Gurgling Sink When Toilet Is Flushed – How to Fix?

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If flushing your toilet causes your sink to gurgle, you may have problems—sometimes big ones! A fully and well-functioning plumbing system shouldn’t make any weird sounds when you drain or flush wastewater down the pipe.

Even though gurgling sounds are a common occurrence in many homes, some people think that they’re part of how the drainage system works. But these issues are in fact an early sign of some type of blockage or damage in the drainage system.

The most common reason for a gurgling sink is blockage in the pipes, which you can easily resolve by using a plunger a few times. You can then check for a damaged or missing P-trap by inserting a wire into the drain. Next, ask your neighbors if they’re experiencing the same issue, and if so, get in touch with your municipality or borough.

Troubleshooting gurgling drains

You don’t have to be a professional plumber to diagnose the underlying cause of your sink gurgling when you flush the toilet. In fact, you can easily determine the cause by inspecting the system yourself.  

#1 Understand how drain pipes are connected (and how they work)

First thing to understand is that the reason you’re hearing gurgling sounds in your toilet, sink or even shower for that matter is because the drain pipes are all connected.

The plumbing system in your bathroom consists of several vents that are strategically placed to provide an adequate amount of air.

In the event these vents get clogged or blocked, variations in pressure occur, both inside and outside the plumbing system.

If large enough, these differences in pressure force the air through the fixture traps in an effort to equalize the pressure.

Clearing the vents or adding additional vents will usually resolve the issue.

Sometimes liquid or solid waste that’s large enough can block the internal pipe diameter, creating a high-pressure zone in the front of the object.

This high-pressure zone may force air out of the traps in the fixture, causing gurgling of your sink when you flush the toilet.

The drainage pipes in the plumbing system are connected to a common vent, and when that’s restricted can cause the sink trap to pull in the vacuum air.

#2 Inspect P-traps

Those U-shaped pipes located in toilets and under your sinks are known as P-traps.

These pipes can be made of either PVC or stainless steel, and are designed to do several things, most notably trap debris and prevent clogging, and sewer gases from getting into your home.

If the P-trap is busted, empty due to an unused shower for long periods of time or is missing, your drain may start to make gurgling sounds.

The gurgling noise that’s coming from your bathroom sink is actually air escaping from the drain. This air applies pressure in the inside of your pipes on the water in your drain’s P-trap, resulting in a gurgling sound.

And if you have a damaged or no P-trap at all, the air will flow uninterrupted through your drain, causing a gurgling sound.

An easy way to check if you have a P-trap or the one you do have is damaged is by inserting a small wire or thread into the drain.

Next, pull the wire back similar to a car dipstick to check oil level, and if it’s wet roughly around four to five inches, then your P-trap is in good working order.

If not, then you may be missing a P-trap (rare) or the one you have is cracked, and is allowing water to escape.

#3 Try closing the drains and plunge the toilet

Next thing to try is closing the sink drain by using duct tape to cover the top, and plunge the toilet.

When you partially block the toilet waste pipe, the air is forced through the sink drain, so plunging the toilet at this time may get rid of the blockage.

Once you’ve done this, you can uncover the drain, and flush the toilet to see if the issue is resolved. You may have to plunge a few times for this solution to work.

#4 Check the main sewer line for blockages

Most of the aforementioned solutions should’ve solved your gurgling sink issue, but sometimes the main sewer line may be the root of the problem.

Just as the name would suggest, the main sewer line transports waste from every toilet, sink and shower in your home to the drainage system that connects to it.

Therefore, if the main sewer is clogged, it will affect every fixture in your house. You can clean out the man sewer line with an auger only if there’s an accessible clean-out.

However, it’s worth mentioning that this is actually a professional job, so if you aren’t sure on how to go about it, it’s best to call in a plumber to get it done right.

#5 Check for any obstructions and blockages in your vent stack

Check the opening of the main vent stack that’s located on the roof for any clogged leaves or debris. If this main vent pipe is clogged, water might not flow through properly through your drainage system.

Check the screen on the vent for accumulated dirt and debris, and clean it. Grab a garden hose and spay direct into the vent pipe, which will clear any debris that’s stuck in the pipes.

If the water splashes back at you, then great chances are that that the vent pipe is still clogged, for which you will can use a tool such as a sewage auger to clean the pipe.

After you clean the top vent and pipe below, check if the gurgling still persists, and if so, you can try replacing the air admittance valve, aka cheater valve connected to the P-trap.

#6 Call your neighbors and ask if they hear gurgling sounds too

As a last resort, you can speak with your neighbors to check if they’re hearing the same gurgling noises.

If they are, you will have to get in touch with your water company, township or borough to inform them of the issue.

Since this is a municipality issue, it will not cost anything to fix, but if your neighbors aren’t facing the same issue, you will probably have to pay for repairs out of your own pocket.

#7 Call a professional if the problem persists

Given that a gurgling issue if not addresses can lead to a much bigger problem, it is recommended that you call in professional help to get it fixed, especially if you’ve tried basic troubleshooting steps with a plunger.

Even though you will have to pay for professional services, you can be rest assured that the job is done properly.

Things you’ll need for this

  • Drainage auger (to get rid of stuck dirt and debris in pipes)
  • Plunger (to unclog the toilet)
  • Flashlight (to inspect the pipes properly)
  • Garden hose (to clean the top vent)
  • Ladder (to climb on if you need to get to the roof)

How to prevent your sink from gurgling after flushing the toilet?

There are however a few things you can do to prevent the gurgling problem from occurring in the future.

#1 Only flush toilet paper down the toilet

Toilet paper dissolves after getting in contact with water, and in most areas you can easily flush it down the drain.

However, you should refrain from flushing non-soluble items down the toilet such as adult and baby wipes, hair, cotton balls and swabs, cigarette butts, cat litter, dental floss and paper towels.

Oils and fats can also clog your drain pipes, so it’s better to dispose of these by pouring them into a sealed container and throw it in the garbage bin.

#2 Keep the vent stack clean

Make sure the vent stack is free from dirt and debris, and keep it covered to keep animals, leaves, etc. at bay.

Vent stacks are connected to the drain piping and prevent water seals from being siphoned out of them each time wastewater is flushed through.

You may not even be aware that vent stacks exist, but it’s highly recommended to inspect them regularly and keep them clean.

#3 Have your plumbing inspected regularly

It’s a good practice to have your plumbing inspected regularly by a professional, because what you think may be a minor problem can cost you hundreds of dollars in repair.