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
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.
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.
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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