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Why Does Sink and Tub Gurgle When Toilet is Flushed?

So you have been ignoring the genteel small burping sounds from your sink and tub when you flush. When all of a sudden, one or both begin gurgling, and maybe smelling a little foul. Instead of just waiting for them to regurgitate a glob of hair mixed with toothpaste, or something worse, read the following to help you find out what may be causing the problem–and better–how to fix it.



What Can Cause Gurgling Drains?

Although plumbing drainage systems can appear to be complicated–and can be quite involved–they really boil down to water and stuff out – air in. Every plumbing fixture in your house should have an air intake and and a P-trap to work properly. (Yes, I know that technically your toilet does not have a P-trap. It is instead, designed to act as a large P-trap.)

Gurgling is usually caused by clogged, or partially clogged, drain pipes. Or blocked air intake pipes. Or both. The actual noise comes from air trying to get through the water in the P-trap.

A P-trap is named for its shape. There should be one under every sink and tub in your house. The purpose of a P-trap is to hold enough water inside to prevent sewer gas odor from entering the house.

Warning Note: Sewer gas consists of methane and hydrogen sulfide among others. Methane is explosive and can cause asphyxiation. People die every year from methane asphyxiation–quite often in septic tanks while trying to repair pumps. Hydrogen sulfide can only cause sinus infections. Take that stink seriously.

For more information on diagnosing and fixing clogs and gurgling, please see our article How to Stop a Toilet Gurgling at End of Flush.


Clogged Sewer Pipes

Clogged sewer pipes can be a bit of a chicken and egg thing. Sometimes you are just trying to flush way too much stuff and it will get stuck. But it could also be blocked or partially blocked air intake pipes that slow down bowl evacuation.

Your toilet is designed to move a lot of stuff and water out quickly. But that only works when there is sufficient air intake capacity for the weight of water to do its job. With somewhere between 1 and 5 gallons of water trying to get out of the toilet, the system will try to get air from anywhere including nearby sinks and tubs. Hence the gurgling sound of air being sucked through the water in the P-trap.


Blocked Air Intake Pipes

Most of us have probably tried to pour from a bottle or container without an air intake. Think jerry can of gas. The liquid does not run smoothly because the air replacing the liquid has to get into the container through the same opening as the outgoing liquid. Create an air intake opposite the pour spout and things smooth out instantly.

When you flush the toilet and the air intake is blocked, the system tries to suck air from other areas to allow the waste to get out. In the bathroom, the closest options are sink and tub. The gurgling sound is the sound of water being sucked out of the P-trap so the system can get the air it needs.


How to Fix Gurgling Drains

Now that you have an idea of the cause of your gurgling, it is time to set about fixing it. 

Note: If your first thought when you hear the word plunger is a French Coffee Press, you might want to call the plumber now.


Use a Plunger to Clear Pipes

As with a lot of things in life, the easiest and most obvious solution should be tried first. Quite often it works. If you don’t have one, get a good toilet/sink plunger and clear the clog in the toilet drain. Depending on the age of your toilet, you may need to get a toilet plunger that will actually fit and work. It seems to me that some toilets are designed in such a way as to prevent an ordinary plunger from forming a seal over the hole and working properly.

Vigorous plunging should move the clog into the main stack which is big enough to allow it to continue on its journey, or pull it back up into the bowl where a little judicious stirring will break it up enough to flush properly.

Occasionally this will not work. You will find yourself pushing and pulling air through the sink and tub drains. When this happens, you will have to seal the drains and overflows in both sink and tub. Although duct tape is a popular choice, I would use painter’s tape–at least to start. Painter’s tape does not leave a residue–unless you forget it for a week or two. Duct tape may leave a sticky residue that will require something like Goof-Off to remove. 


Use a Plumbing Snake

If the arm strong plunger method does not produce results, you may want to try a plumbing snake to dislodge the clog. The snake (or auger) is a wire cable with a hook at the business end that will cut into a clog as it is rotates. They are available in various lengths, but I think 25′ long is realistic. These can be hand operated or drill operated. After 25′ you are probably getting into the main sewer system and may need a professional grade snake. (Many of these cost thousands of dollars.)

Snakes will also work in your sink and tub drains, but realistically almost all problems will be in the P-trap, it is easier to remove the trap, clear it out, re-install it, and gently fill it with water to seal out sewer gas. You can often remove P-traps without tools, or at most with large pliers. Remember to put a pail under it and keep your mouth closed.

Eyeball on a String

If you have not been able to remove the clog using a plunger or snake, you may want to take a look at what you are up against. You can get a fairly decent 50′ sewer camera for less then $200. But for a 100′ cable and a good viewer you will not have much change left over from $500. And they can be significantly more. Before investing that kind of money for what could be a one-time use, you might want to call the plumber. If your problem is more than 5 or 6 feet down the line, it is probably more serious than a simple clog.

Once you know where the problem is, and hopefully what is causing it, you can decide if you want to continue working on it yourself, or if calling for professional help is a better option.


Clear the Vent Stacks

Although each plumbing fixture has a separate air intake, the main air intake is usually a 4″ stack venting through the roof. Each fixture–like sinks and tubs–are connected to the air intake stack–usually with 1 1/2″ ABS pipe. (Our main bath and ensuite are situated back to back and before the drywall went on, the attic above them looked like a giant spider web of 1 1/2″ black pipe.) Incoming air is required to eliminate the vacuum created by outflows of water. 

Older homes can have plumbing stacks a small as 2″ in diameter. These could have worked well when installed, but the addition of newer fixtures, or more fixtures, can increase the demand on small stacks–sometimes making them the blockage just by size alone.

Very rarely do the individual smaller pipes get clogged. But the 4″ stack can be a weak point because it is open to the sky. It can be plugged by all kinds of things–leaves, branches, pine cones, dead birds, live birds, animals, and my favorite, snow. (Heavy, wet snowfall and a short stack are not a good combination.)

  • Clean out the clog if it is close to the top of the pipe. If you can see it but can’t reach it, you can use your snake or a straightened coat hangar to pull it out.
  • If you have a sewer camera, use it to take a look down the stack.
  • Use a garden hose to spray water into the top of the stack to try drive whatever is plugging the pipe down and out of the sewer. More pressure is better.

Note: I have seen suggestions for the use of boiling water to melt an ice clog. This may not be a good idea if the stack is ABS plastic. Cracking a pipe running through the attic will usually need a plumber.

If you are not comfortable climbing onto a roof–specially one with a steeper pitch–you should definitely call a plumber. 

Once you have removed the clog, install Oatey Mushroom Vent Caps on all plumbing stacks to prevent future problems.


Call a Plumber

If none of these suggestions work, you will probably have to call a plumber. Your problems could be something else.

  • Huge Clog. You might have a drain pipe clog that is too big or solid for a hand held snake to clear. Plumbers have more expertise and usually better equipment.
  • Inadequate Venting. I think most modern plumbing codes specify a 4″ diameter main stack and 1 1/2″ – 2″ branch vent pipes. I have seen old plumbing with only 2″ stacks. There is just not going to be enough air intake to adequately provide for drainage if 2 or 3 toilets, sinks, tubs are in use at the same time.
  • Damaged Pipes. Occasionally plumbing drain and vent pipes can get damaged by things such as construction or frost heaves or collapsed pipes. Replacing piping may be above your pay grade. Making a call to the plumber an attractive alternative.

Contrary to popular opinion, plumbers do not go to school for years to learn just 3 things: stuff runs downhill, payday is on Friday, and don’t lick your fingers. Plumbing, as with heating, is a fairly technical system that requires knowledge and understanding to make it operate properly. Having a good plumber fix your problems is never a bad thing.


Preventative Maintenance

A little chemical preventative maintenance every month or two will go a long way towards keeping P-traps and pipes working properly. I dump some Liquid Plumr into our sinks and tubs every couple of months. I also use CLR in the toilet tanks regularly to get rid of the undissolved solids that build up. (Slower flushes make clogs more likely.)



Terry Schutz

I have worked as a contractor, sales person, and business owner in the construction industry for over three decades--mostly in home renovations and also as a home builder. I have been married to the same wife for 46 years. We have 3 children and 4 granddaughters. I have also been writing semi-professionally for about 20 years--construction articles, personal stories, and politically incorrect social commentary.

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