Jaytech Plumbing | Guelph Plumber

What is Wet Venting

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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  1. In the first instance you say that 1.5 is okay on the wet vent for both the sink and toilet, I thought all toilets had to have a 2″

    • Hey Hank. Sorry if it was confusing, but it says that a 1.5″ can not serve as a wet vent for a toilet. So you are correct to say that it should be 2″. Thanks.

  2. Hi! What a great site! Thanks for your help. I’m looking at your 3rd example of wet venting. I am planning a 3piece basement bathroom, with drainage tying into main waste drain (3″ cast iron), and the option of venting tying into 1.5″ vertical vent which currently vents laundry sink.

    Question: I have a vanity sink upstairs, with a drain that currently runs across the basement ceiling, into the main stack drain at head height. I am wondering if I can cut and reroute this upstairs sink drain, down the basement wall and under the floor, (using 2 inch abs) and tying the basement shower into it as wet vent/drain before it ties into main drain. The 3rd wet vent example given in your ‘wet venting’ discussion is exactly the layout, except that the wet vent/sink drain would have several turns in it, on it’s way down. (The basement toilet and sink would tie into the main drain separately, and each vented with a horizontal line to the laundry vent)

    Alternatively, I’ve thought about leaving the sink drain separate, and venting the shower separately with a horizontal around the walls to the laundry tub vent, but this seems a waste of material if the first example is acceptable.

    • There may be another option for you Jamie and I’ll do my best to picture it here… All three fixtures for your basement bathroom can be vented through one vent: 3″ toilet drain has a 2″ y fitting to pick up the sink drain which continues up as your vent after the sink (Much like example 1 but there will be some horizontal pipe underground on the 2″). As for your shower, if you tie it into your 2″ underground within 5ft of the trap then it doesn’t need a separate vent. This would save you a lot of work. Let me know if that makes any sense to you.

    • Awesome! Thanks again for your help and great site!

  3. I recently put in a new bathroom and tied into the old outside vent stack, it flushes slow and will back up after the shower runs about five minutes. I can send pics if you can advise, thanks. I tied all in and put a very good vent cap on the upper side of the farthest unit, the sink. All runs together. Sink went to 2 inch in wall and has breather cover on the wall so its getting air i guess

    • If things are vented well and your drain backs up after 5 minutes of use, then I would assume that your old drain is a bit plugged. You should get a plumber to snake it out or rent one from the local tool rental store. All the best Brad.

  4. where should the vent go for the toilet if the toilet is more than 4ft away from main drain, front or back and how far from the toilet should the vent be?

  5. HI,, installing a three piece bathroom, plus picking up a washer machine and floor drain in my basement. I need to install a vertical vent from the roof,. All this will be going to Sewage Basin, than pump up to feed to the septic system. The question is how It would be easier for me to bring the vent down to the ceiling of the basement, than travel 10 ft 90 * than another 10ft horizontal then down the wall to the floor,,, would this be ok, . I will be installing 3 inch ABS from the toilet to the sewage basin, with everything else Y’ing into this.

    • If I understand correctly your vent from the roof seems ok, but just make sure everything in the bathroom is vented correctly and then tied into that vent. As a note, the sewage pit needs a 2″ vent.

  6. I may be understanding the first diagram incorrectly but, isn\\’t the layout incorrect according to the wet venting rules you laid out. To me it appears that the toilet is upstream of the sink.

    • Thanks for the question John. If you look carefully you’ll notice that the drain from the sink also serves as a vent for the toilet. This is actually why its called a wet vent.

  7. Hi mike,

    I ran across this page when searching for information for a similar project. I have a 3rd story loft in my home and I would like to add a small bar sink up there. I had ordered the rough ins from the builder but they missed it and I didnt make a big deal in time. The location I want it at is directly above the main bathroom. The wall I want it on has a side attic on the other side that I can access through a 2nd floor trapdoor almost exactly behind the bathroom. I am certain that the vent for that bathroom is going right past the spot I want to rough in. I would be tying in approx 11 to 12 ft above the toilet is this feasible? Worst case scenario, I can probably run another vent up to the roof , but id rather avoid that if possible. I havent looked up there yet, but I know the toilet vent atleast goes straight up. Now whether hot or cold water is accessible will probably be the sticking point….

    • It shouldn’t be a problem to connect the vents together but depending on finished walls and access you’ll likely have a harder time finding a drain to tie in your bar sink too. Can you get the drain to the other bathroom?

  8. If I am installing a 1/2 bath (sink/toilet) that is 20′ from the main stack what are my options for venting?

    I assume that I would do a 2″ wet vent all the way up to the roof. My plumber is saying that we can go 2″ all the way up to the attic and then tie into the main stack up there for a vent outlet. Seems odd to do it that way, is this ok?

    • There isn’t any problem with tying vents together in the attic. I prefer this rather than making multiple protrusions through the roof.

  9. Hi. Great information here! I have a single story house with plans to add a bedroom and bathroom in the attic. The new bathroom will be located directly above an existing bathroom. There is a 3″ pvc pipe in the attic that vents that bathroom and the utility room. Is it acceptable to tie into that 3″ vent for waste? Or, should I run a separate waste line down into the main trunk line (which would involve chipping much concrete to get under the slab)?

  10. Thank you! Very helpful.

  11. Mike…..Installing a basement bathroom with a sewage ejector. Running 3″ pvc straight to toilet (about a 5′ run). Between the pump and toilet I’m branching off with a 3×2 wye to pick up the shower. Right before the shower trap will be a 2″ sanitary Tee which will vent the shower and wet vent the toilet. I’m planning on running the 2″ up into the wall and then reduce down to 1 1/2″ to pick up and existing vent. Does this sound ok. The sink will be tied into the 3″ trunk with a 3×1 1/2 sanitary tee right after the pump. The sink has its own vent which I plan on tieing the shower into in the ceiling. The ejector needs to have its own vent up threw the roof. Would tieing into my stack which is 4″ with a 4×2 tee just before it exits the roof be sufficient. I would hate to cut another hole in the roof.

    • Just as you described sounds like it’ll work fine and yes by all means tie in your vent before it exits the roof. The less penetrations the better. Nice work Paul.

  12. Mike,
    I am in the process of finishing my basement bathroom. I have a slab floor and live in Colorado. I need to cut out a chunk of concrete so i can move the plumbing around. The builder was not very kind on leaving them in the correct spot.
    Ho would i go about venting these pipes. do i have to go out the side of the house or can you vent 55\\\” up the wall in the framing studs?? need help on this. I have done sum plumbing but never a finish basement.

    • Hey Joe. You can put the vent up in the outside wall. No need to bring it out the side… as long as it protrudes through the roof somewhere.

  13. To properly vent a septic pumpout in the basement, I understand I’ll need to use 2″ pipe. I will run this pipe to the attic vent before tying it into the existing 3″ stack from the main floor plumbing. Is it necessary or advisable to make that horizontal connecting portion (less than 5 feet) to be on an upward angle (using 45 degee connectors) to accommodate possible frost build up and subsequent melting so that any melt water will drain back down the 2″ pipe? Also, can I wye in a basement laundry tub to this same 2″ pipe and use it as a wet vent?

    • No need to slope it, frost won’t be an issue. As far as the wet vented laundry is concerned, it’s technically not permitted here in Ontario code but it should work fine. You may need to check you’ll local code on that.

  14. Hi, first i wanna say that i appreciate your hard work. Really helpful.my concern is the 3rd diagram.it looks like the lav drain which is actually wet venting the shower has the shower draining into it whereas it should be lav draining into the shower fixture drain,downstream of the shower trap for the lav drain acting as a wet vent….

    • It’s really the same difference actually… Connect the two together with a y fitting. Either way the sink drains and the shower vents.

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