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Combining Plumbing Vents | All You Need to Know

Plumbing can get complicated when you don’t have much knowledge on the subject. There is a lot of nuance to the angles, connections, and sequencing of fixtures, vents, and pipes, so this is understandable.

Thankfully, the International Residential Code (IRC) lays it all out. However, it requires some interpretation and knowing where to look in the codes, which is where I come in. If you are ever unsure about plumbing, you should consult a professional to ensure everything is safe and correct.

A plumbing system is interconnected to begin with. The drains all lead to sewers, and the vents lead to one or more stack vents to remove sewer gases and let air into the system to balance the pressure. It’s vital to follow IRC and local regulations when combing plumbing vents.

Can You Tie Plumbing Vents Together

There are different types of plumbing vents and various components involved in the plumbing system. Plumbing vents are allowed to be tied together. Indeed, it is actually necessary because it is a system and must be connected to function effectively and efficiently.

Plumbing Vent | What Is It and How Does It Work

Multiple Fixtures Can Share One Vent

Common Vents

“A single pipe venting two trap arms within the same branch interval, either back-to-back or one above the other.”

Section R202 (IRC)

Unless you’re familiar with plumbing, this might not mean much. A common vent is an individual vent that is shared between two fixtures, such as sinks or bathtubs.

The vent connects to fixture drains or wet stacks with a sanitary cross/double sanitary tee or a wye/Y fitting.

Double Sanitary Tee and Wye Fitting Illustration

These fixtures can join back-to-back (so on opposite sides of the common vent) or one above the other (at two different points along the common vent). However, a common vent can only connect fixtures on the same floor.

Double wye and sanitary cross plumbing illustration

When you connect the fixtures vertically with an upper and lower fixture, you count it as a vent for the lower fixture.

This means that the size of the common vent must accommodate this fixture. In addition, the upper fixture cannot be a water closet, which refers to a fixture that removes body waste (i.e., a toilet).

Common vents lead to a horizontal branch, the pipe that runs laterally (sideways) to the building drain, which is the lowest point in the piping system that takes all the waste away from the house to the sewer system.

Maximum Number of Fixtures on 2” Common Vent

A common vent is only allowed to vent two fixture drains. This is the drain from a fixture trap between the weir and vent fitting.

When you have two fixtures vertically sequenced, the upper fixture is only allowed a maximum discharge of 4 drainage fixture units (d.f.u.) on a 2” pipe, according to Table P3107.3.

Maximum Number of Fixtures on 3” Common Vent

A 2.5-3″ common vent is still only allowed to vent two fixture drains. However, since the pipe is bigger, the upper fixture is only limited to 6 d.f.u.

Wet Vents

Vent pipes contribute to the system in terms of air and gases, while the drain pipes carry water through the plumbing.

Wet vents are unique in that they are both a vent and a drain pipe.

They can also serve multiple fixtures. However, they must be sequenced appropriately to prevent sewer smells and gases from coming up through the fixture groups. This differs according to whether you are using horizontal or vertical wet vents.

Horizontal wet vent plumbing illustration

Horizontal wet vents are permitted by Section P3108.1 of the IRC for combinations of fixtures from “two different bathroom groups” on the same floor.

The fixtures need to have a dry vent connection or can connect to a horizontal branch via a horizontal wet vent (Section P3108.2).

Vertical wet vents are also permitted for combinations of fixtures on the same floor (Section P3108.4).

In this orientation, the wet vent fixtures must all have independent connections to the vertical wet vent, and toilets must connect at the same level.

The dry-vent connection must be individual or common, meaning it is serving 1 (individual) to 2 (common) fixtures.

All Vents Must Be Tied to a Vent Stack

Branch, individual, circuit, and wet vents must all connect to a vent stack. A vent stack is defined by the IRC (Section R202) as:

“A vertical vent pipe installed to provide circulation of air to and from the drainage system and that extends through one or more stories.”

Vent stacks are also called vent pipes, and these connect the lower part of the plumbing sewer line to the outside.

The termination portion of the vent stack is called a stack vent, which extends outdoors where it can dispose of waste gases.

By now, we know what a wet vent is, but let me quickly clarify the others:

  • An individual vent connects to a single fixture (becomes a common vent if connected to two) and another vent system or terminates outside.
  • A branch vent connects two or more individual vents to a vent stack or a stack vent.
  • A circuit vent connects 2-8 fixture traps (or trapped fixtures that connect to a battery) to a horizontal branch.

Vent stacks are essential because of the air they allow in and how they balance air pressure in the plumbing system.

Without these channels for air to go in and out of the plumbing, you can end up with negative pressure in the pipes. This impacts how well drains remove water and waste from fixtures and can cause backflow.

Exception 1: AAVs

Individual, branch, and circuit vents are allowed to terminate with an air admittance valve (AAV) provided they only vent fixtures on the same floor and as long as the fixtures are connected to a horizontal branch drain (Section P3114.3).

Air admittance valve in the plumbing system illustration

An AAV allows air into the plumbing system to prevent a negative pressure system that would create a problem with water being siphoned from traps in the plumbing. It prevents air from escaping at this point though, so sewer air cannot get into the home.

Air is unlikely to build up at the AAV, which would be concerning, since the airflow into the valve would move them away from that point and into the rest of the system where they can be disposed of correctly.

The AAVs add helpful air into the system to ensure the plumbing works as effectively as possible with a balanced air system. The difference is that you don’t need to add a vent stack where it is inconvenient or impractical as long as there are other vent stacks at some point in the system.

Exception 2: Directly Vented Outside

Individual, branch, and circuit vents can also terminate directly outdoors if they don’t use a vent stack, a stack vent, or an AAV, according to Section P3104.1 of the IRC.

The vents wouldn’t need to connect to a vent stack because fixtures or a group of fixtures are removing sewer gases at that point in the plumbing, instead of having it move to a different area in the system to allow this to happen.

When your fixture is within 5 ft of the main vent stack, but you give it an independent vent, this is direct vented. If the fixtures are too far from the main vent, then it requires its own vent stack.

Direct Venting pluming illustration

It’s helpful if you are adding a bathroom and don’t want to tie the vent into the main plumbing vent, then this is ideal. But, unfortunately, the more direct vents you have, the more holes you have in your home.

Vent Stacks Can Be Tied Together

You can tie vent stacks together if you comply with a few conditions that you can find in the IRC:

  • Section P3102.1: there must be at least one vent that extends outside in the venting system. This is because there must be a place for the sewer gases to be expelled from the venting system and for air to get into the pipes to prevent backflow and incorrect drainage.
  • Section P3104.2: the vent needs the appropriate grading, connection, and support to allow moisture and condensation to drain back to the waste/soil pipe.
  • Section P3104.5: the connection for the stacks (or any other vent) must be at least 6” above the overflow level of the uppermost fixture in the line. This prevents the vent from becoming a drain in the case of blockages and flooding.  

The advantage of connecting vent stacks is that it minimizes the number of protrusions through the roof or the walls in your home. Although, if this is the primary concern, you can also make use of soffits, which is less invasive.


Unfortunately, instead of running a few vents straight up, you will have to go through the hassle of connecting all the vents to the stack. If something happens to that vent stack, you also run the risk that the entire system will suffer compared to when you have more than one vent to the outdoors.

How Many Vent Stacks Can Be Tied Together?

I recommend consulting your local codes and a plumber to find out this answer. Plumbing regulations can have several differences depending on your state, which makes it difficult to determine a general response.

I will say this, the number of vent stacks you connect is reliant on and determined by the diameter of the vent stack pipe. Therefore, the more connections there are, the larger it needs to be.


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