Less is more when it comes to residential venting

By Fred Bretzke


Since I started to teach, I have tried to keep up with the trade by doing the occasional rough-in. Last year I believed that I’d finally done my last one, as my body is getting too old for that stuff. I always tell apprentice students they are like professional athletes – they are at their physical best when they are young. If you don’t want to get traded or cut (fired) you need to be able to rough-in efficiently, safely, and to the code.

Most guys are not still roughing in houses or commercial buildings after they are 50 years old. Usually, it’s your back or knees that go out, and if you are lucky, you become a foreman, owner, teacher, inspector, and so on. If that is the case it’s imperative you know your code. Residential wet venting and circuit venting are just like anything else, the more you do it the better you get.


A word from the wise:

It is a good idea to wear gloves when using ABS glue or otherwise you end up looking like a rookie.



Wet venting is found in clauses and table of the 2015 Canadian National Plumbing Code.* There are around 11 codes on wet venting, which I will explain in simpler terms. 1) a soil or waste pipe is permitted to serve as a wet vent provided

a) you base your fixture unit load by table

b) you can wet vent up to two toilets as long as they are the last connection in the wet vent

c) if two toilets are installed in a vertical wet vent, they are connected with a double sanitary tee wye or in a horizontal wet vent with a double wye.

d) toilets are always installed downstream of all other fixtures, meaning their connections are the last discharging into branch or stack.

e) all trap arms and or fixture drains that connect to the wet vent are no larger than 2 in., except for a 3-in. emergency floor drain.

f) the total fixture unit load does not exceed the table, if you have a separately vented fixture discharging into a wet vent, it cannot exceed two fixture units.

g) a separately vented fixtures hydraulic load that drains into a wet vent are not included in the wet vents continuous vent as it has its own separate vent.

h) when multi-storey wet venting, the total load of each storey above the first storey does not exceed four fixture units

i) you can’t have more than one horizontal offset in a multistorey wet vent. 2 in. or less wet vented offsets cannot exceed 1.2 m. 3 in. or larger wet vented offsets cannot exceed 2.5 m.

j) wet vents are the same size throughout except when a 3-in. emergency wet vent joins into it, then only the upstream part remains the same.

k) the wet vent length is unlimited.


This is a guideline to remember when residential wet venting: a three-piece basement bathroom will typically only have a 2-in. wet vent, as there are usually less than three fixture

units dumping into the wet vent. The toilet is always the last connection on the wet vent. Two toilets have to be symmetrically tied in (double tee wye on vertical or double wye on horizontal, whether it’s a horizontal or vertical wet vent).



Typically, residential circuit venting is harder than commercial circuit venting as the fixtures in a house are laid out all over the place.

This leads me to the next scenario, which is when the homeowner doesn’t just have typical three-piece bathrooms. Many houses have at least two and a half bathrooms. One two-piece guest bathroom on the main floor, one three-piece main bathroom on the upper floor, and many times a four- or five-piece ensuite bathroom.

Bigger houses sometimes have five bathrooms with at least two bathrooms that have four or five plumbing fixtures in each of them making it essential to know residential circuit venting.

It’s also important to realize that the joists have to run parallel to the circuit vented branch, which is typically a 3-in. CVB to allow the best install. Circuit venting is found in and Table of NPC 2015. Clause c is also important when circuit venting.

Typically, residential circuit venting is harder than commercial circuit venting as the fixtures in a house are laid out all over the place. Commercial circuit venting is easier because it was typically designed for a group of toilets all in a row. However residential circuit venting becomes easier if you pretend it’s like running fixtures all in a row.

There are also 11 rules in circuit venting, which is basically glorified wet venting. Two major differences are that the toilets do not have to be the last connection, and you cannot multi-storey circuit vent.

1) A part of a horizontal branch can be circuit vented, as long as

a) there is a circuit vent attached to its most upstream fixture

b) the circuit vent is only on one storey

c) no soil or waste stack is connected upstream of it

2) 1 ¼-in. and 1 ½-in. fixtures shall be separately vented or separately circuit vented from 2-in. and 3-in. fixtures

3) A relief vent is always required and should be installed downstream of the most downstream circuit vented fixture in the circuit vented branch

4) You can dump up to six fixture units into an acting relief vent.

5) A combined symmetrically connected relief vent can be used for two circuit vented branches as long as you do not exceed eight fixtures between the circuit vents and the combined relief vent.

6) Additional circuit vents shall be required if you have more than a 45-degree horizontal change of direction in the circuit vented branch, or if you have more than eight fixtures between the circuit vent and relief vent.

7) The wet portion of an additional circuit vent will always end up being 2 in. in size

8) Circuit vents and additional circuit vents must conform to (Fixtures Draining Into Vents

9) A circuit vented branch including the fixture drain downstream of the circuit vent must be sized in accordance to the branch table and this is not less than the short cut rule

a) 2-in. CVB where the traps are 1 ½ in. and 1 ¼ in.

b) 3-in. CVB where the traps are 2 in. or larger

10) ACVs shall be sized in accordance to the individual vent table and accordance to clause 1) The dry portion of the ACV is sized one size smaller than the dry portion of the circuit vent, and need not be no larger than 2 in.

11) The fixture unit load counted on the CVB shall include all fixtures dumping into the branch served by the circuit vent and not include any fixture units dumping into the relief vent or acting relief vent as they are downstream of the Circuit Vented Branch. The cool thing about residential circuit venting is that you can use less vents for more fixtures and you can get quite code creative. After all, the person with the least vents wins.

*Author’s note: The 2020 code is out but is not required to be implemented until schools and Red Seal exams have been updated. We don’t expect to be teaching it until September 2022 or January 2023.