A drip under the sink starts slow — a damp cabinet bottom, maybe a little mildew smell — and then you open the door one morning to find a puddle and a rusted-through P-trap. I’ve replaced under-sink drain pipes in probably eight different kitchens and bathrooms over the years, and the job breaks down into two categories: the ones where the slip nuts turn with a wrench and you’re done in twenty minutes, and the ones where corrosion has welded everything together and you need penetrating oil, patience, or a phone number.

Most under-sink leaks come from the P-trap (the curved pipe that holds water to block sewer gas) or the supply lines that connect your shutoff valves to the faucet. Both are designed to be replaceable without cutting into walls. If your cabinet isn’t rotted and the connections are accessible, this is a $15-$45 fix that beats a $200 service call — but only if you can identify what you’re working with and know when to stop before you crack something expensive.

What you’ll need

Tools:

  • Adjustable wrench or slip-joint pliers
  • Bucket or shallow pan
  • Old towels or rags
  • Pipe wrench (only if connections are severely corroded)
  • Hacksaw (if you need to cut stuck PVC — rarely)
  • Flashlight or headlamp
  • Penetrating oil (PB Blaster or equivalent, for stuck metal connections)

Materials:

  • Replacement P-trap kit (PVC or ABS, sized to match your drain — usually 1¼” or 1½”)
  • Replacement supply lines if needed (measure existing length; braided stainless is the standard)
  • Plumber’s tape (Teflon tape) for threaded connections
  • Optional: P-trap with cleanout plug for future clogs

Prerequisites:

  • Working shutoff valves under the sink (test before starting)
  • Access to the under-sink cabinet
  • No signs of major corrosion on the sink drain tailpiece or wall stub-out

Before you start

Identify your pipe material. This determines how hard the job will be. Look at the P-trap and drain pipes under your sink:

  • PVC (white or cream-colored plastic): Most common in homes built or remodeled after 1980. Slip nuts are hand-tighten-able plastic. This is the easiest material to work with — connections rarely seize, and replacement parts are everywhere.

  • ABS (black plastic): Common in some regions, functionally identical to PVC for this job. Uses the same slip-nut connections.

  • Chrome-plated brass or galvanized steel (metallic gray or showing rust): Common in homes built before 1980. Slip nuts are metal and can corrode or mineral-lock to the threads. If you see rust, heavy mineral buildup, or the pipes feel rough and pitted, budget extra time for stuck connections — or call a plumber if the corrosion is severe.

According to building code standards, both PVC and ABS are approved for residential drain-waste-vent applications, so you can replace galvanized with plastic if local code allows homeowner work.

Turn off the water. If you’re replacing supply lines, shut off both hot and cold valves under the sink and open the faucet to relieve pressure. If the valves are seized or won’t turn, stop — call a plumber to fix the valves.

Check local code. Most jurisdictions allow homeowner plumbing repairs without a permit, but if you’re replacing drain lines that go into the wall or adding new fixtures, check your local rules. Replacing a P-trap or supply lines usually doesn’t require a permit.

Clear the cabinet. Move everything out. You’ll be working on your back with your head in the cabinet, and you’ll drip on whatever’s left in there.

Step 1: Drain and disconnect the P-trap

Place your bucket directly under the P-trap. The trap holds water by design, and it will all come out when you loosen the slip nuts.

Using your adjustable wrench or slip-joint pliers, loosen the two slip nuts that hold the P-trap in place: one connecting to the sink tailpiece (the vertical pipe coming down from the drain), and one connecting to the drain stub-out in the wall. Turn counterclockwise.

If the nuts won’t budge: This is the most common blocker. For plastic slip nuts, try hand pressure first — they’re often just stuck from grime, not actually tight. For metal nuts that are corroded or mineral-locked:

  1. Spray penetrating oil on the threads where the nut meets the pipe. Soak both connections thoroughly.
  2. Wait 15-20 minutes. For heavy corrosion, wait an hour or apply a second coat.
  3. Try again with the wrench. Use steady pressure, not jerking force.
  4. If it still won’t move and you can see rust flaking or the nut deforming, stop. You’re about to crack the tailpiece or snap the wall stub-out, both of which turn this into a bigger job. This is plumber territory.

PVC threads strip easily if forced — snug is enough. Once both nuts are loose, pull the P-trap down and away. Pour any remaining water into the bucket. Expect some sediment, hair, and possibly a smell.

Step 2: Inspect and clean the connection points

Check the tailpiece and wall stub-out for damage, cracks, or heavy corrosion. Wipe both threads clean with a rag. If the rubber slip-joint washers (the black or white rings inside the slip nuts) stayed on the old P-trap, remove them — you’ll use the new washers that come with the replacement kit.

For galvanized or brass pipes: Look closely at the threads. If they’re pitted, rusty, or you can see missing thread peaks, the new slip nut may not seal properly even with a new washer. That’s a sign the tailpiece or stub-out needs replacement — manageable for the tailpiece (it’s under the sink), but if the wall stub-out threads are damaged, you’ll need a plumber to open the wall.

If the tailpiece itself is cracked or corroded, you’ll need to replace it too. That usually means loosening the nut directly under the sink drain, which can be tight. If you’re not set up for that, this is a reasonable place to stop and call someone.

Step 3: Assemble and install the new P-trap

Water actively dripping from leaking P-trap under sink, illustrating common plumbing failure.
Photo by Rajesh S Balouria on Pexels

Most P-trap kits come in two pieces: the trap bend (the U-shaped part) and the trap arm (the straight pipe that connects to the wall). Slide the slip nuts onto each pipe first, then slide on the rubber washers with the tapered side facing the direction of the connection.

Connect the trap bend to the tailpiece first. Push it up, slide the washer snug against the threads, then thread the slip nut on by hand. Tighten finger-tight, then give it another quarter-turn with the wrench — snug, not gorilla-tight. The Spruce’s P-trap installation guide emphasizes this: PVC doesn’t need much force and will crack if overtightened. You’re compressing the rubber washer, not torquing metal threads.

Attach the trap arm to the wall stub-out the same way. The trap arm may need minor angling to align — that’s normal. Once both connections are snug, check that the trap bend is oriented correctly (the curve should be at the bottom, holding water).

Step 4: Replace kitchen sink supply lines (if needed)

If you’re only replacing the P-trap, skip to Step 5. If your supply lines are leaking, corroded, or more than ten years old, replace them now while you’re down there.

With the shutoff valves closed, use your wrench to disconnect the old supply line from both the shutoff valve (bottom connection) and the faucet tailshank (top connection, under the sink deck). Expect a little water to drip out even with the valve closed.

Attach the new braided supply line to the shutoff valve first. Hand-tighten the nut, then snug it with a wrench — about one full turn past hand-tight. Don’t use plumber’s tape on these connections; the internal washer seals it.

Connect the top end to the faucet tailshank the same way. Make sure you’re connecting hot to hot and cold to cold (hot is usually on the left).

Step 5: Turn the water back on and run the first leak test

Open the shutoff valves slowly. Go one full turn, pause, check for drips at both the valve connection and the faucet connection. If it’s dry, open the rest of the way.

Turn on the faucet and let it run for a full two minutes — longer than feels necessary. You’re not just checking for immediate leaks; you’re pressurizing the connections and filling the P-trap to operating level. Watch every connection you touched while the water runs.

What you’re looking for:

  • Dry connections: Perfect. Move to the next test.
  • A single drop forming slowly at a slip nut, not falling: This is minor weeping from the rubber washer still settling. Tighten the nut another eighth-turn and watch for 30 seconds. If it stops, you’re good.
  • Drips falling steadily, even slow ones: The connection isn’t sealed. Shut off the water, disconnect, check that the washer is seated correctly with the taper facing the right direction, and reconnect. Don’t just keep tightening — PVC will crack before it seals if the washer is wrong.
  • Spray or stream: You’ve cross-threaded, the washer is missing, or the threads are damaged. Shut off and start over.

Step 6: Test the drain

Adjustable wrench alongside coiled stainless steel supply line on work surface.
Photo by mefodiy on Pexels

Fill the sink about halfway and pull the plug. Watch the P-trap connections while the water drains. Look for drips at both slip nuts and around the trap bend itself.

If you see a drip at a slip nut, try tightening it another quarter-turn. If it still leaks after that, shut off the water, disconnect, and check that the washer is seated correctly and not damaged. Do not keep tightening past resistance — overtightening is the number one cause of cracked PVC P-traps in DIY installs, and the crack often doesn’t show up until hours later when the stress fracture opens.

Step 7: Run the extended leak-detection protocol

This is the step most first-time installers skip, and it’s where slow leaks reveal themselves before they damage the cabinet.

15-minute check: Leave the cabinet doors open. After fifteen minutes, get down and inspect every connection again with your flashlight. Run your fingers along each slip nut and supply line connection — even a connection that looks dry can have a thin film of moisture that means it’s weeping. If you find any moisture, address it now.

1-hour check: Check again. Wipe down all the pipes with a dry towel first, then come back in an hour. Any new moisture means a slow leak.

24-hour check: Leave your bucket under the sink overnight. Check it the next morning. A dry bucket and dry pipes mean the job is done. Even a few drops in the bucket means you have a slow leak that will get worse — track down which connection and re-seal it.

According to This Old House’s plumbing guidance, most under-sink leaks that appear days or weeks after installation were actually present from the start as minor weeping that the installer didn’t catch. The extended test catches them while you still have your tools out.

Verify it worked

The new P-trap should hold water in the curve with no drips at the connections. The supply lines should show no moisture or corrosion at either end. After your 24-hour test period, everything should be completely dry to the touch. Run the faucet for a full minute, then open the cabinet and inspect with your flashlight — no drips, no puddles, no moisture film on any connection.

Troubleshooting

Problem: Slip nut won’t tighten — it just spins. The washer is probably misaligned or missing. Disconnect, check that the rubber washer is seated inside the nut with the taper facing the right direction, and reconnect.

Problem: P-trap connections are tight but still dripping. Check the washer for cracks or damage. If the washer looks fine, the threads on the tailpiece or stub-out may be damaged. You may need to replace the tailpiece or call a plumber if the wall pipe threads are stripped.

Problem: New supply line leaks at the faucet connection. Make sure you’re connecting to the correct tailshank (hot to hot, cold to cold) and that the internal washer in the supply line nut is present and seated. If it’s cross-threaded, back it off and start again carefully by hand before using the wrench.

Problem: Shutoff valve won’t close all the way or leaks when I open it again. If the valve leaks or won’t hold, you’ll need to have it replaced by a plumber. Don’t try to force a stuck valve.

Problem: Metal slip nut rounded off when I tried to loosen it. The nut has corroded and lost its shape. If you’ve already rounded it, you’ll need to cut the P-trap off with a hacksaw (cut the pipe, not the nut) and then deal with removing the stuck nut from the tailpiece or stub-out. If the stub-out is in the wall and the nut won’t come off, call a plumber — you’re headed for wall work.

Problem: PVC connection was fine for an hour, now it’s dripping. You probably overtightened and stress-fractured the slip nut or the pipe itself. Examine the nut and the pipe end closely for hairline cracks. If you find one, you’ll need to replace that component. This is why the “snug, not tight” rule matters on plastic.

When to call a professional

  • The shutoff valves are seized, leaking, or missing entirely
  • The drain pipes going into the wall are galvanized steel and crumbling with corrosion
  • You find active mold, rot, or structural damage in the cabinet or wall
  • The sink drain flange or tailpiece is fused to the sink and won’t budge
  • You’re not confident identifying which pipes are supply vs. drain
  • The leak is coming from inside the wall, not from a visible connection under the sink
  • Slip nuts are corroded to the point where they deform or round off when you apply normal wrench pressure
  • You’ve applied penetrating oil, waited, and the connections still won’t turn — forcing them will break the tailpiece or wall stub-out

If you’re replacing a P-trap and discover that the wall stub-out is broken or that the trap arm doesn’t align because someone installed the sink wrong, that’s also plumber territory.

FAQ

How long does a P-trap last?

PVC and ABS P-traps typically last 20-30 years if installed correctly. Chrome or brass traps can corrode faster depending on water chemistry — expect 10-15 years in hard water areas. If you see visible corrosion, cracks, or persistent leaks even after tightening, replace it.

Can I reuse the old slip nuts and washers?

You can reuse the slip nuts if they’re not cracked or stripped, but always use new washers. The rubber degrades over time and old washers are the most common cause of leaks on a reinstalled P-trap. New washers cost about a dollar and come with most replacement kits.

Do I need plumber’s tape on P-trap connections?

No. Slip-joint connections (the kind used on P-traps) seal with the rubber washer, not the threads. Plumber’s tape is for threaded pipe connections like the ones on supply lines or galvanized fittings, but even there it’s often not needed if there’s an internal washer.

How often should I replace braided supply lines?

Replace them every 10 years as preventive maintenance, or immediately if you see corrosion, kinks, bulges, or moisture at the connections. Braided stainless steel supply lines rarely fail, but when they do it’s usually catastrophic — a burst supply line can dump gallons per minute until you shut off the valve.

What’s the difference between PVC and ABS pipe for drains?

PVC is white or cream-colored, ABS is black. Both work fine for drain applications under a sink. Use whichever matches your existing drain line if you’re only replacing the P-trap. If you’re starting fresh, PVC is slightly more common and easier to find in most hardware stores, but it’s not a meaningful performance difference for this job.

Can I replace galvanized drain pipes with PVC?

Yes, as long as local code allows it and you’re comfortable with the transition fitting. Most jurisdictions permit homeowners to replace old galvanized P-traps with PVC using a rubber slip-joint washer to bridge the materials. If the galvanized stub-out in the wall is in good shape, you can connect a PVC P-trap directly to it. If the stub-out itself is rotted, you’re into wall work and should call a plumber.

The first time I replaced a P-trap was in a rental property where the previous tenant had apparently been dumping grease for months — the trap was half-full of solidified shortening and the slip nuts came off without a wrench. The second time was in a 1960s house with galvanized steel everything, and I spent forty-five minutes with penetrating oil and a pipe wrench just to get the nuts to turn. Both worked, but I learned to check what I’m working with before I commit. If your pipes are plastic and accessible, this is a solid Saturday-morning fix. If they’re metal and crusty, give yourself an extra hour and know when to stop. For more drain-related repairs, see how to unclog a sink drain and fix a garbage disposal jam. And if you’re tackling a full faucet upgrade next, best kitchen faucets under 200 has some solid options that won’t fight you on installation day.