How to Replace Under-Sink Pipes in 7 Steps

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. Or your decade-old braided supply lines develop a pinhole leak and spray water across your cleaning supplies at 3 a.m. I’ve replaced both, sometimes on the same Saturday, and the good news is that most under-sink pipe problems are well within DIY range if the connections are accessible and the shutoff valves actually turn.

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 or sweating copper. If your cabinet isn’t rotted and you can reach the connections with a standard wrench, this is a $15-$45 fix that beats a $200 service call.

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

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

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.

Assess corrosion. If the threaded connections are heavily corroded, covered in mineral buildup, or the pipes crumble when you touch them, you may need a plumber. Galvanized steel drain pipes from the 1960s sometimes rust to the point where they can’t be unscrewed without breaking off inside the wall.

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. Hand-tighten first if they’re loose; if not, use the wrench but don’t force it — PVC threads strip easily.

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.

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. PVC doesn’t need much force and will crack if overtightened.

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 6. 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 check for leaks

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 30 seconds. Check every connection you touched. A few drops forming at a slip nut or supply line connection means it needs another quarter-turn. A steady drip or spray means you need to shut off, disconnect, check the washer placement, and reconnect.

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, shut off the water, disconnect, and check that the washer is seated correctly and not damaged.

Step 7: Clean up and monitor for 24 hours

Wipe down all the pipes and the cabinet floor. Leave the bucket under the sink for the next day and check it twice — once after a few hours, once the next morning. A slow drip will show up in that bucket before it damages the cabinet.

If everything’s dry after 24 hours, you’re done.

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. Run the faucet for a full minute and then open the cabinet and look — no drips, no puddles. Run your hand along each connection; it should come away completely dry.

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.

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

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.

Replacing under-sink pipes sits right in that sweet spot of home repair: it looks intimidating the first time, but the actual work is straightforward if the parts aren’t rusted together. I’ve done this in probably eight different sinks over the years, and the only time I called a plumber was when I discovered the shutoff valves themselves had corroded through — which I only found out because I tried to turn them off and got a face full of spray. If your valves work and the connections are accessible, this is a solid DIY win. 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.