How do I find exactly where the leak is coming from under my sink?

Dry all the pipes completely with a rag, then run water and watch closely. Put a flashlight under the cabinet and look for drips forming at joints. For a slow leak, place dry paper towels under each joint section—the wet spot will tell you exactly which connection is leaking. Supply line leaks typically appear at the valve or at the faucet connection; drain leaks appear at slip joints, the p-trap, or where the drain exits the wall.

Do I need to turn off the water to fix a leaking p-trap?

Not always. The p-trap and drain pipes are not under pressure—water only flows through them when you're using the sink. You can replace a p-trap with the water supply on, as long as you're not doing any work on the supply lines themselves. That said, it's good practice to turn off the supply valves under the sink before any plumbing work.

What is pipe dope and when should I use it?

Pipe dope (also called pipe joint compound) is a paste applied to threaded connections before assembly. It lubricates the threads so they tighten fully, fills microscopic gaps, and prevents leaks. Use it on threaded metal-to-metal connections, such as supply line connections to angle stop valves or where a trap arm threads into a drain stub-out in the wall. Don't use pipe dope on PVC slip joints—those use washers or rubber gaskets instead.

Why does my p-trap keep leaking even after I tighten the slip nuts?

If tightening doesn't stop the leak, the slip joint washers inside the nuts are worn, cracked, or missing. Disassemble the joint, inspect the rubber or neoprene washer, and replace it. Washers harden and shrink over time, especially with hot water and chemical drain cleaners. A new washer costs under a dollar and solves most persistent slip joint leaks.

What's the difference between a compression fitting and a slip joint?

A slip joint uses a large plastic nut with a rubber washer that slides over the pipe and tightens against the fitting—it's hand-tightened and easy to disassemble. A compression fitting uses a brass ring (called a ferrule) that bites into the pipe when the nut is tightened with a wrench—it creates a metal-to-metal seal and is used on supply lines and copper pipe. Compression fittings are stronger and more permanent; slip joints are for drain lines and are designed to be serviced.

When should I replace the entire p-trap assembly vs. just tightening connections?

Replace the whole p-trap if the pipe shows visible cracks, if the plastic has yellowed and become brittle, if there's significant corrosion on metal traps, or if the pipe has been patched before and is leaking again. A full p-trap kit costs $8–$15 and takes 15 minutes to install. Tighten connections first only if the pipe itself looks solid and the leak is clearly at a joint.

How do I fix a leaking pipe under the sink myself — step by step?

Step-by-step under-sink pipe leak repair: (1) Dry everything and identify the source: wipe all surfaces dry with a towel. Fill the sink with water, then drain it while watching with a flashlight. Mark the wet spot immediately — leaks look larger once water spreads. (2) Identify the pipe type: white or black plastic slip-joint pipes = PVC/ABS drain lines. Braided steel or copper lines connecting to the supply valves = supply lines. Chrome or plastic curved U-shape = P-trap. (3) For supply line leaks: shut off the shutoff valve under the sink. If it won't close fully or drips, shut off the main. Check the connection at both ends (valve end and faucet end). Tighten by hand first, then snug with pliers (never use a pipe wrench on supply lines — 1/4 turn past hand-tight is enough). If the braided line itself has a pinhole or crack, replace it — new supply lines cost $5–$12 and attach in minutes. (4) For P-trap or drain line leaks: put a bucket under the P-trap. Unscrew the slip-joint nuts by hand (clockwise to tighten, counter-clockwise to remove). Inspect the rubber or plastic washer at each connection — if it's flattened or cracked, replace it ($0.50 each at hardware stores). Reinstall hand-tight plus 1/4 turn with slip-joint pliers. Do not overtighten plastic slip joints — they crack. (5) Test: fill and drain the sink 2–3 times while watching from below. A repair is successful when no drips appear during or after draining.

How much does a plumber charge to fix a leaking pipe under the sink?

A plumber charges $100–$300 to fix an under-sink pipe leak: P-trap replacement (most common): $100–$200. The part itself costs $8–$15; the rest is minimum service call labor (typically 30 minutes of work). Supply line replacement: $100–$175. A $5–$12 part; labor is the majority of the cost. Multiple pipe sections / complex drain assembly: $200–$400. Older homes with corroded metal P-traps or non-standard configurations take longer. When to DIY: any standard PVC or ABS drain line repair is homeowner-territory — no tools beyond slip-joint pliers needed. Supply line replacement also requires only a wrench. Under-sink repairs are the best starter plumbing project because water is off before you start, the repair area is fully accessible, and you can see exactly what you're doing. When to hire: if the leak is at a copper soldered joint (requires torch and solder), if the supply valve itself is leaking or corroded, or if water has caused cabinet damage that needs to be assessed.

How to Fix a Leaking Pipe Under the Sink: P-Trap, Supply Lines, and More

A wet cabinet under the sink is one of those problems you might ignore for a few weeks—until you reach under for dish soap and your hand comes back soaked. Under-sink leaks are among the most common household plumbing problems, and most of them are beginner-friendly repairs that don’t require a plumber.

This guide covers the full range of under-sink leak repairs: diagnosing the source, fixing slip joint drain connections, replacing a p-trap, repairing or replacing supply lines, tightening compression fittings, and using pipe dope correctly on threaded connections.

What You Need

Have these tools and materials ready before you start:

P-trap replacement kit with slip joint fittings — fits 1-1/4 inch and 1-1/2 inch drain lines
Flexible braided supply lines — for kitchen and bathroom faucet connections
Pipe joint compound (pipe dope) — for threaded metal connections
PTFE thread seal tape (Teflon tape) — for threaded pipe connections
Slip joint pliers (channellock pliers) — for tightening slip nuts without marring plastic
Plumber’s putty — for sealing drain flanges to sink basin

You’ll also need a bucket to catch water when you remove the trap, old towels or rags, a flashlight, and a basin wrench if you’re working on faucet supply connections in a tight space.

Step 1: Diagnose the Leak Source

Never start wrenching until you know exactly where water is coming from. Open the cabinet, dry everything thoroughly with a rag, and then run water in the sink for 30 seconds. Use a flashlight to watch every joint and connection while water is flowing.

The most common leak locations, in order of frequency:

Slip joint connections on the drain: These are the large plastic nuts on either end of the p-trap and where the drain connects to the trap arm. Leaks here drip from the nut area when the sink is running.

P-trap itself: The curved U-shaped section can crack from impact, chemical damage, or simple age. Look for drips from the middle of the trap.

Supply lines: The flexible tubes running from the angle stop valves (shutoffs) up to the faucet. Leaks appear at either end—the valve connection or the faucet connection. These drip even when the sink is off if the valve is open.

Angle stop valve connections: Where the supply line attaches to the shutoff valve, or where the valve itself connects to the water supply stub-out in the wall. A leak here may be slow and only visible as mineral deposits or a damp spot.

Drain basket (sink strainer) or drain flange: Leaks here appear inside the cabinet as a drip from the very top of the drain pipe, even when the sink isn’t running—they’re caused by water sitting in the sink or a failed plumber’s putty seal.

Step 2: Fix Slip Joint Drain Connections

Slip joint connections use a plastic nut with a rubber or neoprene washer inside. Hand-tight plus a quarter-turn with pliers is all they need. If a slip joint is leaking, there are three possible causes: loose nut, bad washer, or misaligned pipe.

Step 1: Place a bucket under the trap to catch water.

Step 2: Hand-tighten the slip nut at the leaking joint, then add a quarter-turn with slip joint pliers. Don’t overtighten—plastic threads strip easily. Run water and check.

Step 3: If it still leaks, remove the slip nut completely. The washer inside should be a soft, pliable ring. If it’s hard, cracked, flat on one side, or missing, replace it. Take the old washer to the hardware store to match the size (usually 1-1/4 or 1-1/2 inch).

Step 4: Reassemble with the new washer, hand-tighten the nut, then snug it with pliers. Test by running hot water—hot water reveals most gasket leaks that don’t show with cold.

Step 5: Check pipe alignment. A slip joint can leak if the pipe isn’t fully seated in the fitting before the nut is tightened. Disassemble, ensure the pipe slides fully into the fitting hub, then reassemble.

Step 3: Replace a Leaking or Cracked P-Trap

If the p-trap is cracked, deeply corroded, or was patched before, replacement is faster and more reliable than repair.

Step 1: Turn off the water supply at the angle stop valves under the sink. This isn’t strictly necessary for trap work, but it’s good practice.

Step 2: Place a bucket under the trap to catch the standing water inside it (there will be some—that’s normal and intended; it blocks sewer gas).

Step 3: Unscrew the two slip nuts. One connects the trap to the tailpiece (the straight pipe coming down from the drain basket), and one connects the trap to the trap arm (the pipe going into the wall). Turn the nuts counterclockwise by hand or with pliers. The trap will drop free.

Step 4: Remove the trap arm from the wall stub-out if it’s also being replaced. It typically either pulls straight out (if it’s friction-fit) or unscrews (if threaded). Note how far it was inserted and at what angle.

Step 5: Take the old trap to the hardware store and match the diameter (usually 1-1/4 inch for bathroom sinks, 1-1/2 inch for kitchen sinks) and the style (J-bend or P-trap). Complete replacement kits include the trap, trap arm, and all necessary slip nuts and washers.

Step 6: Install the new trap arm into the wall stub-out first. If it’s threaded, wrap the male threads with two to three layers of PTFE tape (wrap clockwise looking at the thread end) before threading it in. Hand-tighten plus a quarter-turn with pliers.

Step 7: Install the P-trap. Slide the slip nut and washer onto the tailpiece (nut facing down, washer below), then bring the trap up, connect it to the trap arm, and tighten both slip nuts. The trap should hang level or slope very slightly toward the wall—never slope away from the wall or it won’t drain completely.

Step 8: Run water and inspect every joint for 60 seconds. Check the trap arm connection at the wall, both slip joints on the trap, and look for drips from the trap itself.

Step 4: Fix or Replace Supply Lines

Supply lines are under constant water pressure, so even a slow drip will worsen over time. Supply line replacement is one of the easiest plumbing repairs: you just unscrew two connections and screw on new ones.

Signs a supply line needs replacement:

Bulging, discolored, or wet braiding on the exterior
Corrosion at either end connection
Dripping at the valve end or faucet end
Any supply line more than 10 years old (replace proactively)

Step 1: Turn off the angle stop valve under the sink by turning it clockwise until it stops. Open the faucet to release pressure and drain the line.

Step 2: Place a rag under the supply line connections to catch residual water.

Step 3: Unscrew the supply line at both ends. The bottom end connects to the angle stop valve with a compression nut; the top end connects to the faucet inlet with a similar nut. Both turn counterclockwise. If a nut is corroded and won’t turn, use two wrenches—one to hold the valve body, one to turn the nut.

Step 4: Measure the old supply line length and note the thread size at each end (most residential supply lines use 3/8-inch compression at the valve end and 1/2-inch or 7/16-inch at the faucet end). Modern braided stainless steel lines include rubber washers in the faucet end nut—do not use pipe dope or tape on these connections, the washer seals them.

Step 5: Install the new supply line. Thread the valve end nut onto the angle stop valve outlet and hand-tighten, then snug with a wrench—one full turn past hand-tight is usually enough. Thread the faucet end up to the faucet inlet and hand-tighten, then snug with a wrench.

Step 6: Turn the angle stop valve back on slowly and check both ends for drips. A slight weep often stops as the rubber washer seats; if it doesn’t stop within a few seconds, tighten another quarter-turn.

Step 5: Tighten or Repair Compression Fittings

Compression fittings are used at the angle stop valve and on some older drain connections. They use a brass ferrule (ring) that compresses against the pipe when the nut is tightened, creating a watertight seal.

For a leaking compression fitting at the angle stop valve:

Step 1: Turn off the main water supply or close the angle stop valve further upstream.

Step 2: Tighten the compression nut with a wrench—one quarter-turn at a time, checking for leaks after each turn. Overtightening will crush the ferrule and cause more leaking.

Step 3: If tightening doesn’t stop the leak, the ferrule is damaged and the fitting needs to be disassembled and rebuilt. Replacement compression ferrules are available, but if the brass has any deformation, replace the entire angle stop valve—they’re inexpensive and the repair is straightforward.

Never reuse a compression ferrule once it’s been fully tightened—the metal has permanently deformed to the pipe, and it will leak if reinstalled.

Step 6: Using Pipe Dope and Teflon Tape Correctly

Pipe dope and PTFE tape are not interchangeable—each has specific applications.

Use PTFE tape when:

Connecting plastic threaded fittings (like a trap arm into a plastic waste tee)
Connecting threaded metal fittings where a sealant is needed
The manufacturer specifies it
Wrap 2–3 times clockwise around the male threads, overlapping each wrap by 50%

Use pipe dope when:

Connecting metal-to-metal threaded fittings with standard tapered pipe threads (NPT)
You want extra sealing assurance on a critical joint
Apply a thin, even coat to the male threads, avoiding the first thread to prevent compound from entering the water supply

Don’t use either one on:

Slip joint connections (use rubber washers only)
Compression fittings (use no sealant—the ferrule creates the seal)
Connections where the manufacturer’s instructions say “do not use sealant”

Don’t use both at once. Tape and dope together don’t provide double protection—the tape can bunch up in the threads and actually cause a leak. Choose one and apply it correctly.

When You’re Done: Check Everything

After all repairs, take five minutes for a thorough final inspection:

Run both hot and cold water at full pressure for 60 seconds
Fill the sink and drain it completely—this tests the drain system under load
Check every supply line connection, every slip joint, and the trap itself with a dry rag
Open and close the angle stop valves a few times to confirm they operate freely
Leave the cabinet door open for an hour, then check for any slow seeps you might have missed

A repair done carefully the first time means you won’t be back under the sink next week with another bucket.

How to Fix a Leaking Pipe — all pipe leak types: supply, drain, compression, soldered
How to Fix a Leaking Pipe Joint — threaded and compression fitting leaks
How to Fix a Leaking Pipe in a Wall — when the leak is hidden inside the wall
How to Unclog a Drain Without Chemicals
How to Fix a Running Toilet
How to Replace a Kitchen Faucet
How to Fix a Kitchen Sink Sprayer — fix the sprayer hose connection if the leak is at the spray head

⏰ PT4H 💰 $10–$50 🔧 SharkBite push-to-connect fittings, PEX or copper pipe, Pipe cutter, Teflon tape, Channel-lock pliers

Diagnose the Leak Source
Never start wrenching until you know exactly where water is coming from. Open the cabinet, dry everything thoroughly with a rag, and then run water in the sink for 30 seconds.
Fix Slip Joint Drain Connections
Slip joint connections use a plastic nut with a rubber or neoprene washer inside. Hand-tight plus a quarter-turn with pliers is all they need. If a slip joint is leaking, there are three possible causes: loose nut, bad washer, or misaligned pipe.
Replace a Leaking or Cracked P-Trap
If the p-trap is cracked, deeply corroded, or was patched before, replacement is faster and more reliable than repair.
Fix or Replace Supply Lines
Supply lines are under constant water pressure, so even a slow drip will worsen over time. Supply line replacement is one of the easiest plumbing repairs: you just unscrew two connections and screw on new ones.
Tighten or Repair Compression Fittings
Compression fittings are used at the angle stop valve and on some older drain connections. They use a brass ferrule (ring) that compresses against the pipe when the nut is tightened, creating a watertight seal.
Using Pipe Dope and Teflon Tape Correctly
Pipe dope and PTFE tape are not interchangeable—each has specific applications.

How to Fix a Leaking Pipe Under the Sink: P-Trap, Supply Lines, and More

What You Need

Step 1: Diagnose the Leak Source

Step 2: Fix Slip Joint Drain Connections

Step 3: Replace a Leaking or Cracked P-Trap

Step 4: Fix or Replace Supply Lines

Step 5: Tighten or Repair Compression Fittings

Step 6: Using Pipe Dope and Teflon Tape Correctly

When You’re Done: Check Everything

Free: 10-Point Home Maintenance Checklist

Save this for next time.

How to Fix a Leaking Pipe Under the Sink: P-Trap, Supply Lines, and More

What You Need

Step 1: Diagnose the Leak Source

Step 2: Fix Slip Joint Drain Connections

Step 3: Replace a Leaking or Cracked P-Trap

Step 4: Fix or Replace Supply Lines

Step 5: Tighten or Repair Compression Fittings

Step 6: Using Pipe Dope and Teflon Tape Correctly

When You’re Done: Check Everything

Related Reading

Free: 10-Point Home Maintenance Checklist

Save this for next time.