Emergency Services

What’s All the Plumbing Under the Sink?


Shamrockplumbing 2023 03march infographic under the sink header-100

When it comes to your home plumbing, there are things you can fix yourself and things you need a plumber for. Plumbers are necessary for anything behind the wall or under the floorboards, but if you’re handy with a wrench and sealing putty, you can fix a wide range of accessible plumbing features DIY, including the pipes under your sink.

Whether you’re rescuing a dropped item in the trap or fixing a leak, if you are in the process of repairing a sink, you’ll need to know what you’re looking at. The professionals at Shamrock Plumbing are here to help you break down all the components below the sink so that you can buy replacement parts and follow repair instructions with confidence.

Shamrockplumbing 2023 03march infographic under the sink

The Structure Under Your Sink

When you open the cabinet below your kitchen or bathroom sink, you’re going to see a few essential pieces. The sink drain comes down into a tailpiece (straight down pipe) followed by a curved trap and then a drain outlet that extends horizontally into the wall before joining your vertical sewer pipes shared by the whole house. You may also see your water supply lines, which lead to the faucet.

If it’s your kitchen sink, then there may be a garbage disposal where the tailpiece goes, and possibly a dishwasher drain line in a t-junction before the trap hook.

You can get a clear view of each piece by looking at our infographic as we break down the name and function of each piece of plumbing below your sink.

Main Water Supply Line

The main water supply line is your inlet pipe, usually smaller and sometimes hidden behind the wall or cabinet housing. If you can see them, there will typically be two water supply lines, hot and cold, which run directly into the underside of your faucet and are regulated using knobs or a lever handle. These bring fresh water into your property, and the hot water runs through your water heater first.

Shut-Off Valves

At the base of your water supply lines should be a shut-off valve, used to control the flow of fresh water to the sink.. This is how you turn off water to your sink while conducting repairs on the faucet so that you can change out the tap or handles without spewing water into the kitchen. If needed, you can access the valves under the sink to turn off to avoid flooding or additional plumbing damage.

It’s a good idea to test your shut-off valve at least once a year, and to replace it if you see significant rust or corrosion to ensure that it works and doesn’t leak if/when you need it.


The tailpiece is the first part of the drain structure, which is much larger than your water supply lines. The tailpiece extends straight down from the sink’s drain and may be sealed to the drain basket in a kitchen sink. The tailpiece typically connects the drain to the p-trap, carrying waste water from the sink down to the trap and then out to the main drain line of the house.


Also known as “The Trap”, your P-Trap is the U-shaped pipe below the tailpiece. That U can catch fallen items, prevents debris backflow into the sink, and is meant to be full of water most of the time to prevent drain and sewer gasses from entering the home. The P-Trap is what you will likely want to remove if you have a stubborn clog or lose something important down the sink. Just be sure to place a bucket below before you disconnect either side, as it is always full of water.

Drain Line

The drain line carries waste water from the P-trap to the main drain and sewer below the house. All plumbing in the house shares drain lines based on where each fixture is located, and they converge into a single sewer access. You can use regular drain flushing and drain snakes to keep the drain line clear. Your drain line under the sink is likely horizontal as it enters the wall.

Air Admittance Valve

The air admittance valve is a special valve that points upward from the horizontal drain line. It allows air to enter the drainage system to balance pressure in the pipes and prevent siphonage of water from the trap if there is ever negative pressure in the system. It is also part of preventing backflow.

Vent Pipe

If you see a pipe heading upward from your drain line, this may be a vent pipe. Vent pipes are used to circulate air and prevent clogs in the drain pipe. Your vent pipes typically will share one exterior vent release so that sewer gasses that might build up or release inside the house are carried safely outdoors.

Strainer Basket

The strainer basket in a kitchen sink has two parts. One half is puttied into the underside of your sink, while the top half is a removable metal basket designed to catch food debris and prevent items from clogging the drain. In many models, the two halves can also fit together to close the sink so you can fill it with water for washing.

Garbage Disposal

Kitchen sinks may have two additional pieces, the garbage disposal and/or dishwasher drain line. Garbage disposals are used to dispose of food waste and are  typically installed in the place of the tailpiece, while the dishwasher drain line is typically a flexible hose that connects at a t-junction in the tailpiece before the P-trap.

A dual-basin sink may have a garbage disposal that connects via t-junction to a second tailpiece below the non-disposal basin.

Professional Sink Repairs with Shamrock Plumbing

Understanding your sink can help you with maintenance and minor repair tasks. However, if the issue is beyond your confident DIY skills, don’t hesitate to call a professional. Homeowners in the Salt Lake region of Utah can rely on our skilled team of plumbing pros for sink repairs and much more.

Are you currently having issues with your bathroom or kitchen sink? Shamrock Plumbing can help! Contact us at (801) 505-9505 to schedule your appointment!

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