Understanding the Under Sink Pipes in Your Bathroom

Pipe Down

We take for granted that every time we turn the bathroom faucet on, water comes pouring out. Which it normally does, thanks to our under sink pipes. These under sink pipes are usually easily accessible in case you ever need to turn the water off and/or perform a repair. If you aren’t familiar with your bathroom’s under sink pipes, it’s a bit of knowledge that can prove helpful. At the very least, you need to know how to shut the water off from the bathroom sink if you want to avoid what happens in Fantasia, The Sorcerer’s Apprentice.

Let’s take a look at the various parts of under sink pipes. This way, you’ll be better prepared to buy a part or talk to a plumber about what is going on beneath your bathroom sink with the under sink pipes:

  • Shut-Off Valve
  • Supply Lines
  • Tailpiece
  • P-Trap
  • Drain Pipe
  • Vent Pipe

With the exception of the supply lines, all these under sink pipes are usually made of white  PVC plastic, although under sink pipes are also made in black ABS plastic or chromed brass or copper metal.

Shut-Off Valves

While not an under sink pipe, shut-off valves provide crucial control of the flow of water to the sink. As the name states, you use this valve to shut off the water to the sink without needing to shut off water to the entire house. This is particularly convenient if there is a problem with any of the under sink pipes where you need to replace something or there’s a leak. However, if you ever need to replace a shut-off valve, you do need to shut off water to the entire house beforehand.

Shut-off valves are small and are usually made of metal, though sometimes they are made of plastic. Look for a football-shaped handle or sometimes a straight lever.

There are two shut-off valves to your bathroom sink, one for hot water and the other for cold water. Some shut-off valves feature a red or blue marking to indicate hot or cold water.

Shut-off valves are usually located between the incoming supply lines and the pipes that connect to the tailpieces (see below) on the sink faucet. Most shut-off valves use compression fittings, which means the valves are installed without needing to do any soldering.

Supply Lines

There are two supply lines, one for hot water and the other for cold water. These narrow tubes are variously made of white plastic mesh, gray solid plastic, chromed copper, or braided mesh. It’s not uncommon for these supply lines to fail if they are particularly old.


The tailpiece is a straight under sink pipe connected to the bottom of the sink drain fitting. If the sink has a pop-up drain, the lever rod of the drain assembly connects to a port on the back of the tailpiece. The tailpiece attaches to the drain fitting with a threaded slip nut. If you ever need to remove the tailpiece, you can often easily unscrew the slip nut by hand, or worst case, by using channel pliers. Note that beneath the nut is a tapered plastic washer, which is used to provide a watertight seal. If you ever need to remove the tailpiece, don’t forget the washer when you reinstall it. You’ll know right away if you’ve forgotten because the tailpiece will leak without the washer.


The P-trap is a curved section of pipe that looks like, you guessed it, a sideways “P.” There are two parts to a P-trap, a U-bend and a trap arm connected with slip nut joints.

The purpose of the P-trap is exactly that, to trap water. The P-trap’s bend holds a small amount of standing water left behind each time you use your sink. Each time you run water down your bathroom drain, the old water in the bend is flushed out and is replaced with new water.This water seals the drain opening and  prevents gasses from your sewer rising up the pipe and entering your home. Not only is the P-trap crucial for maintaining a barrier between the sewer system and your indoor living space, it also provides a convenient trap in case you ever drop a ring or other small object down the drain.

Drain Pipe

The drain pipe carries wastewater from the sink to the main sewer system. It connects to the P-trap and typically has a slight downward slope to ensure proper drainage. It also uses a slip nut for easy disassembly.

Vent Pipe

The vent pipe allows air to enter the plumbing system, preventing a vacuum from forming in the pipes. This helps maintain proper water flow and allows for the release of sewer gasses. Vent pipes usually extend through the roof to the outside of your house.

Getting a Grip on Under Sink Pipes

We hope you haven’t found this discussion of under sink pipes too draining. But understanding the layout of these under sink pipes is essential for troubleshooting and performing basic plumbing maintenance tasks, and to help you avoid calling a plumber more than is necessary. If you need to work on your plumbing or if you encounter issues, it’s important to turn off the water supply using the shut-off valves and to follow proper safety procedures.

If you do encounter a plumbing problem with under sink pipes that you can’t fix yourself or aren’t sure you can fix yourself (and if you aren’t absolutely sure, fixing it yourself sometimes leads to more serious, and more expensive, problems), we’re here to help. Benjamin Franklin Plumbing of Kansas City offers complete plumbing services to provide prompt and efficient solutions for all your under sink pipe and plumbing needs. We deliver the highest quality plumbing repair services that result in complete customer satisfaction, with no hidden costs or surprises. Our plumbers are punctual, professional, and equipped with the latest tools and techniques to provide reliable repairs.