What is Wet Venting 104

Wet venting is a plumbing term and is a method of protecting the trap on a drain pipe on multiple plumbing fixtures.  It is a useful method of venting and can save pipe and fittings, time and money. If you don’t know what venting is or why it’s needed, first read What is Venting?


Wet Vent Definition: A waste pipe that also serves as a vent pipe.

Wet venting is most common in conjunction with toilets and sinks; the drain for the sink is also the vent for the toilet. It can also be used for a variety of other applications but due to the following rules this is the most convenient and common situation to run into.

Wet Venting Rules:

The Ontario code book is written probably by lawyers and is very confusing, so I’ll try to simplify it a bit and include just the most common and relevant parts.

1) A waste pipe may serve as a wet vent provided that,

a) There’s not too much hydraulic load on the wet vent… 1.5″ pipe can not serve as wet vent on a toilet and max 2 fixture units. A 2″ pipe can serve a toilet and a maximum 4 fixture units draining into it.  (Sinks, tubs, showers all have 1.5 fixtures units each). As a general rule, you will just be able to vent 2 fixtures on a toilet wet vent.

b)Toilets must be installed downstream of all other fixtures

c)The wet vented portion of the pipe may not be reduced in size

d)The length of the wet vent is not limited

Practical Examples:

1) Toilet and Sink:

The toilet is vented through the sink drain. The toilet drain should be 3″, the sink drain is 1.5″, the shared sink drain/toilet vent area should be 2″, and the vent going up should be 1.5″. Also note that where the 2″ drain connects to the 3″ at the bottom the fitting should be a y instead of a tee as shown.


2)Toilet, sink and tub (or shower)

Like the previous picture the toilet and sink are wet vented together.  This time the wet vent connects to the vertical leg of the toilet drain which is also good. Also this time we have a tub connected to the wet vent. Here it is shown to have it’s own vent off the drain. If the tub was less than 5ft from the wet vent, however, then you wouldn’t even need to have that extra vent on the tub.  This is a great way to save money on materials as well as saving time.


3) Shower and sink

In this picture it is vented exactly the same as the first example only you will not have the option to tie in the drain to the vertical leg. Also the wet vent area (shared drain and vent section of pipe) can remain 1.5″ if both the sink and shower traps are also 1.5″.



Let me know you have any questions by dropping a line in the comments section. I can also update the blog to better clear confusing points.

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104 thoughts on “What is Wet Venting

  • kenji sasaki

    Hi Mike, thanks for the explanations and the forum! I have a bathroom remodel with lavatory, shower, and WC, but walls are closed/tiled.
    Right now, all three are in a line like: vented lav, WC, shower, then soil stack.
    Can these be correctly connected without an additional vent installed?
    I cannot use a 2-to-3 heel-in from the sink to WC line, right? it has to be a horizontal-connected sanitary T?
    thanks again!

    • Mike

      Hi Kenji,
      More than likely you’ll need to separately vent something. By the sounds of it, I would wet vent the toilet and sink together and dry vent the shower.
      All the best

  • Simon Parry

    I want to connect a shower drain to a toilet drain. (I am pretty sure the toilet is already vented according to Ontario Plumbing codes of the early 90’s which is when this house was built.)
    The shower drain will have a P trap on it and the drain pipe will run across (under the concrete floor) to link up with the waste pipe from the toilet. It sounds as if this connection cannot be downstream from the toilet. If this is the case it will have to connect into the toilet (soil) pipe into the 90 degree curved pipe under the toilet. (I’ve never seen such fitting in Home Depot.)

    Your advice in this will be most helpful.
    Thanks very much.

    • Mike

      Hey Simon,
      There might be a bit of confusion there, but you can tie into the horizontal toilet drain with a ‘y’ fitting. Just make sure that your shower is still vented.
      Thanks for the question and good luck with your project.

  • Rick Deis

    Great info Jay. Thanks for sharing. 1. a) states ” a) There’s not too much hydraulic load on the wet vent… 1.5″ pipe can not serve as wet vent on a toilet and max 2 fixture units. ” I trust you meant 1.5″ pipe “can” serve instead of “can not” ?
    After years of the basement toilet making gurgling sounds and overflowing a couple of times I’ve torn the entire bathroom out and plan on redoing everything. I’ve discovered a couple of things which I believe have caused my problems. I’ll try and give you an idea of my setup prior to asking my questions.
    Our bungalow is L shaped with main stack on one segment of the L and the basement plumbing/sewer exit on the extreme end of the other segment. All the main floor sewer lines flow into a 3″ horizontal pipe and then vertically drop straight down into the basement floor in line with the bsmt bathroom wall and then exits the house beneath the opposite bthrm wall (outside wall). There is a cleanout on the bottom of the vertical (soil stack) and a 1.5″ drain pipe coming from a laundry sink ptrap/standpipe/ptrap combination .
    The 7′ x 7′ bsmt bathroom plumbing consists of a toilet 15″ off the wall and directly in line with the under slab sewer (13.5″ from left concrete wall) next is the sink plumbing which consists of 1.5″ drain /1.25″ Vent about 4″ off the left concrete wall. This 1.25″ vent eventually ends up as a 2″ vent exiting the roof. I’m not sure if the main floor laundry ties into this vent. Next is a 2″/1.5″ bathtub drain (Originally intended for shower as it is 20″ off the left concrete wall and 18″ off the sewer exit concrete wall.
    1./ Inadequate venting? Will the sink venting serve to wet vent the shower drain ?
    2./ Toilet T directly into sewer is a no no?
    3./ Can the soil stack/waste stack coming from the main floor be used for the bsmt plumbing venting?
    Hears what I am planning; can you comment on my plan or any easier way?
    a) Install shower instead of bathtub.
    b) Remover the laundry sink/standpipe plumbing and somehow use it for my water softener drain(previously drained into sink). Maybe like a the hookup for a dishwasher drain?
    c) Relocate the toilet and tie in a vent from it (in the ceiling) into the 1.25″ sink vent

    Rick D

    • Mike

      Hi Rick,

      Question 1:
      A toilet always needs a 2″ wet vent. Only when it’s dry vented can it be 1.5″

      Question 2:

      I feel you should be getting a plumber in to take a look at this one Rick. There’s too much going on to communicate over a blog post. Thanks for the question anyway.


      • Rick Deis

        Thanks for the info Mike. Unfortunately my rural location makes even getting a quote somewhat expensive. I’m better served finding out exactly what I need so I can get several quotes.
        If you could just clarify one thing for me and that’s in regards to determining what is the effective vent size? In my case where the basement sink vent goes from 1.5″ and then reduces to 1.25″ in the wall above the flood line and then continues to the attic where it ties into a 1.5″ kitchen sink vent. That 1.5″ pipe then ties into the 2″ main floor laundry vent which terminates through and above the roof. What size is basement sink vent?



          • Rick Deis

            Thanks Mike but there appears to be a bit of a misunderstanding. The question was “what is the effective vent size” Is it the size that exits the waste pipe( 1.5″) or is it the smallest size it gets reduced to along its path (1.25″) or is it the size that exits through the roof (2″) ? Seems to me somewhat pointless to have reduced it to 1.25 as that size is actually more expensive than 1.5″ or 2″. Plumbing was done sometime after 1970 and prior to 2002 when we purchased the house. I expect it was early 70’s as its all copper. If you choose to ignore I understand.



          • Mike

            The appropriate vent size is dependant on the load serving it at any given point in the system. The further down you get and the more fixtures are tying in and adding load, the bigger the vent size needs to be. Getting a plumber in to determine things for you is probably a good idea.

    • Mike

      Sorry Luis but that case won’t work out. Try dry venting the tub or shower and wet venting the other two with a vanity sink or something. That’s usually the best case scenario.
      All the best!

  • Dennis Patterson

    Hi, thanks for the great articles. I’ve read a lot of the code, and done a lot of research but I’m not sure if my plan will meet code and I’m hoping you can help me out. Part of the trouble is reading too much online I’m sure. I’m converting the basement to an apartment and therefore have to add laundry and a kitchen sink. I have a basement washroom(toilet, sink, shower) and a main floor laundry on an aux vent at the back of the house. The main stack is about the middle of the house. Toilet has a 2″ vent, can I add a clothes washer in the basement to this 2″ toilet vent? Can I add a kitchen sink in the basement onto the main stack (3″ stack add a 3x3x2″ fitting for the drain and vent it out the back aux vent ( about 6 m away)? Hope I’ve left enough detail for you to answer.

    • Mike

      Hey Dennis. I believe you can tie in your fixtures to the drains that you wanted as long as you still separately vent them and tie those vents into existing dry vents. Let me know if that makes sense to you.

      Mike – Jaytech Plumbing

  • Andrew

    I’m putting in a basement bathroom. All units; sink, toilet and shower, are draining into a sewer pump pit in which I must install a pump that is supposed to have a 2″ vent. I’m wondering if I can ty into an existing 2″ drain pipe as the vent for this pump pit… The 2″ drain pipe services only the kitchen sink. Can you help me answer this question?

    • Mike

      Hey Andrew. According to code you are supposed to tie it into a dry vent that is the same or greater size. More than likely you’ll need to tie it in at the attic if there wasn’t a vent previously provided for this purpose. All the best man.

      Mike – Jaytech Plumbing

  • Robyn

    I’m an amateur DIY plumber. Just to make sure. The wet vent size servicing the sink and the toilet can be 1.5 inches, or is it 2 inches. Someplace I read said it has to be 3 inches??? Somewhere along the line, the shower is vented too. My husband re-routed the shower drain for curbless wheelchair and I’m not sure how he did the vent for the shower drain. Robyn

    • Mike

      Hi Robyn.
      The combined sink drain/ toilet vent part of the pipe needs to be a 2″ pipe. The toilet drain is 3″ and the dry vent part is 1.5″.

      Thanks for the question.


  • Jason

    I am moving my washer to a shared wall with a bathroom. All bathhroom fixtures have their own vent. (Sink 1.5″, tub 1.5″ and toilet 2″) i want to cut the laundry tub drain into the tub vent and the 2″ washing drain into the toilet vent. Can i do this? If not, can I vent the washer into the sink vent and drain it into the toilet vent? It is a single story.thank you for your time.

    • mike@jaytechplumbing.com Post author

      I would probably wet vent the washer and laundry tub together, put the drain into a drain below the floor and tie the vent into one of the existing vents in that wall.

      • Daniel

        I have a similar situation as Jason. Trying to tie in a washer drain into a 2″ toilet vent. This is located on a slab so going below the concrete is not an option.