Vent check

[Stereo]

I'm a little iffy on the venting of the toilets in a back-to-back 3/4 bath rebuild because of the combination of vertical and horizontal attachments to a horizontal drain line. The photo shows a VERY simplistic drawing of the layout. I have two toilets entering the horizontal drain horizontally. I also have two lavs and two showers entering the horizontal drain vertically. The layout is the way it is because of structural limitations so I am not asking for alternative layouts. I also know the proper fittings to use. I have just ONE straightforward question I would appreciate an answer to. Does the dry vent of the upper toilet also serve as a (wet) vent for the downstream toilet since it's less than 6' away (it's about 5')? 2018 IPC. Thanks for your help.

 

[Stereo]

"Does the dry vent of the upper toilet also serve as a (wet) vent for the downstream toilet since it's less than 6' away (it's about 5')? 2018 IPC.

Maybe not. I just came across this: "Keep in mind that any additional fixtures outside of the [horizontal] wet vent that are to be drained into the same branch must be connected to the branch downstream of the wet vent." https://www.iccsafe.org/building-sa...ps-in-the-2021-international-plumbing-code-3/

If I'm interpreting that statement correctly, the downstream WC has to have its own vent because of the other fixtures entering the horizontal branch in between the downstream toilet and the upstream vent. Do you agree?

 

[Sponsor]

 

[wwhitney]

The IPC, in force in Colorado, allows horizontal wet venting for up to two bathroom groups at once.

So yes, your downstream WC is properly wet vented as drawn.

In general, each fixture (except WCs) needs to have a vent connection before the trap arm has fallen more than one pipe diameter. (The WC still needs a vent connection, it just doesn't have the elevation limit that every non-siphoning fixture has). That vent connection can be a dry vent, or it can be a wet vent, if the fixture is a bathroom group fixture and joins a horizontal branch carrying only bathroom group fixtures, one of which is dry vented.

So given the connectivity order of your diagram, you only need one dry vent for the two upstream most fixtures, the shower and the WC. That dry vent could be on the shower, or if the shower trap is at the same elevation as the horizontal branch, it could be the dry vent you show between the upstream shower and upstream WC. You don't need both, although it's fine.

Individual lavs always need their own dry vent, as their traps are above the floor, so the horizontal branch is too low in elevation to wet vent them.

With proper shower trap arm routing (staying horizontal until it hits the lav drain), the downstream most shower does not need a dry vent, it can be wet vented by the lav it is connected to.

Cheers, Wayne

 

[Stereo]

So yes, your downstream WC is properly wet vented as drawn.

Thank you, Wayne! I appreciate the thorough explanation. You saved me some work.

 

[Stereo]

After stewing about this, I'm wondering if the upper toilet dry vent really can serve as a vent for the lower toilet. There's that statement from the ICC that's bugging me: "Keep in mind that any additional fixtures outside of the [horizontal] wet vent that are to be drained into the same branch must be connected to the branch downstream of the wet vent." Yes, the IPC allows horizontal wet venting, but that assumes all fixtures join the horizontal branch at the same level. Intervening vertical drains will - at least temporarily - block the air flow between the lower toilet and the upper toilet vent. I think I'll err on the side of caution and add a vent to the lower toilet.

 

[Stereo]

Yep. I need a separate vent for the downstream toilet. This video explains it well.

I'm posting for anyone having the same question.

 

[wwhitney]

There's that statement from the ICC that's bugging me: "Keep in mind that any additional fixtures outside of the [horizontal] wet vent that are to be drained into the same branch must be connected to the branch downstream of the wet vent."

The horizontal wet vent in your diagram in the OP is allowed to be both bathrooms groups, so the downstream WC is not "outside the wet vent," and the above does not apply. The above would be referring to something like a washing machine, kitchen sink, etc.

Cheers, Wayne

 

[wwhitney]

Yep. I need a separate vent for the downstream toilet. This video explains it well.

No, you don't. And here are my step by step responses to the points in the video:

Example 1 claims that having the WC drain entering the top of the wet vent is a violation (which is not a condition that your drawing shows). This is true for the IRC if your jurisdiction has adopted the plumbing sections of the IRC. Namely, P3108.1 of the IRC says in part "Each fixture drain shall connect horizontally to the horizontal branch being wet vented or shall have a dry vent."

However, if you are subject to the IPC, or UPC, this is not true. The equivalent sentence in IPC 912.1 just says "Each wet-vented fixture drain shall connect independently to the horizontal wet vent." The only requirement is independently, no mention of horizontally. Likewise, UPC 908.2 has no mention of the fixture connecting horizontally.

Now in jurisdictions that have adopted both the IRC plumbing sections and the IPC, you need to check the scoping sections of the IPC and IRC as adopted to see whether when working on a building that is subject to the IRC you must follow the IRC or have the option to follow the IPC.

In the case of CO, the exception to IPC 101.2 Scope says that detached one and two family dwellings "shall comply with the International Residential Code." No choice in the matter. So you would be subject to IRC P3108.1, although again that's not an issue in the the diagram in the OP.

Chapter 1 Scope and Administration: Scope and Administration, Colorado State Plumbing Board Plumbing Code 2018 | UpCodes

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Chapter 31 Vents: Vents, Colorado Residential Code 2021 | UpCodes

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

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Example 3 shows a horizontal double wye, which is a bad idea, as you can not adjust the slope of the two branch inlets independently. Best case you get the two inlet exactly level to each other, but then their slope is only 71% of the slope of the barrel of the fitting. So you'd want to slope the barrel at least 3% to get 2% on the inlets. But more likely, one inlet will end up a little lower than the other, and that will be a problem. So just use two single wye fittings, one after each other.

Example 9 refers to a requirement that the WC is the last fixture on a horizontal wet vent. That requirement only exists in the UPC, not in the IPC.

Examples 10 and 11 show a dry vent takeoff where the dry vent turns horizontal under the floor. This is a violation, a dry vent has to stay vertical (which includes up to 45 degrees off plumb) until at least 6" above the fixture flood rim. Since you are subject to the IRC, the reference is P3104.4.

Chapter 31 Vents: Vents, Colorado Residential Code 2021 | UpCodes

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Cheers, Wayne

 

[Stereo]

No, you don't. And here are my step by step responses to the points in the video:

Wayne, thank you for the details and references. Much appreciated!