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What Is A Wet Vent In Plumbing

If you have ever manually installed a toilet, sink, or drain yourself, then you are familiar with terms or parts like a P-Trap. The vaporous byproducts also need somewhere to go. Should sewer gases not be vented properly, they could enter the home via your sink, shower drain, or all the other fixtures, including the toilet.

When vented correctly, vent pipes in a plumbing system stop waste water from escaping your p-traps. However, should your p-trap end up dry, there won’t be a water barrier to stop gases from leaking into your home. A pipe serving as a vent and a waste pipe is called a “wet vent” plumbing arrangement.

Bathroom ventilation is necessary and using wet vents can save time and money. Although it has been used for years, it is now something that needs to be added as part of the plumbing codes.

wet vent

In our guide, you can learn more about understanding horizontal wet venting without scouring over confusing wet venting diagrams. By the end, you’ll know about vertical wet venting and if you need to add one to your home or if you already have a wet vent layout installed. (Learn How Long Does PVC Glue Take To Dry)

What is A Wet Vent System?

As is known in the plumbing industry, a wet vent connection is a technique for safeguarding the trap on a drain pipe on several plumbing fixtures. A vent pipe simultaneously acts as a waste pipe to stop sewer gas from flowing back.

The most common use of wet venting is with toilets and sinks; the toilet’s vent and sink’s drain are the same.

Wet Venting Rules:

A waste pipe can act as a wet vent if:

  • The wet vent doesn’t face too much hydraulic load. A 1.5″ pipe is used for two fixture units, not a toilet wet vent. For toilets and up to four fixtures (six drains), a 2-inch drain pipe can be used as wet vents to cover an entire bathroom.
  • Toilets must be installed downstream to other fixtures.
  • The wet-vented section of your pipe cannot be reduced in size.
  • The wet vent’s pipe length isn’t limited

Examples of Wet Vents and Plumbing Fixtures

1. Toilet and Sink Drains:

Through the sink drain, both a drain line and toilet will be vented. There should be a 1.5′′ sink drain, a 3′′ toilet drain, a 2′′ shared sink drain/toilet vent area, and a 1.5′′ vent going up.

Note: The bottom fitting where your 2″ drain joins your 3″ needs to use a Y-piece rather than a tee-piece.

2. Toilet, Sink & Shower, or Tub

The toilet and sink are wet-vented together, as in the preceding example. This time, the wet vent trap arm is connected to the toilet drain’s vertical vent pipe, which is also a plus.

This time, the shower or tub plumbing fixture is connected to the wet vent, although it should have its own vent and drain here.

However, you wouldn’t need to have the extra vent on the tub if it was closer to the wet vent than 5 feet. This is a fantastic way to save money on materials for adding more fixtures to your existing bathroom plumbing.

shower and sinks

3. Showers & Sinks

These are like the previous example, as you can see, but you won’t have the option to tie the drain to the vertical vent. If the sink and shower trap plumbing system are both 1.5 inches, the wet vent area of the standard drain and vent portion can remain at 1.5 inches.

Dry Vent Vs. Wet vent

It’s simple to think you’re hearing a lot of nonsense when folks talk about plumbing. Flange? A macerating toilet, soil stack, and more. Especially if you’re new to the plumbing world, there’s a lot to learn at first.

What distinguishes a wet vent from a dry vent? A wet vent system lets the air out of one or more fixtures while venting the water from one fixture.  Only air is moved in a dry vent system, and no water is carried.

Wet vents may drain one fixture while venting air from another, whereas dry vents typically only vent air from one fixture at a time. Wet venting saves time, money, and a lot of material costs, so many prefer it to a dry vent system as part of their septic system.

Building new drain lines, for instance, might be a pain for someone adding a new bathroom to their home, especially if you must travel through a closet. Wet venting can help with an individual vent line, venting the horizontal drain line main drain in this situation. (Learn How Long Does It Take For Pipes To Unfreeze)

How Do You Use Wet Venting?

A group of bathroom fixtures, including your toilet, sink, shower, water closet, and other plumbing fixtures, use wet venting. When you use wet venting, all the fixtures in a bathroom group share the same vent.

  • Although you will discover other uses for this type of plumbing, bathroom fixtures will be the primary uses, as plumbing codes now play a massive part in this.
  • Even though wet vent plumbing can vent two or more-bathroom fixtures at once, they all must be on the same floor, as it isn’t possible for them to be on separate floors to connect two toilets.
  • If you’re installing a toilet in a plumbing system with a wet vent, the toilet must be installed lower than any other fixtures using the vent. This rule does not have any exceptions.
  • Each 4-foot length of horizontal plastic pipe needs a pipe supporter. Typical pipe configurations are also quite helpful.
  • Most plumbing codes and building rules set a minimum size for your wet vent.
  • Wet vent rules state a pipe can be if needed because you can find hydraulic overload in short pipes. So, before you start your project, do some research on plumbing codes in your area.
  • Your wet vet trap arm must connect to a horizontal drain line at a specific angle of at least 45 degrees.
  • Make sure your wet vent pipe is big enough, as a wet vent pipe that is too narrow could cause obstructions, hydraulic overload, leaks, and other problems.
  • If unsure, choose a 4-inch horizontal waste pipe and a wet vent pipe of 3 inches or larger.

Your plumbing system should pass local plumbing codes and work for years if you abide by these wet venting regulations.

Wet Vent Plumbing System Inspections

Even if you follow all the wet vent rules to the letter, it isn’t enough, and you will need a wet vent plumbing inspection.

Most plumbing regulations mandate you schedule an appointment with your local plumbing inspector so they can conduct water and air tests once you have done installing your wet vent drain line and plumbing system.

When your neighborhood plumbing inspector visits your home, they’ll pay close attention to whether the water pressure is safe. So, in addition to being for your protection, getting this test done could help you avoid a lot of trouble down the road. (Learn How Long To Fill Hot Water Tank)

Wet Vent to Main Drain

Plumbing standards are strict regarding the angle at which both a drain that is a wet vent or dry vent for all the fixtures connecting to a horizontal drain line. To ensure that your drain layout will operate as an actual vent, ensure enough space for wet vents to enter drain lines.

Connections to the main drain lines are made differently for dry vent and wet vents. A dry vent must connect at an angle of 45 degrees or more, calculated from a line cut through the pipe horizontally in the middle. A wet vent cannot connect to this line at an angle greater than 45 degrees since it transmits air and water.

If you have two-bathroom groups, it is wise to get the help of experts as too many fittings in a bathroom group can impair the performance of the fittings and both a drain line and wet vents, and even a dry vent.

What Will Happen With a Poorly Built Wet Vent?

If you wish to build a wet vent, you should know two key issues: the plumbing and the building code inspection. If you don’t build your wet vent system properly, both can seriously harm your home or your cash.

The post-build inspection is the first problem that you might know about. You can be obliged to rip down the plumbing and rebuild it if your municipal inspector determines you built the wet vent incorrectly. A horizontal drain line may be easy, yet a vertical leg may have restricted access in your home.

If this occurs, your building will be deemed unfit for habitation until it is reconstructed. That is a significant amount of money wasted.

If the wet vent is not equipped to handle the expected workload, other problems can occur. If this occurs, you can encounter obstructions, sluggish drainage, odd noises from your wet vent, and unpleasant odors.

Even if you have additional plumbing vents through the roof, extreme circumstances could lead to methane gas entering your bathroom.

How To Avoid Pitfalls Of A Wet Venting System?

You can do a few things to guard against having a negative time with wet vents, assuming you decide to go with a wet vent installation. The most crucial thing to remember is that you must adhere to all local building codes and follow the set-out wet vent rules.

Another thing you ought to do is err on the side of caution wherever you can. For instance, getting the next size up could be wiser if you believe you can get away with a 2-inch pipe.

Similarly, ensure you use pipes of the highest caliber in your wet venting system. Dents and other dings are, therefore, strongly discouraged.

When in doubt, always review your renovation plans with licensed plumbing and the local building code board when in doubt. It’s the simplest way to guarantee that you make the most of your construction time. (Learn How To Loosen PVC P Trap That Is Stuck)


The codes are highly stringent and must be followed for wet vent plumbing. Because of this, it’s best to check them over and adhere to every instruction. If not, your project won’t be approved by your local plumbing inspector, and you’ll probably have to start over.

This is troublesome, besides being pricey. Do your study before starting your wet venting project to avoid frustration.

What Is A Wet Vent In Plumbing (2)