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Plumbing Vents | How Many Are Necessary? (Code Examined)

Every plumbing fixture in your house needs to be vented in some way. If each were to have its own individual vent, then your house would be riddled with plumbing pipes. The professionals who compiled the building codes are well aware of this, so they have made provisions that mean fewer vents in total.

So, let’s dissect the plumbing code and get to the bottom of what vents are needed to have a properly functioning plumbing system without the hassle of multiple unnecessary pipes.


One stack vent is required. All traps and trapped fixtures in the home need to be vented, so the number of vents depends on the number of fixtures. However, they don’t need to be vented individually. Two or more fixtures from the same floor can be vented together with a common vent or a wet vent.

There Has to Be One Main Stack Vent

The International Residential Code (IRC) states:

“The vent system serving each building drain shall have not less than one vent pipe that extends to the outdoors.”

Section P3102.1

This section describes the vent stack and stack vent part of the plumbing vent system.

The vent stack is the main vent pipe that connects to the soil stack and into which the smaller individual vents, branch vents, loop vents, etc., feed. Multiple vent stacks can also be connected.

The uppermost portion of at least one vent stack must end in a stack vent. The stack vent is the portion of a vent stack that extends above the highest drain point and through the house to the outside through the roof, wall, or soffit.

Bathroom-plumbing-system-with-branch-vent-vent-stack-stack-vent-and-soil-stack

The necessity of at least one stack vent comes from its purpose.

Why the Stack Vent Is Necessary

The stack vent is an essential part of a plumbing system.

The primary purpose of the stack vent is to allow sewer gases to safely escape to the outside, where they do not pose a health or fire risk. Even if it did not build to such levels, the smell would be very unpleasant.

In addition, the stack vent, just like all the other vents in a plumbing system, allows air to enter the vent stack to equalize air pressures in the drainage system.

Without a stack vent and proper airflow, pressure vacuums could form within the system. This would lead to your drains not being effective or malfunctioning—no one wants toilet backflow!

All Traps and Trapped Fixtures Need to Be Vented

When it comes to traps and trapped fixtures, the IRC outlines the following guidelines:

Every trap and trapped fixture shall be in accordance with one of the venting methods specified in this chapter.”

Section P3101.2.1

In plumbing, traps are small devices that capture some water and prevent gasses that form in the pipe from reentering the home through the drain. Typically, they will look U- or S-shaped. 

Eastman 35304, White S-Trap Drain Pipe, 1-1/4 inch, 3.5 x 5.5 x 15

They are attached to fixtures with a drain, including your kitchen sink, shower/bathtub, bathroom basin, toilet, washer, and dishwasher.

Therefore, each plumbing fixture in your home will need to be vented. Typically, this means at least four vents (shower, toilet, kitchen sink, and bathroom basin) in a home.

Why They Need to Be Vented

Traps must be vented to ensure that they remain functional.

Their primary purpose is to keep the air pressure in and around the trap or fixture such that the water drains readily and in the right direction (i.e., no backflow) and that the trap remains filled with water.

Secondarily, these vents also provide a path for the sewer gases, typically taking these gases to the vent stacks.

Multiple Fixtures Can Share a Single Vent

Now, let’s say that you have two full bathrooms, a washer, a laundry sink, a kitchen sink, and a dishwasher. This means that you will need 12 vents if you had to vent each individually.

Luckily, you do not need to plan different vent paths for each fixture because they can, under certain circumstances, share vents.

This makes sense considering that the entire plumbing system is connected and, in the end, all vents lead to the main vent stack and stack vent.

However, the IRC does dictate how vents can be shared if they are shared.

Common Vents

According to Section P3107.1 of the IRC, two fixtures can be vented with one individual vent, which then become known as a common vent, but only if they are on the same floor level and the vent is sized correctly (Table P3107.3).

Wet Venting

Wet venting is where the vents also serve as drains. While wet vents are still technically plumbing vents, in terms of counting the number of plumbing vents, we tend to only refer to the ones that terminate in an AAV or vent stacks.

Horizontal Wet Vents

Horizontal wet venting is detailed in IRC Section P3108.1.

Horizontal wet venting is when the wet vent runs horizontally and all the fixtures without individual vents connect into this horizontal pipe via their drains.

The wet vent leads to a “main” drain but the other end bends up into a vertical dry vent, which is typically an individual or common vent for one or two fixtures.

REPLACE WITH LICENSED IMAGE OR ORIGINAL SKETCH

In terms of how many fixtures can be connected via horizontal wet venting (they must be connected individually and on the same floor), you will need to look at the drainage fixture unit (d.f.u) load and the size of your wet vent pipe.

Section P3004.1 gives a table listing the d.f.u. for various fixtures and Section P3108.3 provides the maximum d.f.u that can be wet vented along with the size that the wet vent pipe would have to be in order to accommodate this load.

Here is the table from Section P3108.3.

Wet Vent Pipe SizeDrain Fixture Unit Load (d.f.u)
1.5″1
2″4
2.5″6
3″12
4″32

Technically, you may only need one or two plumbing vents per bathroom or per floor and then the stack vent.

Vertical Wet Vents

Vertical wet venting is detailed in IRC Section P3108.4.

Vertical wet venting is when the vent pipe stands straight and the drains from multiple fixtures all connect at different points to this pipe.

The lower portion of the pipe, the wet vent, is connected to the drain, and the upper portion, the dry vent acts and terminates like a normal vent pipe.

REPLACE WITH LICENSED IMAGE OR ORIGINAL SKETCH

The water from the fixtures will obviously go downwards, flowing through the wet vent portion, but the dry portion will still act as a vent for all the fixtures.

The fixtures all have to be on the same floor but they can be from up to two different bathroom groups on that floor.

In terms of how many fixtures can be vertically vet vented, you need to look at the same tables as used for horizontal wet venting.

Sources

https://civiconcepts.com/blog/types-plumbing-trap

https://inspectapedia.com/plumbing/Plumbing_Trap_Repairs.php

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