What you're actually replacing under the sink

Kitchen sink plumbing replacement means swapping out the pipes and fittings that sit in the cabinet below your sink. This includes the P-trap (the curved pipe that holds water to block sewer gas), the supply lines (the flexible hoses bringing hot and cold water to the faucet), and sometimes the drain tailpiece (the pipe that drops straight down from the sink). You are not replacing the faucet itself or the pipes inside your walls—just the visible assembly under the cabinet.

Most homeowners can do this work in an afternoon with basic tools. The job is straightforward because these connections are designed to come apart by hand or with a wrench, and you do not need to solder, glue, or cut into walls. The main reason to replace this plumbing is age (pipes crack or corrode), leaks at the connections, or damage from a clogged drain that required cutting into the line.

Key Takeaways

  • Turn off the water supply at the shutoff valves under the sink before you disconnect anything, and place a bucket underneath to catch standing water in the pipes.
  • Disconnect the supply lines first (the flexible hoses), then unbolt the P-trap from the wall drain, and finally remove the tailpiece from the sink basket.
  • Measure your old P-trap and supply lines before buying replacements, because cabinet depth and sink height vary, and the wrong length will not reach.
  • Reassemble in reverse order, hand-tighten all slip nuts first, then use a wrench to snug them—over-tightening cracks plastic fittings and strips metal threads.
  • Test for leaks by running water for two minutes and checking every connection; if water drips, tighten the slip nut a quarter turn and retest.

Shutting off water and draining the old pipes

Look under the sink and locate the two shutoff valves on the supply lines—one for hot water, one for cold. They are usually oval knobs or small levers on the pipes coming out of the wall. Turn both clockwise (or flip the levers to the off position) until they stop. Then go to the sink and turn on the faucet to release pressure and drain the lines.

Place a bucket under the P-trap to catch the water still sitting in the pipes. Loosen the slip nuts (the large nuts connecting the pipes) by hand first to break them free, then use an adjustable wrench to unscrew them the rest of the way. Water will spill out—this is normal. If the P-trap is bolted to the wall drain with a large nut, loosen that too.

Disconnecting the supply lines and tailpiece

The supply lines are the flexible hoses running from the shutoff valves up to the faucet. At the faucet end, they connect with a slip nut (a large nut you can usually turn by hand). Unscrew both the hot and cold supply lines and set them aside. If they are stuck, wrap a cloth around the nut for grip and use a wrench, but do not force it—you can crack the fitting.

Next, locate the tailpiece, which is the straight pipe dropping down from the sink drain. It connects to the top of the P-trap with a slip nut. Unscrew this nut and pull the tailpiece out of the sink basket. If your sink has a garbage disposal, the tailpiece will be bolted to the disposal flange instead—use a wrench to remove the bolts. Once the tailpiece is free, the old P-trap will come out easily.

Measuring and buying the right replacement parts

Before you go to the hardware store, measure three things: the distance from the sink drain to the wall (this tells you how long your tailpiece needs to be), the distance from the wall drain to where the P-trap will sit (this determines P-trap length), and the distance from the shutoff valves to the faucet (this determines supply line length). Write these down. Bring a photo of your old P-trap and supply lines too—the store staff can match the connection types.

Most replacement kits come with a tailpiece, P-trap, and slip nuts, but supply lines are usually sold separately. Buy stainless steel or braided supply lines rated for your water pressure (usually 80 to 100 PSI for residential homes). Avoid cheap plastic P-traps if your cabinet gets warm or wet—they crack easily. If you have an old metal P-trap, you can often reuse it if it is not leaking; ask the store whether it is worth keeping.

Installing the new tailpiece and P-trap

Start by inserting the new tailpiece into the sink basket. If it has a rubber washer, make sure the washer sits between the tailpiece and the basket—this prevents leaks. Hand-tighten the locknut under the sink, then use a wrench to snug it. Do not over-tighten; you only need it tight enough that water does not drip when the sink is full.

Next, slide the new P-trap into place. The curved part should point downward, and the straight end should point toward the wall drain. Connect the tailpiece to the top of the P-trap with a slip nut—hand-tighten first, then use a wrench to snug it. Connect the bottom of the P-trap to the wall drain the same way. The P-trap should sit level or slope slightly downward toward the wall; if it is tilted sharply, water will not drain properly and you will get clogs.

Reconnecting the supply lines

Screw the new supply lines onto the shutoff valves by hand first. The hot line goes to the hot valve (usually on the left), and the cold line goes to the cold valve (usually on the right). Once both are hand-tight, use a wrench to snug them—again, do not over-tighten. Then run the lines up to the faucet and connect them to the inlet ports (the small threaded holes where water enters the faucet). Hand-tighten, then wrench-snug.

If your supply lines are too long, they will coil up in the cabinet and trap air, which can cause sputtering at the faucet. If they are too short, they will be under tension and can crack. Coil any excess line loosely and find it with a zip tie to the cabinet frame, leaving slack for movement.

Testing for leaks and troubleshooting

Turn the shutoff valves back on by turning them counterclockwise (or flipping the levers to the on position). Go to the sink and run hot and cold water for two minutes. Watch every connection under the sink—at the supply lines, the tailpiece, and the P-trap. If you see water dripping, turn off the water, place the bucket underneath, and tighten the leaking slip nut a quarter turn with a wrench. Turn the water back on and retest.

If water still drips after tightening, turn off the water again and unscrew the slip nut completely. Check that the rubber washer inside is seated properly and not cracked. If the washer is damaged, replace it (they cost a few cents). Screw the nut back on and test again. If the leak is at a supply line connection and tightening does not stop it, the fitting may be stripped—you will need to replace that supply line.

Once all connections are dry, fill the sink and let it drain while you watch underneath. The water should flow smoothly and the P-trap should stay full (you should see water in the curved part). If the drain is slow or backs up, the old clog may still be in the wall drain—you will need to snake the line or call a plumber.

When to call a plumber instead

If the shutoff valves do not turn or are stuck, do not force them—they can break and flood your home. Call a plumber to replace them. If the wall drain is corroded, cracked, or leaking where it enters the wall, that is beyond sink plumbing and requires professional work. If you cannot reach a connection to unscrew it, or if the pipes are soldered together instead of using slip nuts, stop and call a plumber—your home may have an older setup that needs a specialist.

Also call a plumber if you turn the water back on and it does not flow, or if water comes out of the shutoff valve itself. These are signs of a problem inside the wall or at the main line, not under the sink.

Frequently Asked Questions

Can I replace just the P-trap and leave the supply lines alone?

Yes. If your supply lines are not leaking and the P-trap is the only problem, you can replace just the P-trap. Measure from the tailpiece to the wall drain, buy a matching P-trap, and follow the steps above to swap it out. The supply lines can stay in place.

What is the rubber washer for, and where does it go?

The rubber washer sits between the tailpiece and the sink basket to seal the connection and prevent leaks. It goes on the tailpiece before you insert it into the basket. If your old tailpiece had a washer, remove it and check if it is cracked or hard. If it is, replace it with a new one from the hardware store—they are inexpensive and come in standard sizes.

My P-trap is leaking at the slip nut. Do I need to replace the whole thing?

Not necessarily. Turn off the water, place a bucket underneath, and unscrew the leaking slip nut. Check the rubber washer inside—if it is cracked or missing, replace it. If the washer is fine, the threads on the fitting may be stripped. Try a new slip nut of the same size; if that does not work, replace the P-trap section that is leaking.

How tight should the slip nuts be?

Hand-tighten first until the nut stops, then use a wrench to snug it another quarter to half turn. You should not be able to turn it easily by hand, but you also should not need to strain with the wrench. Over-tightening cracks plastic fittings and strips the threads on metal ones, making leaks worse.

What if water comes out of the shutoff valve when I turn it off?

The valve may be worn out or stuck. Do not force it. Turn it gently and stop if you feel resistance. If water still leaks from the valve itself (not from the pipe below it), the valve needs to be replaced—call a plumber, because this requires soldering or compression fittings that are beyond basic sink plumbing.