Troubleshooting Gurgling Drains

Last Updated on May 7, 2022 by Cheryl Khan

Getting to the Root of Gurgling Drains

Help, my shower gurgles when I flush the toilet! Gurgling drains are usually caused by obstructions in the venting system. The gurgling sound is caused by air being forced through water in your drain trap. It’s like pouring milk too quickly (glug, glug glug).

While the gurgling sound often comes from the shower/tub, it can also come from your vanity sink or toilet. Gurgling drains are a sign of improper ventilation of your plumbing system. This post will address the following questions:

• Why does my sink gurgle when I flush the toilet?
• Why does my shower drain gurgle when I flush the toilet?
• Why does my toilet gurgle when I flush?

This photo shows what your plumbing looks like underneath the bathroom. You can see the p-trap in plain view. (Photo Credit: Nate Vack)

Why are my Drains Gurgling?

Let me try to illustrate what happens when your drains are gurgling. When you flush, water is displaced by air in your water pipes (water leaves and is replaced by air). This water needs to be replaced or else a vacuum will be created and water will not flow properly. To make this possible, every bath plumbing system has a venting system that allow water to exit through the roof instead of through other drains in the house.

Try this: Turn a soda bottle upside down and open the cap. The water will flow in waves as air passes in cycles (gurgling sound). If you poked a small hole in the bottle, water and air could enter and exit simultaneously without any disruption or gurgling.

What Causes the Toilet to Gurgle?

There are 2 causes for a gurgling drain. The second cause is more common but can be easier to fix so we’re going to start troubleshooting this area first.

Shower Drain P-Trap

A gurgling drain can be caused by a shower drain that does not have p-trap, the p trap is busted, or the p-trap is empty because the shower has been unused for long periods of time.

The p-trap prevents methane-rich sewage gases from rising into your bathroom. It also effectively closes your shower drain (with the water pressure contained in the p-trap), and prevents air from exiting your plumbing system this way instead of through the venting system. Without it, air would constantly flow up through your tub drain and make you hear gurgling.

You can test to see if you have a p-trap (or if it’s busted) by inserting a small wire or thread down into the drain. When you pull the wire back up, you should find about 4-6 inches of water on the wire. This lets you know that there is water being held in the p-trap as it should be. If not, you either do not have a p-trap installed (rare) or there may be a crack that is allowing water to escape. Have this fixed immediately.

Plumbing Venting System

Plumbing vent systems allow air to displace water through a separate pipe that will not obstruct the constant flow of water and hence prevent gurgling.

How it works: When there is no water in your plumbing pipes, air fills the pipes. All plumbing systems have vent systems that transfer air out through the roof. Using our soda bottle example, when water exits, air is also moved. If either the drain of the vent is clogged, the air will flow to any possible exit (i.e. your shower drain). Since there is only a small amount of pressure created by the water in your p-trap, flushing your toilet can cause water to flow up and out of your shower drain and gurgle while it passes through water that is held in the p-trap. It’s the same as if you were to blow air through a pipe filled with water.

Obstructions in your plumbing vent system can cause bathroom drains to gurgle. Bathroom plumbing pipes are all connected to the same system. When a connecting branch to a fixture group becomes clogged (or the vent is clogged) you’ll hear a gurgling sound. Because of the way that plumbing systems are designed, proper function of each drain is somewhat connected.

How to Fix a Gurgling Toilet

There are two ways to fix a gurgling toilet.

Step 1: Plunge Your Toilet

The plunging method is the easiest and quickest gurgling toilet fix. Tape the shower drain (air tight) as well as the sink drain and overflow drains shut. Then plunge your toilet to jar loose any obstructions in your ventilation system. The reasoning behind this, if the air cannot flow up and out of your other drains, it will be forced to flow out of your ventilation system (the correct way). If this does not work, proceed to step 2.

Step: Clear Venting System

Obstructions in the plumbing vent lines can occur at the main stack opening that is located on the roof. Most of the time, debris will fall from the vent opening and collect in an elbow in the ventilation system. You will need to clear this out to make the gurgling stop. In rare cases, birds or small animals can climb in these vents. You can clear out the air vents by using a garden hose or a plumbing snake. Learn How to Use a Plumbing Snake.

Final Words

Understanding the relationship between air and water in your bathroom plumbing system is the key to solving gurgling drain symptoms. Whether your experiencing gurgling from the toilet, shower, or sink, they all happen because of the same thing; poor or improper venting of the plumbing system. If the problem still isn’t fixed, a plumbing contractor should be able to handle this task and solve the issue. You can look up services like drain cleaning in San Jose, CA to help you get the job done.

Additional Troubleshooting Guides

1. The Answer to All of Your Basic Plumbing Questions
2. Troubleshooting Guide : How to Repair Your Toilet
3. How a Toilet Works When You Flush

About the Author
Cheryl Khan is a contributing author to the Tradewinds Imports Blog. She likes to roll-up her sleeves and take life by the horns! Always ready for a new adventure, she is a social butterfly who loves to spark up a good conversation.

Cheryl Khan