Kitchen Sink Plumbing: How the Under-Sink Assembly Connects
Kitchen sink plumbing runs as two separate systems under one cabinet: supply lines that push hot and cold water up to the faucet, and a drain assembly that carries wastewater down and out. The drain path goes strainer, tailpiece, P-trap, then trap arm into the wall. A garbage disposal and a dishwasher hose tie into that same drain. Related: the kitchen plumbing rough-in.
What are the parts of kitchen sink plumbing?
Kitchen sink plumbing breaks into three groups: the drain assembly below the basin, the disposal and dishwasher connections that feed into it, and the supply lines with shutoff valves. The table below lists each part, what it does, its typical size, and how it usually fails, for both a single bowl and a double bowl with a disposal.
| Part | What it does | Typical size | Common failure |
|---|---|---|---|
| Strainer basket (basin drain) | Seals the drain opening to the basin and catches food scraps | 3-1/2 in opening | Worn putty or gasket leaks at the basin; loose locknut |
| Disposal mounting flange (disposal basin) | Locks the garbage disposal to the basin drain opening | 3-1/2 in opening | Dried plumber’s putty seeps at the rim |
| Tailpiece | Straight pipe from the strainer down to the trap | 1-1/2 in | Cross-threaded or loose slip-joint nut drips |
| Continuous waste (double bowl) | Tee or wye that joins both basins to one trap | 1-1/2 in | Misaligned slip joints weep at the tee |
| P-trap | U-bend that holds a water seal against sewer gas | 1-1/2 in | Dried seal lets in odor; grease clogs the bend |
| Trap arm | Connects the trap to the wall drain stub-out | 1-1/2 in (2 in on some branches) | Back-pitch or a belly causes slow draining |
| Garbage disposal | Grinds food and discharges into the trap | 1-1/2 in discharge | Leak at the discharge gasket; hum or jam |
| Dishwasher drain hose / air gap | Carries dishwasher water to the disposal or a waste tee | 5/8 in hose to 7/8 in inlet | Knockout plug left in the disposal; backflow with no loop or air gap |
| Supply lines and shutoff valves (angle stops) | Feed hot and cold water to the faucet, with a local off switch | 3/8 in riser, 1/2 in stop | Valve seizes; compression nut weeps at the wall |
Sizes reflect common US residential practice and may vary by fixture and local code. For how this drain and supply set fits the rest of the house, see our residential plumbing diagram guide.
How does the under-sink kitchen plumbing layout connect?
Picture the drain from the top down. Waste leaves the basin, drops straight through the tailpiece, hits the U-shaped P-trap, then rises into the trap arm and heads into the wall. On a double bowl the two basins meet at a continuous waste tee before the trap. Supply lines sit off to the sides, feeding up to the faucet.
- Water and food waste leave the basin through the strainer basket seated in the drain opening.
- On a disposal basin, the flow passes through the disposal and out its discharge tube instead of a plain tailpiece.
- The tailpiece or discharge tube drops down to the trap connection.
- On a double bowl, a continuous waste tee joins the second basin before the trap.
- The combined flow enters the P-trap, whose U-bend keeps a standing water seal.
- Water rises out of the trap into the trap arm, sloped about 1/4 inch per foot toward the wall.
- The trap arm enters the wall drain stub-out, which carries waste to the branch drain and up the vent.
Where the supply lines and shutoffs sit
Two supply lines rise from the back of the cabinet, one hot and one cold. Each passes through a shutoff valve, usually an angle stop, then a flexible riser runs up to the faucet tailpieces. Turn these valves off before you touch the faucet. If a valve drips or the faucet leaks after, our guide to fix a leaky faucet covers the repair by faucet type.
How is a double-bowl kitchen sink with a disposal plumbed?
A double bowl adds a second drain that has to merge with the disposal before a single trap. The disposal mounts under one basin and grinds waste into a discharge tube. That tube meets a tee or continuous waste fitting where the second basin’s tailpiece drops in. The joined flow then runs through one shared P-trap and out the trap arm.
Both basins should sit at the same height so neither backs up into the other. Keep the whole assembly at 1-1/2 inch and slope the horizontal run toward the trap. If the disposal hums, stays dead, or leaks at the discharge, see our guide to a garbage disposal that is not working for the reset and diagnosis steps.
How does the dishwasher drain tie into kitchen sink plumbing?
The dishwasher drain hose connects to the garbage disposal inlet or to a dedicated waste tee on the tailpiece. Before you attach it to a new disposal, punch out the knockout plug inside the disposal inlet, or the water has nowhere to go. Backflow protection is required so dirty sink water cannot siphon back into the clean dishwasher.
Two methods handle backflow. A counter-mounted air gap gives a physical break in the line and is required in Uniform Plumbing Code states such as California, Washington, Minnesota, and Hawaii. A high loop, where the hose is fastened as high as possible under the cabinet (often near 32 inches), is accepted in many International Residential Code areas. Confirm which your local code allows.
What are the common failure points in kitchen sink plumbing?
Most under-sink trouble shows up at three spots: the slip-joint connections, the P-trap, and the disposal or dishwasher tie-in. Slip nuts loosen and weep, the trap collects grease and clogs, and a dried-out trap seal lets sewer gas into the room. Catching a slow drip early keeps the cabinet floor and the base dry.
Snug a leaking slip nut by hand plus a quarter turn, and swap the washer if it still weeps. A stubborn drip at a threaded or soldered joint upstream calls for more than a nut turn; our guide to fix a leaking pipe walks through temporary and permanent repairs by leak type. Clear a clogged trap by unscrewing the slip nuts and emptying the bend over a bucket.
Frequently asked questions
What is the plumbing under a kitchen sink called?
The pipes under a kitchen sink are the drain assembly, or trap assembly. It includes the strainer basket, the tailpiece, the P-trap, and the trap arm that ties into the wall drain. On the intake side sit two supply lines and their shutoff valves, often called angle stops.
What size pipe is a kitchen sink drain?
A kitchen sink drain uses 1-1/2 inch pipe throughout the visible assembly: tailpiece, continuous waste tee, P-trap, and trap arm. Bathroom sinks use smaller 1-1/4 inch pipe. Never reduce a kitchen drain below 1-1/2 inch, since the narrowest point becomes the first place food waste clogs.
Does a kitchen sink need a P-trap?
Yes. Every kitchen sink needs a P-trap by code. The U-bend holds a small pool of water that seals the pipe and blocks sewer gas from rising into the room. Without that seal you get a rotten-egg smell at the sink. Two basins may share one trap through a continuous waste fitting.
How does a double sink with a garbage disposal drain?
The disposal mounts under one basin and grinds waste into a discharge tube. That tube ties into a tee or continuous waste fitting where the second basin drops in. The combined flow then runs through a single shared P-trap and out the trap arm. Both basins sit at the same height so they drain evenly.
Do you need an air gap for a dishwasher?
It depends on local code. California, Washington, Minnesota, and Hawaii follow the Uniform Plumbing Code and require a counter-mounted air gap. Many International Residential Code areas accept a high loop instead, with the drain hose fastened as high as possible under the cabinet, often near 32 inches. Both prevent dirty water from siphoning back into the dishwasher.
How far can the P-trap be from the wall drain?
The trap arm between the P-trap and the vented wall drain is limited by code. For 1-1/2 inch pipe the Uniform Plumbing Code often caps it near 42 inches, while the International Plumbing Code allows longer runs. The arm must slope about 1/4 inch per foot toward the wall. Check your local code, since limits vary.
Last reviewed: September 2026.