P-Trap Code (2024 Requirements)

I know the feeling.

You are trying to install a new p-trap, or wondering if the existing trap is code compliant.

We all know that building codes can be notoriously complicated, but p-traps are an essential plumbing component—keeping out foul sewer smells.

Keep reading to learn all about p-trap plumbing codes and how to avoid common problems!

What Are The P-Trap Code Requirements?

P-trap code states what is required for traps for toilets, sinks, tubs, floor drains, and other plumbing fixtures.

If the p-trap is improperly installed, it will be more likely to clog with debris, cause water damage to a home, and even allow nasty sewer smells into the home.

P-trap plumbing code are detailed in the International Residential Code (IRC) which is a ‘model code’ for many U.S. states and countries around the world. The IRC is updated every 3 years.

Many U.S. states have exactly the same code requirements as laid out in the IRC.

However, your state may have exceptions or a few key differences with the IRC, so always check it out for yourself. Also, your local city or state code will always take precedence over the International Residential Code.

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P-Trap Code (2022 Requirements)

  1. Approved Pipe Used For P-Traps
  2. P-Trap Diameter Size
  3. Vertical Distance From Trap Weir To Fixture
  4. Horizontal Distance Of P-Trap To Plumbing Fixture
  5. Maximum Number of Fixtures Per Trap
  6. Prohibited P-Traps
  7. Floor Drain P-Traps

1. Approved Pipe Used For P-Traps

P-traps should always be made with a smooth interior pipe.

A corrugated style p-trap will catch debris and lead to a clog.

P-traps can be made out of approved plastics, copper, copper allow, cast iron, or copper. If copper is used, it needs to be at least 0.8-mm thick.

2. P-Trap Diameter Size

The diameter of the p-trap piping should not be larger than the drain pipe that it goes into.

If the tailpiece (the pipe from p-trap to the fixture) is larger than the p-trap size required by code, then the p-trap needs to at least match the size of the tailpiece. Below is a table of the required p-trap sizes for common fixtures:

Plumbing FixtureP-Trap Minimum Size
Clothes Washer2″
Floor Drain2″
Kitchen Sink1-1/2″
Utility Sink1-1/2″

3. Vertical Distance From Trap Weir To Fixture

The maximum distance from the trap weir to the fixture outlet is 24-inches.

The ‘weir’ is the point where water begins to flow into the drain pipe as it exits the p-trap. And the ‘fixture outlet’ is basically the bottom of the sink where the drain pipe connects to the sink.

4. Horizontal Distance Of P-Trap To Plumbing Fixture

The maximum horizontal distance from the fixture to the p-trap is 30-inches.

If you draw a vertical line down the center of the plumbing fixture, and measure it to the inlet of the p-trap, this is the measured horizontal distance.

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5. Maximum Number of Fixtures Per Trap

Every plumbing fixture should have its own trap.

The exception to this code is for multiple kitchen sinks, bathroom sinks, and laundry tubs if they are in the same room. An example of this exception would be two sinks in a master bathroom.

If there are three sinks in a large bathroom, then the trap should be installed under the center sink. And the sinks should not be more than 30-inches from each other (measured from the centerline).

6. Prohibited P-Traps

The most common prohibited fixture trap is the one called an s-trap. A normal p-trap, as the name implies, has a P-shape in its design.

S-traps are similar to p-traps except that the drain outlet doesn’t go out horizontally, and makes an S-shape rather than a P-shape. 

These improper traps are usually installed by homeowners and they are more likely to clog and cause problems. A few other types of prohibited traps include older style bell traps, drum traps, or traps with moving parts.

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7. Floor Drain P-Traps

Emergency floor drain traps need to have a minimum 2-inch liquid seal (maximum 4-inches).

The seal is important in a trap because it prevents gases and odors from seeping into the home. 

Since emergency floor drains aren’t directly connected to the homes drain or water system, it will eventually evaporate and need a way of re-filling the liquid seal. Some approved devices that refill the liquid seal include…

  1. Potable water primers
  2. Gray water primers
  3. Wastewater primers
  4. Barrier style primers

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Final Thoughts On P-Trap Plumbing Code

P-trap code requirements are laid out in the International Residential Code but your state will probably have some exceptions and differences.

The most important p-trap codes include the pipe diameter, vertical distance, horizontal distance, and the type of material. The p-trap is essential to prevent sewer gas from entering the home, and an improperly installed p-trap can also lead to a clog.

If you have any doubts about your p-trap, consulting with a local plumber is recommended.

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