Plumbing Under Sink: Every Part, How It Works, and How to Fix It

Open the cabinet under a kitchen or bathroom sink and the tangle of pipes underneath can look intimidating. It isn’t. Almost every residential sink in the United States uses the same six or seven parts arranged in the same configuration, and once you understand which piece does what, fixing leaks and replacing components becomes a straightforward DIY job. Plumbing under sink follows a predictable layout regardless of whether you’re working in a powder room, a kitchen, or a basement utility sink.

This guide breaks down each part, explains what it does, and walks through the most common repairs. Most fixes take 20-45 minutes with basic hand tools and parts from any hardware store totaling $5-30. The major exceptions — supply line replacement involving the shutoff valve and drain reconfigurations — get specific notes below.

The Six Main Components

Working from the water source out to the drain, here’s what you’ll find under almost every sink:

1. Shutoff valves — angle valves that turn the hot and cold water on or off
2. Supply lines — flexible braided stainless steel hoses connecting the shutoffs to the faucet
3. Faucet tailpieces — threaded shanks extending down from the bottom of the faucet
4. Sink drain (basket strainer or pop-up) — the drain fitting in the sink basin
5. P-trap — the U-shaped pipe that holds water to block sewer gas
6. Drain arm and waste outlet — the pipe leading from the P-trap into the wall

Kitchen sinks add a few more pieces: a garbage disposal, a tee fitting for double-bowl drains, and sometimes a dishwasher branch tailpiece. Bathroom sinks usually include a pop-up drain assembly with a lift rod connected to the stopper.

Shutoff Valves and How to Use Them

The shutoff valves are your first line of defense for any plumbing emergency. Standard angle stops sit on 3/8-inch or 1/2-inch supply tubes coming out of the wall. Two valves per sink — one hot, one cold — control water flow to the faucet.

Modern quarter-turn ball valves (BrassCraft G2 series, around $9 each) are far more reliable than the older multi-turn compression valves they replace. If your valves haven’t been touched in 5+ years, they may be seized. Exercise them gently — full open to full closed and back — every 6 months to keep them working.

To shut off water before a repair: turn the valve handle clockwise (right) until it stops. Open the faucet to relieve pressure. If water keeps flowing after the valve is closed, the valve has failed internally and needs replacement.

Supply Lines — When and How to Replace

The braided stainless steel hoses connecting your shutoffs to your faucet are the most common failure point in residential plumbing. They look bulletproof but the rubber inside ages out, typically after 8-10 years.

Replace supply lines when you see:

• Rust spots or kinks in the braid
• Bulging in the rubber liner visible at the ends
• Any moisture or weeping at the connections
• White mineral deposits on the fittings

Replacement is simple: shut off the valves, unscrew the old lines (an open-end wrench works, or use a basin wrench for tight spaces), and hand-tighten the new ones a quarter-turn past snug. Use a 20-inch line for most kitchen installs and a 12-inch line for bathroom sinks. Fluidmaster Click Seal lines ($12-18) are dependable.

The P-Trap — How It Works and Why It Leaks

The P-trap is the U-bend below the sink. Its only job is to hold a small amount of water (about a cup) that blocks sewer gas from drifting back up through the drain.

P-traps come in two materials:

• PVC (white plastic) — most common in modern installations, around $4-7 per assembly
• Chrome-plated brass — used when the trap is visible (pedestal sinks, exposed bathroom plumbing), around $15-30

P-trap leaks almost always come from one of three places: the slip nut at the top connecting to the tailpiece, the slip nut at the bottom connecting to the drain arm, or a crack in the trap itself. Tighten slip nuts hand-tight plus a quarter-turn with pliers — over-tightening cracks the plastic.

If a P-trap leaks after tightening, replace the rubber washer (slip joint washer) inside the nut. A pack of assorted slip washers costs $3 and solves most trap leaks.

Clearing a Clogged P-Trap

The P-trap is the single most common clog point in a sink drain. Hair, soap scum, food particles, and grease accumulate at the bottom of the U-bend. Clearing it is one of the most useful DIY skills:

1. Put a bucket or shallow pan under the trap
2. Loosen both slip nuts by hand or with channel-lock pliers
3. Lower the trap and dump the contents into the bucket
4. Use a bottle brush or pipe cleaner to scrub the inside
5. Reinstall with the washers seated properly
6. Run water and check for leaks

This 10-minute job avoids hundreds of dollars in plumber callouts. Don’t use chemical drain cleaners before clearing a trap — they pool in the trap and create a hazardous splash when you open it.

Drain Arm and Wall Outlet

The horizontal pipe running from the P-trap into the wall is the drain arm or trap arm. It must slope downward at 1/4 inch per foot toward the wall. If the slope is wrong, water and waste sit in the pipe and create chronic slow drainage.

The connection at the wall is a sanitary tee inside the wall cavity. From there, the drain runs down to the building’s main waste stack. Issues at this point — settling, broken pipes, root intrusion — are wall-opening jobs and not DIY territory.

Garbage Disposal Plumbing (Kitchen Sinks)

If your kitchen sink has a disposal, the drain layout is slightly more complex. The disposal replaces the basket strainer on one side of a double bowl, and a tee fitting joins the disposal discharge with the other bowl’s drain before connecting to the P-trap.

Common disposal-side issues:

• Leak at the sink flange — re-bed the flange in fresh plumber’s putty
• Leak at the dishwasher inlet — replace the rubber hose clamp
• Leak at the discharge tube — replace the rubber gasket inside the discharge elbow
• Humming with no spin — clear a jam with the included hex wrench from underneath

When to Call a Plumber

Most under-sink work is well within reach for any homeowner with basic tools. Call a professional when:

1. The shutoff valves themselves need replacement — soldering or sharing-off on existing supply lines is involved
2. You find galvanized or lead piping in the drain system — these need full replacement
3. The drain is clogged past the trap and a 25-foot hand auger doesn’t clear it
4. You discover mold or rotted cabinet bottom — water damage repair is needed first
5. The waste arm doesn’t have proper slope or runs uphill

Routine work — trap clearing, supply line swaps, faucet tailpiece replacement — saves $150-300 per plumber visit and takes less time than waiting on an appointment.