When determining drain vents, 2" can have a vent 8' away and 3" can have the vent 12' away, how does that work in terms of wet venting? I have a toilet flange connected to a line that has a Wye which picks up my sink approximately 4' down stream.

So technically, I have a vent connected to my toilet trap approximately 4' down stream, however it is a wet vent so does that mean I have to also add the length of drain line to the 12' maximum distance?


1 Answer 1


Updated Answer

Definition of a wet vent: a drain pipe that also serves as a vent pipe and extends from the most downstream wet-vented fixture connection to the most upstream fixture connection (the green item 4 in the below sketch).

Wet vents are typically used when plumbing a bathroom group. So yes the shower can also be vented by the wet vent along with the toilet. There is one major stipulation when wet venting multiple fixtures when a toilet is one of them: the toilet must be the last fixture connected to the wet vent. This means that in some situations you must use a symmetrical fitting (such as a double wye) in order to not break that rule. (Also, there can only be a max of two toilets connected to a wet vent).

Typical Bathroom Wet Vents:

enter image description here

The left sketch shows a typical arraignment when the toilet (WC) is located in-between the sink (LAV) and shower (SH). This arraignment relies on the use of a double wye fitting to avoid lines crossing while maintaining the toilet as the last fixture connected to the wet vent.

The right sketch shows how a bathroom wet vent might be arraigned without the use of the symmetrical fitting. There are many ways that a wet vent can be arranged, it all depends on the location of the fixtures and the direction the drain is coming from.

Components of a Typical Bathroom Wet Vent:

  1. Branch: Distance is unlimited but must be graded as per the diameter of the pipe.
  2. Trap Arm / Fixture Drain (shower): Distance is limited by the diameter of the pipe as the fall cannot exceed the pipe diameter. If 2" the distance would be 8' measured from the connection to the wet vent (where blue meets green).
  3. Fixture Drain (toilet): Distance is limited by the diameter of the pipe as the fall cannot exceed the pipe diameter. If 3" the distance would be 12' measured from the connection to the wet vent (where violet meets green).
  4. Wet Vent Branch: Distance is unlimited but must be graded as per the diameter of the pipe.
  5. Trap Arm (sink): Distance is limited by the diameter of the pipe as the fall cannot exceed the pipe diameter. Measured from the connection to the vent.

Issues With Your Sketch

The shower is connected to the toilet's fixture drain meaning that the the toilet is no longer the last fixture connected to the wet vent. (The issue with the toilet not being the last fixture is that the rush of water when the toilet is flushed could choke the vent connection of the other fixture (in your case the shower) and could result in siphoning out the fixture's trap.)

If you were to run the vent for the shower as shown in grey, the toilet would now be wet vented by the shower. The pipe downstream of the vent connection to the shower's fixture drain and the connection to the toilet would be the wet vent. The grey vent would be a dry vent, however the issue here would be that in most locations, you are not allowed to run the dry vent horizontal until you are 6" above the flood level of the fixture is serves and at minimum 42" above the floor.

The solution in your case would be to use a double wye fitting to symmetrical connect all three fixtures, similar to the left sketch.

I assume that the other fixture shown in the bottom left of your sketch in separately vented.

Original Answer

No. The portion from the wet vent connection (wye) to the 90 elbow under the toilet has a max distance determined by the grade of the pipe (3" pipe, 12' distance). The wet vent portion, from the vent connection at the sink to the wye that connects the toilet has no distance limit (it actually goes vertical). The portion from the vent connection to the sink is limited by the grade of the pipe (distance will vary as the pipe could be 2", 1-1/2", or 1-1/4" for a bathroom sink).

  • I added a picture above to add clarification. I'm thinking the toilet vent is fine, but also want to know if it's considered okay to vent my shower with the straight branch off of a Wye (where once again, the vent itself is not "elevated" very much. Otherwise I use a 45 bend there and use a wye rolled up 45 degrees to vent the shower separately because I don't think the sink would serve it.
    – CCCBuilder
    Jun 17, 2017 at 22:27
  • See updated answer.
    – pdd
    Jun 18, 2017 at 2:12
  • A double Wye is a special order item here, and with this being a weekend that means even worse. I have a lot of fittings so I am thinking I need to work with what I have and make it right. Assuming I can not use the wye on it's side to vent the shower like shown in grey, I could attempt to: 1.)Vent shower with a San-Tee on back, then a street 90 to make the elevation as low as possible 2.) Use a Wye on it's back and a street 90 to get a tiny bit lower elevation 3.) ??? A better idea without using a double Wye?
    – CCCBuilder
    Jun 18, 2017 at 3:37
  • Your option 1 in your comment would work fine. It may not be to code if you happen to live in an area that does not permit flat venting (horizontal vent below flood level rim), but I have seen it done before. The concern with flat venting is that if your system backs up, derbies could get logged in the flat vent portion (in a wet vent, water would flush the line). If posable, add a cleanout on the vent in the wall (or attic) so that you can easily cleanout the vent if it does backup.
    – pdd
    Jun 18, 2017 at 3:52
  • Another option is to run the shower line to the wall, turn vertical with two 45 fittings, then use a sanitary tee with the outlet pointed to the shower drain. The vent would continue off the top the the tee. This would allow compliance to the code but requires that you have adequate vertical space.
    – pdd
    Jun 18, 2017 at 3:53

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