How a Sink Drain Works – Plumbing Diagrams

How Does the Drain in a Sink Work?

Both your bathroom and kitchen sink are connected to the water supply system and the drain-waste-vent system (DWVS). The water supply system supplies water to the sink faucets while the DWVS carries wastewater out of the sink.

The main difference between the 2 is that the water supply pipes are smaller in diameter than drainpipes, but they supply pressurized water (usually between 40 and 70 psi) while drainpipes depend on gravity to carry waste out of the sink.

Image: Home Depot

Sink drains work by utilizing the force of gravity. The sink drainpipe and the sewer line connecting the house to the public sewer system or septic tank are installed at a sloping angle. As the water moves down from the sink, it picks up speed which allows it to flow past the P-trap and out into the sewer line

Sinks and toilets have separate branch drainpipes that are then connected to the main house drain stack which is in turn connected to the horizontal sewer line carrying waste out of your house. Toilets, sinks, showers and all water fixtures in your house therefore drain into the same place.

To have an understanding of how the whole house plumbing works, let us look at a plumbing diagram and see how your sink drains are connected to other fixtures and the drain-waste-vent system.

Understanding Your Home Plumbing

I often have people ask me if sinks and toilets drain to the same place. Others even ask if the kitchen sink drains to the sewer line. I reckon most of these people imagine each fixture draining to the sewer line independently.

Image Credit: Nations Home Inspection, Inc

A kitchen sink drains to the sewer line just like your toilet, washing machine and shower/tub. Although each of these fixtures has a separate drainpipe, the drainpipes are all connected to the main drain stack which is connected to the sewer line.

To have a clear picture of how a house plumbing looks like, think of it like a tree. A tree has one big trunk and several branches.

In this example, the truck represents the main house drain stack while the branches represent the different fixtures and their drainpipes.

The main house drainpipe is a vertical 4-inch pipe that gathers all the waste from the house and carries them out. Above the fixture on the highest level, the drain stack is connected the plumbing vent.

A plumbing vent is the vertical pipe which runs through the roof of the house. It has 2 main functions:

  • Vent stacks carry sewer gases/odors out of the drainage system. Without it, the gases would be forced out through your drains.
  • It introduces air into the system. By introducing air in the system, the sinks and other fixtures are able to drain faster and toilets to flush better. If you ever encounter slow draining fixtures and weak flushing toilets, there is big likelihood that the plumbing vent is clogged.

As you can see, all your fixtures drains in your house are interconnected. If the main drain stack or plumbing vent is clogged, the impact will be felt throughout all the fixtures. A sign of the same include:

  • Tub/shower drains gurgling when the toilet is flushed.
  • Toilet bubbling when the sink or shower drains.
  • Sink gurgling/backing up when the toilet is flushed or shower drains.
  • Tub/sink backs up when the washing machine is running.

The main drain stack in the house is then connected to the lateral sewer line outside your house. This sewer line is installed underground and sloping towards the street where it drains to the public sewer lines with the help of gravity. If you have a septic system it will slope towards it.

Lateral sewer lines can also clog, meaning waste cannot flow out to the public sewer system or septic tank. When that happens, the waste accumulates inside the sewer line and if the problem is not fixed, raw sewage will start to back up through your drains.

In most houses, there is a 4-inch pipe sticking out of the ground very close to the house. Some times 2 pipes. This pipe is called a sewer cleanout.

A sewer cleanout provides access to the sewer line, allowing plumbers to insert their drain augers and/or sewer cameras when fixing sewer line problems.

Without a cleanout, the plumber would have to remove the toilet to gain access to the sewer line, or do so from the roof of the house via the plumbing vent.

How Plumbing In Kitchen/Bathroom Sink Works

bathroom sink drain

 A kitchen sink drain works just like a bathroom sink. It is just that you can add other fixtures on a kitchen sink drain like dishwasher and garbage disposal.

If you have a double kitchen sink, each sink drain opening will have a separate drainpipe but the 2 drainpipes will be connected together using a piece of pipe called a tee. Double kitchen sinks therefore use the same drainpipe.

The main problem with this design is that if there is a clog in the drainpipe, neither of the 2 sinks will be able to drain wastewater. You will therefore notice that the 2 sinks will be backing up into each other.

One main component of a sink drain is the P-trap. A P-trap is the U-bend part of the pipe under the sink. It is named as such since it looks like an inverted P.

Actually, all fixtures in your house have one. The one on the toilet is part of the toilet itself and that is why you are able to have water at the bottom of the bowl at all times. You however cannot see the P-trap in your washing machine or tub/shower drain but it is there.

A P-trap has 2 main functions:

  • Due to its U-shape, it traps potential drain clogs which are easy to remove, preventing them from clogging the drainpipe farther away where it would be more difficult to unclog.
  • It is always full of water. This water acts as a barrier preventing sewer gases from coming up through the drain, but to instead do so from the plumbing vent.

If you ever experience a sewage smell from your kitchen or bathroom sink, there is a problem with the P-trap or plumbing vent.  When your travel away from home for a few days, the water in the P-trap will evaporate allowing sewer gases to come up through the drain unrestricted.

To fix the problem, turn on the faucets for a few seconds to replenish the water. Do the same in all the water fixtures in your house including flushing all the toilets.

When the plumbing vent is clogged, sewer gases have nowhere to go, but force themselves through the drains. The sink and other fixtures will be draining slowly since air cannot flow into the drains. Also, as the sink drains, a vacuum will be created since there is no air coming in.

As a result, the water in the P-trap will be siphoned out and you will hear a gurgling sound from the P-trap. The sink will also gurgle when draining the tub or flushing the toilet.

Usually, when a sink is clogged, most people are quick to use chemical drains cleaners to clear it. There is however a better and faster way.

When you have a clogged or slow draining sink, the clog is usually inside the P-trap. Removing and cleaning the P-trap will fix the problem in no more than 10 minutes. It is also the same method you would use to replace a leaking P-trap.

Here is how you do it:

  • Remove the items you have stored under the sink.
  • Place a small container under the P-trap. This will prevent water from spilling on the floor.
  • As you will notice, the P-trap has a short and long leg. To help the water inside flow out via gravity, start by disconnecting the short leg. Most of the connections are usually hand-tight and you should therefore try to loosen them with your hand before reaching out for a wrench.
  • Once disconnected, clean it and connect it back. Before connecting it, check for clogs in the pipe leading away into the wall.

P-trap vs S-Trap

Most people use P-trap and S-trap interchangeably but they are not the same thing. An S-trap forms an S-shape with the drainpipe and is actually illegal in modern plumbing.

If you carefully look at a P-trap, you will notice that it drains out into a horizontal drainpipe while an S-trap is connected to a vertical drainpipe. The horizontal pipe in a P-trap allows air to flow into the P-trap when the sink is draining.

The main reason why S-traps are banned is because as the water drains out through the vertical section of the pipe, there is no way for air to flow in. As a result, a vacuum is created which sucks water from the P-trap.

If you are always having sewer gases in your kitchen or bathroom sink, check underneath your sink if you have a P-trap or S-trap. Installing an air admittance valve will help fix the problem.


When waste leaves the sink’s P-trap, it flows to the main house drainpipe where it mixes with waste from other fixtures. The waste in the main drain stack is emptied into the lateral sewer line which is buried in your yard,

The lateral sewer line then is then connected to the public sewer line at the street or septic tank. The homeowner is responsible for the sewer line on their side of the property while the city is responsible for the public sewer line running along the street.

And basically that is how a sink drain works.

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