What is Wet Venting 120

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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120 thoughts on “What is Wet Venting

  • Ben


    I’m doing a basement reno, and trying to figure out if my laundry room needs a vent.

    What I think makes my situation a bit different is my laundry tub (without P-trap) drains into a sump pit (uncovered), the waste is then pumped out to an old existing French drain in the yard. It’s an old house and I’m assuming this was common in that era. I should mention my washing machine drains into the laundry tub. Currently everything seems to drain well, and no sewer gas becuase it’s an independant system.

    What I plan to do is drain the washing machine into a recessed water box (to clean up the look), which would drain into the sump pit. And have the laundry tub drain (the laundry tub located closer to the sump pit) drain into the line connecting the washing machine drain and the sump pit.

    1)Since the laundry tub and washing machine are the only appliances draining into an uncovered pit, will they need a vent?
    2)Would this need a vent if the pit was covered?
    3)And should any of these appliances have a P-trap, and why?

    Thank you very much!

    • Mike

      Hi Ben.
      Those pits aren’t legal any more. I can’t comment on how to vent a pit that’s potentially draining contaminants into the ground.
      The best would be to have a sewage pit or laundry pump and drain it into the plumbing system.

      Thanks for the question though.

  • Brad


    I am completing a shower stall update on my parents bathroom for them. Existing shower is plumbed with 1-1/2” ABS from shower drain to the stack. I am assuming it is tied into existing bath tub or vanity sinks on its path back to the main stack.

    That said, the new shower stall will have (1) 10” railhead and (1) standard wall mounted hand wand. The shower stall is 36” x 60” prefabricated acrylic base. My question is how to tackle the 1.5” existing trap and sanitary line? I cannot replace as I would likely have to tear up the entire floor and re-plumb all fixtures.

    The shower base came with 2” shower drain.(Oaty, No Caulk Drain) What are my options? Can/should I reduce the 2” drain to 1.5” before or after the trap? Should I see if I can get a 1.5” shower drain ( I haven’t had any luck in the last day sourcing one)? I understand a reduction amounts to an impedance for draining and is why I assumed the best option would be to stick with 1.5”. Just not sure how the drain would perform if both railhead and hand wand were used at the same time..then again, shower heads are mostly low flow and more efficient than they were many years ago.

    I know 2” all the way through is preferable but I do not have the option to re-plumb everything. Can you provide any clarity here?

    • Mike

      Hi Brad.

      This is actually a very common scenario and what everyone does (and is totally fine) is bush it down to 1.5″ right at the shower base.
      Thanks for the question.

      God bless.

  • Ron

    The attached sketch shows proposed venting for back to back bathrooms. Sink 2c did not have a vent, WC2A exceeds 6′, WC2B is new and exceeds 6′, Sink 2A and 2B are new, SH2 is existing, Tub is new. 3″ stack needs to be offset 15″.
    The first fl is what I was able to trace and not sure if this is correctly piped for the drain and vent.


  • Roger Spotswood

    I have a 2″ shower drain 6 feet long connecting to a 3″ extension running an another 6 feet to a 2 inch wet vent with two sinks (1.25″ traps on each) with the sinks connected to the 2 inch vertical drain/vent in a double sanitary T. Is the shower adequately vented? I can supply a pdf digram if necessary.

  • jeff

    Thank you for this article! This has been very helpful. I was wondering if I can tie into the main cast iron vent in my house for a half bath upstairs. I noticed all your diagrams have the sink going into the wet vent but not a toilet. Is that okay?

    • Mike

      Hi Adam. Your link didn’t work but if the picture is the same as the comment below it, then it’s done correctly, and if the inspector passed it, then for sure it’s done well.