and it appears to only allow a 2" horizontal line plus a 2" riser to the end of the lavatory trap arm to serve as a went vent. Which means that a 3" line carrying a WC can not serve as a wet vent.

10.16 Vents and Venting: 248 CMR 10.00: Uniform State Plumbing Code | UpCodes

UpCodes offers a consolidated resource of construction and building code grouped by jurisdiction

up.codes

You could try perusing the rest of Section 10.16 to see if there are any other allowances that might help you.

Also, your rendering shows the shower connecting to the 3" line with a san-tee on its side. That needs to be a 3x3x2 combo, but the discrepancy may just be a limitation of the shapes available in your software.

Cheers, Wayne

 

[RedBird52]

@Wayne, thank you so much. (you also came to my rescue about a year ago when I had some questions as well!)
Confirmed what you called out is a 3x3x2 combo, not a san-tee. Its a strange angle of the screenshot.

That link is super helpful. I'm studying it now.

 

[RedBird52]

Man, so after reading 10.16, it appears I can not wet vent through a 3" pipe...(?). Which leads me to the design below, but this is so much less clean/elegant because (1) I am obstructing the water flow by turning it around. (2) I will be hacking up the floor joists real bad (3) my height clearance in the 10" joist bay will be tight.

So here is the 10.16 compliant design. Does this look ok?

If I choose to gamble and ignore 10.16, is my wetvent layout below UPC compliant?

 

[wwhitney]

So here is the 10.16 compliant design. Does this look ok?

Looks like you have the laundry drain intersecting with the lavatory drain. Also see 10.16(5)(a)(1), the wye joining the wet vent to the 3" line is to be rolled up somewhat. Otherwise the venting seems to be OK.

Presumably there's a wall that is carrying the 3" drain from above that joins on the right, could you take a dry vent off for the shower in that wall, and have the shower drain join the 3" line after that 3" wye? Also seems odd that the shower trap outlet is to the left instead of towards the 3" line (or to the right for the above suggestion).

I'm not sure how you could have the laundry drain pass over the lav drain, which has to pass over the WC. As just the pipes themselves are 2.375" (laundry) + 2.375" (lav) + 3.5" (WC) = 8.25". And any holes in the joists have to be 2" clear from the top and bottom of the joists. Maybe you could run the lavatory drain to the left in the wall, so that it enters the floor system to the left of the WC, and all the non-laundry drains can be at approximately the same level? (Other than the WC being a little lower per 10.16(5)(a)(1).)

If I choose to gamble and ignore 10.16, is my wetvent layout below UPC compliant?

No, the UPC also prohibits a wet vent from carrying a WC, so it would require the same lav - shower - WC connection order. It would comply with the IPC, though.

Cheers, Wayne

 

[RedBird52]

What a comprehensive answer. Thank you so much for sharing this knowledge with me
Per your feedback, (1) a separate vent for the shower arm. (2) fixed formerly intersecting pipe (3) Confirmed about rolling Wyes for wet vents.

Im less familiar with these separate vents. . Did I do this ok/correctly? Is blue wye ok? Would blue wye be rolled?

 

[wwhitney]

A dry vent has to come off the top of the pipe and rise vertically (which includes up to 45 degrees off plumb) until at least 6" above the fixture flood rim. So you generally have to route the trap arm under a wall to take off a dry vent. The horizontal section of vent entering the barrel of the blue wye is not allowed. See 10.16(1)(c).

If you are still having difficulty with the layout, it would be helpful to know the joist locations and wall locations, e.g. in a floor plan.

Cheers, Wayne

 

[RedBird52]

Wayne thank you so much for the knowledge.

The horizontal section of vent entering the barrel of the blue wye is not allowed. See 10.16(1)(c).

This is why I asked. My drawing is very wrong then. Thank you for calling me out.

Joists run parallel to the to the horizontal 3" pipe.

Floor plan below. Bathroom is directly over an open-layout kitchen. You can see that I have the drain of the tub, opposite the sink end. There is just no room in the joist bay to fit a tub trap with the stack in there. My thought was to flip the direction of the tub for more working room.

Im contemplating committing the sin of only being IPC compliant... (under my original drawing).

 

[wwhitney]

For MA code compliance, I guess my best idea is based on your first drawing in post #5, with a few adjustments:

- The 3" line needs to be as low in the joist bay as possible while maintaining slope, to allow the 2" lav drain to cross over via holes in the joists that stay at least 2" clear from the top edge.
- Once the laundry drain enters the joist bay with the 3" line, it needs to drop down to approximately the same elevation as the 3" line, so that it can pass under the lav drain. For connecting to the 3" line with a 2" street 45 into a 3x3x2 wye, the center to center distance between the parallel drains will need to be approximately 4.9".
- Turn the shower trap outlet 90 degrees to point towards the 3" line. Note that the u-bend and outlet elbow of the trap can be rotated independently, the combination does not need to be planar.
- To join the lav to the shower, do it in the joist bay next to the WC joist bay, as the WC joist bay is too crowded. The lav drain does a 180 via a LT 90 into the side entry of a combo, with the shower going straight through the barrel of the combo.
- The shower drain hits a LT90 in the WC joist bay. It wants to end up again about 4.9" center to center from the 3" line, and at at least a slightly higher elevation to comply with 10.16(5)(a)(1). Given the dimensions of a LT90, that could mean you need to adjust its location within the joist bay with a couple 22.5 degree elbows before it joins the 3" line.

Cheers, Wayne

 

[Jeff H Young]

maybe run the vent up the laundry wall and connect with that vent for yoour tub/shower vent connection are you doing no dry wall opening for any of this ? boston code requires ipc and upc requirements both ? i dont know your codes but assuming outside of boston it goes to a massachucets code ?

 

[RedBird52]

Wayne, THANK you so much for your expert knowledge. You are a master at this, and I am greatful to be learning.

- Turn the shower trap outlet 90 degrees to point towards the 3" line. Note that the u-bend and outlet elbow of the trap can be rotated independently, the combination does not need to be planar.

I think I achieved your suggestion except for the trap articulation. two issues, my software does not support the trap articulating (although i understand what you are saying) and with a 5ft tub (possibly 5.5ft non standard tub in this strange bathroom layout) and the wrap around of the lav/vent I am now potentially more than 6 linear feet to the vent? (Vent is measured from the lav-Tee correct?)

are you doing no dry wall opening for any of this?

assuming outside of boston?

All the drywall (plaster and lath) can be opened in this space. Happy to bust walls in the bathroom, just want to avoid it outside the bathroom.
I am in Brookline, MA Now. (account was opened while i was in boston) This is a good point. I am bound to the MA code not the boston code. But still am not supposed to wetvent per my other drawing.

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I Think my next step is to lay the parts out on the floor, and see if I'm within that 6 foot magic number, and how the joists would allow me to fit all the stuff in it. I just don't think I'll be able to fit it reasonably.

 

[wwhitney]

(Vent is measured from the lav-Tee correct?)

No, trap arm length is measured from the tub trap outlet to the wye where the wet vent (lav drain) joins the tub fixture drain. And while length is regulated, what matters for the physics is that the total fall on the trap arm is limited to one pipe diameter. Which gives a length limit from the minimum fall rate of 1/4" per foot.

With a tub, it's fine to use a 2" trap to help with the above, even if 1-1/2" is more common.

Cheers, Wayne

P.S. If it were important for some reason, the lav drain could traverse left or right within the wall before it enters the floor system.

 

[RedBird52]

P.S. If it were important for some reason, the lav drain could traverse left or right within the wall before it enters the floor system.

Thank you so much Wayne! That is also a great call out --my problem is how tight it is the corner where the stack comes up into the floor system. I also can not access any of the pipes from below, as it is in a newly renovated kitchen. (best to keep the wife happy!)

 

[RedBird52]

Hello Experts!
Does the Wye (in Red) need to be rolled up? It services a bathroom on the floor above. (or can the wye (in red) be laid horizontally/flat?)

Having trouble hosting images today... here is a link https://i.postimg.cc/K8YdGS3S/Screenshot-2026-03-27-at-9-41-07-AM.png


ill try again later

 

[wwhitney]

The only constraint on the red wye is that the two inlets should have at least 1/4" per foot fall. Other than that it can be "flat" or upright or whatever.

Cheers, Wayne

 

[RedBird52]

red wye is that the two inlets should have at least 1/4" per foot fall. Other than that it can be "flat" or upright or whatever.

Thank you so much Wayne!

 

[RedBird52]

So, this is an image of the entire system I am building.
The green trap is an 'emergency' floor drain which will be on the floor under the laundry. (ill probably fill the trap with a bit of oil, so it does not evaporate.

Is the green trap ok to be used as a floor drain like this?

 

[Reach4]

The dry vent line at upper right needs to stay within 45 degrees of plumb. To do that, run the trap arm over to the vent at the wall, vent it, and then route it horizontally toward where it needs to go.

 

[John Gayewski]

The only constraint on the red wye is that the two inlets should have at least 1/4" per foot fall. Other than that it can be "flat" or upright or whatever.

Cheers, Wayne

Isn't that red wye a stack carrying suds? Does Massachusetts not have a "10 pipe diameter" or equivalent rule at its base for suds?