Under-Sink Plumbing Diagram: Complete Guide
Open the cabinet under your kitchen sink and you will see a tangle of pipes, hoses, and valves that looks more complicated than it is. This under sink plumbing diagram guide names every part, explains what each one does, and shows you how to handle the most common problems — leaks, slow drains, and that mystery smell — without guessing. Once you can identify the parts, most under-sink jobs become straightforward, and you will know exactly what to tell a plumber when you do need one.
Every Part Under Your Kitchen Sink
Here is the full cast of characters, top to bottom. The sink strainer or basket sits in the drain hole and catches food debris; below it, the tailpiece is the straight pipe dropping down from the strainer. The tailpiece connects to the P-trap, the U-shaped (actually J-shaped) bend that holds a plug of water to block sewer gas from entering your home. From the P-trap, the drain arm (also called the trap arm) runs horizontally into the wall, where it joins the drain stack and vent system. On the supply side, two shutoff valves — hot on the left, cold on the right — feed supply lines up to the faucet. If you have a garbage disposal, it mounts to the sink strainer and its outlet connects to the drain via a short pipe; if you have a dishwasher, its drain hose connects to the disposal or to a dedicated drain tailpiece, usually with a high loop to prevent backflow.
Materials matter for repairs. Most modern under-sink drain parts are white PVC or black ABS plastic, joined by slip-joint nuts you can tighten by hand. Older homes may have chrome-plated brass traps, which look nicer but corrode from the inside and are harder to work with. Supply lines are usually braided stainless steel now; older ones may be rigid copper or plastic. Knowing your material tells you which replacement parts to buy — take a photo to the hardware store if you are unsure, because a PVC trap kit and a brass trap kit are not interchangeable.
The Drain Side: Tailpiece, P-Trap, and Drain Arm
The drain side works entirely by gravity, and its geometry matters. Water falls from the sink through the tailpiece into the P-trap, which holds about two inches of standing water as a gas seal. From there it flows through the drain arm into the wall drain. The drain arm must slope slightly downhill toward the wall — about a quarter inch per foot — so water flows out instead of sitting in the trap. If the arm slopes uphill, the sink drains slowly and gurgles.
The P-trap is also your best friend for lost items: a ring that slips down the drain usually lands in the trap, and you can retrieve it by unscrewing the trap's slip nuts and emptying it into a bucket. The trap is designed to be removable for exactly this reason, plus cleaning out clogs. When you reassemble a trap, the slip-joint washers must face the right way — tapered side toward the fitting — or the joint will drip. Hand-tighten the nuts, then give them a quarter turn with pliers; over-tightening cracks plastic nuts, which is one of the most common DIY mistakes under the sink. If you are replacing a whole drain assembly, our guide on how to install a kitchen sink drain covers the full sequence.
The Supply Side: Shutoffs and Supply Lines
The supply side is pressurized, which is why its failures cause real water damage while drain leaks usually just make a puddle. Each faucet has two shutoff valves, and every other water-using fixture under the sink — dishwasher, ice maker, filtered water tap — should have its own valve too. The best modern valves are quarter-turn ball valves: a short lever handle that turns 90 degrees from open to closed, with a reliable seal inside. Older multi-turn valves (round handles you spin several times) wear out, seize up, and drip from the stem; if yours are multi-turn, replace them with quarter-turn valves the next time you have the water off.
Supply lines run from the valves up to the faucet. Braided stainless steel lines are the standard in 2026: flexible, durable, and easy to install, with rubber washers that seal the connection. Check them once a year for bulges, rust spots on the braiding, or moisture at the connections — a failing supply line is one of the most common causes of serious kitchen flooding, and replacements cost under $15 each. Never use saddle valves (the clamp-on valves that pierce a pipe) for a refrigerator or dishwasher line; they leak over time and many codes now prohibit them. A proper installation uses a real shutoff valve tee'd into the line.
Dishwasher and Garbage Disposal Connections
If you have a garbage disposal, it hangs from a mounting ring on the sink strainer and grinds food waste into the drain. Its outlet pipe connects to the drain system on the side of the tailpiece or directly to the trap arm via a baffle tee. The disposal's drain connection is a frequent leak point because vibration loosens the slip nut over time — check it when you notice dampness. The disposal also has a dishwasher inlet port on its side: the dishwasher drain hose connects here, and the knockout plug inside that port must be removed at installation or the dishwasher will not drain at all. This is a classic post-installation mystery: a brand-new dishwasher that will not drain usually means the knockout plug was never removed.
The dishwasher drain hose needs a high loop — a section of hose routed up high under the sink and secured there — or an air gap device on the sink deck. The high loop prevents dirty sink water from siphoning back into the dishwasher. If your dishwasher smells bad or leaves dirty water in the tub, a fallen high loop is one of the first things to check. The dishwasher also has its own hot water supply line and shutoff valve, usually shared with the sink's hot valve via a tee. For pricing on dishwasher work, see our dishwasher installation cost guide.
Common Under-Sink Problems and Fixes
Most under-sink complaints trace back to a short list of causes. A slow-draining sink usually means a partial clog in the trap or tailpiece — remove the trap, clean it out, and check the drain arm before calling anyone. A sewer smell under the sink usually means the P-trap has dried out (common in a rarely used sink or after a long vacation) or the trap seal is being siphoned by a venting problem; running water for thirty seconds refills a dry trap, while a recurring smell needs a plumber to check the vent. A dripping faucet connection is usually a worn washer in a supply line or a loose compression nut — tighten gently first, replace the washer or line if tightening fails.
Water stains on the cabinet floor deserve a closer look, because the source is not always obvious. Supply-side leaks drip constantly, even when the faucet is off, while drain-side leaks only appear when water runs down the sink. Put dry paper towels under each joint, run the faucet, and see which towel gets wet first — that tells you which joint to fix. Also check the sink strainer seal: a failing putty seal under the strainer lets water seep around the drain and down the outside of the tailpiece, mimicking a trap leak. Resealing a strainer means removing it, cleaning off old putty, and resetting it with fresh putty or silicone per the manufacturer's instructions.
How to Find and Stop a Leak
When you find water under the sink, work methodically. First, shut off the water if the leak is active and more than a drip — use the fixture shutoffs, or the house main if those fail. Dry everything with towels so new water is visible. Then inspect each joint with a flashlight, feeling for moisture with a dry hand. Start at the top (faucet connections and strainer) and work down to the trap and drain arm. Turn the water back on slowly and watch each joint while the faucet runs, then again with the faucet off — a leak that continues with the faucet off is on the supply side.
For a loose slip-joint nut, hand-tighten and add a quarter turn with pliers, then retest. For a cracked trap or tailpiece, replace the part — a plastic trap kit costs $10-$25 and installs with hand tools in twenty minutes. For a supply line leak, replace the line; do not try to patch a braided line. For a valve that drips from its stem, try tightening the packing nut slightly; if it still drips, replace the valve. Keep a bucket and towels under the sink for a day after any fix to confirm the leak is truly gone before you put cleaning supplies back.
When to Replace Parts vs. Call a Plumber
Homeowner-friendly under-sink jobs include replacing a P-trap, tailpiece, supply line, or shutoff valve; reseating a strainer; and reconnecting a dishwasher hose. These are all slip-joint or threaded connections with no soldering and no wall work. Call a plumber when the drain arm inside the wall is leaking, when the wall drain connection is damaged, when you find mold or a rotted cabinet floor, when a shutoff valve will not close and you cannot isolate the water, or when the problem keeps returning after you have fixed the obvious parts — a recurring trap leak can signal a venting or slope problem inside the wall that needs diagnosis.
A plumber typically charges $150-$400 for an under-sink repair visit in 2026, depending on the part and your market. That is worth it when the alternative is water damage to cabinets and flooring. For bigger kitchen plumbing decisions — like moving the sink or remodeling — see our cost to move a kitchen sink and kitchen remodel plumbing cost guides, or browse the full kitchen plumbing hub. Knowing what is under your sink turns a scary leak into a ten-minute fix — and tells you exactly when it is time to call in help.