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Plumbing Vent | Should It Be Capped?

A vent cap may not sound too worrisome if you don’t know much about plumbing. It may seem like an excellent way to protect the terminal point even. However, protecting a plumbing vent and capping it are very different.

Plumbing vents remove the hazardous sewer gases that can be harmful to your health if they aren’t removed. They also provide fresh air into the system to balance out pressures. This is why it is important to keep the terminal open and free. In this article, I explain why your vent shouldn’t be capped and other options for covering the terminal instead.

A plumbing vent should not be capped as this prevents the vent from ejecting sewer gases and blocks the air supply into the system. Terminals can be protected from animals and snow, but should never be blocked.

Plumbing Vents Shouldn’t Be Capped

A stack vent’s purpose is to exchange air. When you cap (seal) off a plumbing vent, it cannot perform this function by removing gases and allowing fresh air to enter the vent stack.

A stack vent and a vent stack sound similar, and they are connected. However, they are not the same thing.

Vent stacks are plumbing vents that run through the house and help balance the air pressure in the plumbing system. The stack vent is the final portion of the vent stack, above the last drain and which extends to the outside of the house.


If your concern is to keep rain out of the vent, you can rest assured that any rain that can enter the stack poses no issue. It will simply flow into the drains and be carried away with the water in the plumbing system.

Plumbing Vent | What Is It and How Does It Work

What Happens if You Cap off Plumbing Vents?

When your plumbing vent is blocked off with a cap, you will have issues with noxious gases building up in your plumbing, and not enough air will be able to enter the plumbing system. These issues have consequences:

  • When a plumbing vent is capped, it allows all the sewer gases to build up in the pipes instead of being ejected, where they are harmless to your home.
  • Air is necessary in your plumbing system to balance the pressure within and prevent siphoning through the traps, backflow, gurgling noises, slow drainage, etc. Basically, it’s very similar to when you have a clog.

This means that sewer gases will not be filtered out of the house properly, and now these gases can enter your living space. Sewage gases result in unpleasant odors and are not good for your health.

The smell comes from the ammonia and hydrogen sulfide in sewer gases, which also contain carbon dioxide and methane.

These gases irritate the eyes and respiratory system, impair concentration and memory, and cause nausea and vomiting, fatigue, headaches, and dizziness. Some of these symptoms are associated with asphyxiation as these waste gases displace oxygen.

Sewer gases are also flammable, which increases the risk of fire or explosion in your home.

Therefore, it is important to deal with these gases efficiently and correctly to protect your household’s health, so you should never do things like cap plumbing vents or connect bathroom fans to them.

Some Vents Can Have AAVs

AAV stands for air admittance valve and it can be attached to some vents. This mechanical one-way valve is installed when you can’t vent through a stack.

Studor 20341 Mini-Vent Air Admittance Valve with PVC Adapter, 1-1/2- or 2-Inch Connection

These valves are activated by negative pressure in the vents and allow air into the plumbing system, but they prevent sewer gases from leaking into your home.

According to Section P3114.3 of the International Residential Code (IRC), stack vents (along with a few others) can terminate with an AAV.

AAVs are permitted while caps are not because they have opposite functions. A cap seals off any air exchange, while an AAV is designed to let air into the plumbing to prevent problems of a negative pressure system that comes from insufficient air in the vents.

If you have an AAV, then the vent does not have to terminate outside; it can terminate in, say, the attic.

One Stack Vent Can’t Have AAV

At least one stack within a plumbing system must extend to the outdoors for correct venting, as per Section P3114.7 of the IRC. This vent cannot have an AAV as there still needs to be an outlet for the plumbing gases.

As mentioned before, you don’t need to worry about rain getting into the stack that vents outdoors.

Vent Pipe Terminals Can Be Covered

As stated in IRC Sections P3103.1.3 and P3103.1.4, the terminal point of your plumbing vent (roof extension and sidewall terminal) can be covered in order to protect the opening from rodents and birds, which can also block the vent.

The covering cannot interfere with airflow and cannot decrease the size of the vent opening.

The opening must be maintained to allow proper airflow through the vent. If you reduce the size with a cover, you are disrupting the pressure system in the vent and the removal of sewerage gases.  

Plumbing vent with capping on the roof illustration

Where there is an open vent pipe that terminates above a sloped roof and is covered by a roof panel or element (which must also help with snow build-up and wind), the terminal must be at least 2” above the roof surface or according to the state regulations for vent height.

Covering Options

You can use some wire mesh to cover the pipe terminal. You cut a square, fold it over the pipe, and use a hose clamp to secure it. This metal mesh is a good option since it is durable and tough, meaning no animals can chew through it.

A metal mushroom cap is also an option. This allows proper airflow and prevents things from falling or crawling into the vent.


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