Your bathroom is your go-to place for comfort. But it wouldn’t be as comfortable if you hear weird sounds, like when the bathtub drain gurgles after you use the commode.
The reason why the bathtub gurgles when the toilet is flushed may be the blocked plumbing vents, dry P-trap, small drain pipes, and a clogged sewer line or iron pipes.
Don’t worry, you’re reading the right article and we’re here to explain and give solutions to why this noise happens.
Table of Contents
Causes of a Gurgling Bathtub
1. Blocked plumbing vent
The vent pipe can be seen as the protruding pipe up our roof, and it’s an essential part of the plumbing system. The air and pressure from the latter escape through the pipe, allowing the water to flow successfully down the drain.
Home drains connect to the plumbing vent, such as the toilet, bathtub, and sink. If the vent is blocked, the sewage smell will seep through the traps, and water will not flow as it should, resulting in a gurgling sound from the bathtub.
Notice when the toilet is flushed in winter, there are gurgling sounds because the vents are more prone to clogging due to snow.
A clogged plumbing vent calls for a professional plumber’s services since DIY cleaning can be dangerous, especially for the inexperienced.
However, if clearing the vent is currently impossible because of the snow, we can pour water into unused drains as a temporary solution, such as in the following:
- floor drains
Once the snow starts to disappear, immediately call a plumber for vent cleaning.
2. Dry P-trap
If you look below the sink, there is a P or U-shaped pipe; that’s the P-trap. Bathtubs also have this, even the toilets as well. P-traps should always have water in order to keep sewer gases and smells from entering the room.
With that said, if the bathtub gurgles when the toilet flushes, then sewer gases are coming up because the P-trap is dry and empty.
Here is what we can do to solve this problem:
- Aside from using your plumbing fixtures regularly, we recommend pouring gallons of water down the drains in your house more than twice a year.
- Cold or lukewarm water will do; we don’t advise pouring boiling water as this may damage the P-trap and the other pipes.
The bad news here is a dry P-trap can be because of leaks or it’s clogged. This will entail extra work, and we don’t recommend doing it by yourself. If this is the case, a professional plumber is best for the job.
3. Shower drain has no P-trap
Worst case scenario: the shower gurgles when the toilet is flushed because it has no P-trap.
Technically, the water can still go down the shower drain even if it has no P-trap or differently-shaped ones. But again, the trap serves an important function: to block sewer gases—and even sewage—from entering the bathroom.
That’s why P-traps for shower drains are required by the legislation of the United States for health safety. Of course, you will need a plumber to install one.
4. Clogged shower drain
That drain noise when the toilet flushes can also be caused by a clogged shower drain (and yes, with a P-trap).
This is a common scenario since we use shampoo, soap, and other toiletries. Even our hair can cause a clog since it can also go into the drain.
To fix a gurgling shower drain, start by removing visible clog-causing obstructions in the drain cover. Next, let’s try these methods.
- White vinegar and baking soda mixture
- Pour a kettle of warm water into the shower drain
- Mix a cup of baking soda with a cup of white vinegar in a bucket, then pour it into the drain.
- Wait for 10 to 15 minutes, then pour another kettle of warm water to rinse the drain.
- Drain auger
- Push the end of the snake into the drain until you find resistance.
- Rotate the auger clockwise to push further. If it gets difficult, rotate it counterclockwise to give it momentum, then rotate it clockwise again.
- The auger will push the clog through or pull it out.
If you can’t advance the tool anymore, rotate it counterclockwise and pull it out of the drain. Most likely, you’ll also pull the clog with it.
- Rinse the drain with warm water.
5. Congested sewer line
If the bathtub drain is making a noise whenever you flush the toilet, it’s possible that the sewer line is congested.
The roots of the trees are some of the main culprits, but what we do inside our homes also counts. Constantly allowing oil, fat, and grease to go into the sink’s drain will cause a clog in the long run since they harden by the time they reach the sewer line.
Flushing un-flushable things in the toilet, such as toilet paper, is a cause too. And when the sewer line is blocked, we definitely need professional plumbers since they have the necessary tools and knowledge to solve the problem.
6. Small drain pipes
The job of drain pipes is to carry wastewater to the sewers, and they should be big enough for loads of water too. So if you flush the toilet and the bathtub gurgles, it could be because the diameter of the drain pipes is too small.
A narrow drain pipe may suction up air from the sewer, which will cause the water to gurgle. Waste may also accumulate in the said pipes, causing the passage to become even more narrow.
The best solution for this situation is to increase the diameter of the pipe, which will need professional help (unless you’re ready to try using a hacksaw and cement, we recommend leaving this to experienced folks!).
7. Clogged or corroded cast iron pipes
If your plumbing system was created before 1980, the reason for the bathtub drain and making gurgling noise is probably because of your cast iron pipes.
These pipes will rust and cause clogs since rust is also one of the major causes of blocked drains. They cause a suction mechanism with the air and water, resulting in gurgling sounds.
There are varying reasons why a bathtub gurgles when the toilet is flushed. One of them is a blocked plumbing vent that prevents air from escaping from the plumbing system.
Another cause is a dry P-trap or the absence of a P-trap for the shower drain, which results in the sewer gas entering the bathroom. More severe problems include:
- A clogged sewer line.
- Drain pipes with small diameters.
- A clogged plumbing system due to cast iron pipes.
- Reasons why my toilet gurgling on a septic system.
- Why is my toilet bubbles & gurgles when washer drains?
I’m Paulk Webb, and I work as a writer for Saveourwaterrebates. I’m happy to put in the time and effort to conduct market research to identify the most pressing issues faced by households concerning their plumbing. Feel free to check out our guides to get the most informed recommendations for how to solve your problems.