What Is The Purpose Of A "P" Trap?

First, Why “P” Traps?

The reason I thought this subject would be useful and interesting, is because I don’t think most people have an understanding of the function and purpose of a “P” trap. While doing home inspections in San Diego, I have noticed that most people don’t seem to use the bathtub in the master bathroom. I recently discovered a leaking bathtub drain in a second floor master bathroom using thermal imaging. The bathtub was rarely used, so the homeowner wasn’t even aware that the tub drain leaked. I guess the previous home inspector didn’t fill and drain the bathtub.

But the main reason I wanted to write this is to let people know what else can happen with infrequently used plumbing fixtures.  I’ll start by defining the "P" trap.

What is a “P” Trap?

The “P” trap gets it’s name from it’s appearance. It is a combination of two 90 degree couplings with a horizontal drain pipe that gives the entire configuration the shape of the letter “P”.

What is the purpose of the “P” Trap?

A plumbing trap is a device used on your home waste system to prevent the passage of harmful sewer gases into your home, yet not impede the fixture’s ability to discharge to any great extent. The effect of sewer gases on the human body are known, and many are extremely harmful. Additionally, certain sewer gases are explosive.  A “P” trap is just one of the many types, and is also the most commonly accepted type of trap.  All fixtures connected to the household plumbing system is required to have a trap installed

How Does The “P” Trap work?

The purpose of the “P” trap is to hold water and create a seal to prevent harmful sewer gasses passing into the home. The water in the “P” trap acts as the “seal”. The depth of the “seal” is to be 2 inches deep, with a maximum of 4 inches.


So What About Infrequently Used Fixtures?

With infrequently used fixtures, like a master bathtub, the trap will dry out. This happens from naturally occurring evaporation. Once the water evaporates, you no longer have a seal. Without the seal you will now have harmful and odorous sewer gases coming into the home.

What Can I do To Prevent Sewer Gases Entering My Home?

In some jurisdictions they require seldom used plumbing drains, like a floor drain, to have a “self-priming” trap. The self-priming trap will have a line run to it from a more frequently used plumbing fixture, such as a sink, that will supply small amounts of water to the trap to maintain the seal.

However, bathtubs are not typically on the list of drains requiring a self-priming trap. This can easily be addressed by running the water at the tub for just a few seconds every month or so. Or just pouring a couple cups of water down the drain.

If you are smelling something unpleasant in your bathroom, and the toilet has not been recently used, try pouring a little water down the tub drain and see if that fixes it. If it doesn’t, then it may be something more serious and you should consult with a licensed plumber to help you solve the issue.

About the author: Frank Rotte is the Owner of Certified Inspection Services, LLC in Bonsall, CA. Comments are always welcome! Please visit us on Facebook at https://www.facebook.com/homeinspectionsandiego/