Why Does the Kitchen Sink Gurgle When the Toilet is Flushed?

We all flush the toilet at least once a day—I hope, and the last thing you want to hear when you flush the toilet is your sink gurgling. Does your kitchen sink gurgle when the toilet is flushed? If this is happening, it means it’s usually a ventilation problem, or there is debris in the pipes.

In some cases, it means that there are problems with the plumbing hardware, but this is very rare. If the sink drain gurgles when flushing the toilet, you can easily discover the root of the problem. There are also some steps you can take to fix it yourself.

The Anatomy of Your Plumbing System

Every modern plumbing system consists of drain pipes and vents. Plumbing systems today serve 3 primary purposes.

  • Drain waste – pipes are used to move water and debris down to the sewer.
  • Create a water seal – this prevents sewer gas from flowing into the home.
  • Vent air – keeps water free-flowing and protects the water seal through the use of ventilation.

The use of a sink vent is crucial for draining and forming the water seal. The vent stack refers to the configuration of your pipes. One long pipe runs from the sewer to the roof on traditional plumbing systems, known as a wet vent.

With the use of a wet vent, fixtures like toilets and sinks share a pipe that serves to drain the water downward into the sewer pipe and pull air through an outside roof vent at the same time. This is known as external venting.

Installing a vent directly under the sink is another way of ventilating a plumbing system, which removes the need for a wet vent. An air admittance valve, also known as a cheater valve, can be installed on the trap.

Traps and Air Admittance Valves

Sink Trap

Each fixture has its own trap that creates a water seal to prevent sewer gas from rising up into your home. The actual gurgling noise you are hearing is happening in the plumbing trap itself, located under the sink. This trap is a piece called the p trap or s trap.

S traps are no longer used in new structures, but they are common in older homes or cabins. If you have an s trap, it needs to be replaced with a p trap or retrofitted with an air admittance valve installed. Otherwise, recurring problems with the water seal may occur.

Quiet a Gurgling Kitchen Sink

Without access to outside air, the vacuum in the system can pull air from another plumbing fixture. So flushing the toilet could pull air from the sink, removing the water from the trap. This may be the cause if the sink gurgles but drains fine. To fix this problem, you need to:

  1. Make sure that any vents installed are working properly.
  2. Inspect the pipes to see if they are clogged.

If a vent is blocked by snow, ice, or debris, the trap seal can be sucked out with the draining water, causing that gurgling sound. Additionally, the build-up of debris in the trap itself can cause the seal to drain as well.

Strands of hair and fabric inside the trap can slowly cause water to seep out, removing the water seal. Follow the steps below to stop your sink from gurgling and keep your pipes working smoothly.

Check the Main Vent Pipe

Along with kitchen pipes gurgling, the toilet may flush on its own from time to time if you have a clogged vent stack.

  • Use a ladder to go up on the roof.
  • Flash a light down the pipe to look for any blockage.
  • If you can reach it, clear it out. If not, use a snake or auger to do so.

In colder climates, the pipes will often freeze up in the winter and need to be cleared. Climbing up on the roof under icy conditions can be dangerous, and we recommend calling a professional. If you decide to try this on your own, try spraying water from your hose into it or using an auger to clear the ice.

Check for Obstructions Inside the Drainage Pipe

Clogged Drain Pipe

When the drainage pipe does not receive enough flow, the water will drain slowly. If you notice a slow-moving drain, you need to clean your trap. To do this, you must remove the trap and remove any debris inside.

  • Put a bucket directly underneath the trap, and use pliers to remove the coupling that attaches the trap to the sink and the wall fitting.
  • Use a wire brush, bottle brush, or metal hanger to clean it out thoroughly.
  • Rinse it, put it back together, and see if the drain works. This may solve the problem.

Check the Air Admittance Valve

Air admittance valves typically work for at least 20 years without any issues, but they do fail. If you have an air admittance valve installed on your trap, you should check it to ensure it is in proper working order. The gurgle could be from the valve being stuck in the closed position. It could also be due to damage, a clog, or improper fitting.

  • Try removing it and running the sink.
  • If the water drains properly, it may mean that the vent is not working as it should. Don’t run the water without the valve for an extended period of time, or you may flood the pipes and make a mess.
  • Inspect the valve for any damage or debris. Clean it and reattach it to see if the sink drains smoothly.

Check for Waste Inside the Sink Vents

It’s common to find things like leaves or birds’ nests stuck in the vents. If you have any other type of external vent, like one leading to your garden, check those to see if they are blocked. It takes a large amount of pressure to remove these types of clogs, so you will need to use the right tool. Use a plumbing auger to push or pull out all of the material blocking the pipe.