How to Fix a Leaking Kitchen Sink: Drain, P-Trap, Supply Lines, and More

A leaking kitchen sink is one of those problems that seems minor until it isn’t. By the time most homeowners notice water damage inside the cabinet, the slow drip has been going on long enough to swell the cabinet floor, grow mold on the wall, and compromise the drywall behind the base cabinet.

A leaking kitchen sink is one of those problems that seems minor until it isn’t. By the time most homeowners notice water damage inside the cabinet, the slow drip has been going on long enough to swell the cabinet floor, grow mold on the wall, and compromise the drywall behind the base cabinet. The good news is that the vast majority of kitchen sink leaks are simple DIY repairs that require no special skills and are usually fixed in under an hour.

This guide covers every common leak location — the drain basket, supply lines, p-trap connections, and garbage disposal — with step-by-step instructions for each.

What You Need

Stock up on these before you crawl under the sink.

• Danco Sink Drain Basket with Strainer and Gasket Kit — A complete drain basket replacement kit with the basket, mounting hardware, and rubber gasket.
• Eastman Braided Stainless Steel Supply Lines 3/8-inch — Stainless-braided lines are far more durable than the ribbed plastic lines found in many original installations.
• PVC P-Trap with Slip-Joint Fittings 1-1/2 inch — Replace the entire p-trap assembly rather than trying to find individual replacement washers for old plastic traps.
• Oatey Plumber’s Putty 14oz — The standard sealing material for drain baskets on stainless steel and porcelain sinks.
• Slip-Joint Pliers Channel-Lock 10-inch — Essential for tightening and loosening p-trap nuts, supply line connections, and drain locknut.
• InSinkErator Essential XTR Garbage Disposal 3/4 HP — A reliable replacement when your disposal is leaking from the bottom or is more than 10 years old.

Step 1: Locate the Leak

The first and most important step is finding the exact leak source. A single sink can have four or five separate potential leak points, and fixing the wrong one wastes time.

Setup:

1. Empty everything out of the cabinet under the sink.
2. Dry the inside of the cabinet thoroughly with old towels.
3. Place sheets of dry paper towels under each pipe connection and fitting.
4. Run the faucet for 60 seconds, then shut it off and inspect immediately.

The four most common leak locations:

A. Drain basket rim: Water pooling on the cabinet floor directly below the drain opening, with no drip visible on the pipes. Usually worse when the sink is full and draining — the leak is at the putty seal where the basket rim meets the sink bowl.

B. Supply lines: Look for drips at both ends of the supply lines — where they connect to the angle stop valve below, and where they thread into the faucet body above. Mineral deposits (white crust) around a fitting are a sign of an ongoing slow leak.

C. P-trap slip joints: Drips at the plastic nuts where sections of the drain pipe connect. These are very common on sinks more than 10 years old because the plastic washers dry out and crack.

D. Garbage disposal mounting ring: A ring of water around the base of the disposal where it mounts to the sink drain. This is different from an internal seal failure — this leak is at the upper mounting connection.

Mark which connection is wet before you start repairs.

Step 2: Replace the Drain Basket Gasket

The drain basket is sealed to the sink with plumber’s putty (on metal/porcelain sinks) or silicone (on composite/quartz sinks). Over time this seal degrades and allows water to seep between the basket rim and the sink surface.

Removing the old drain basket:

1. Under the sink, locate the drain basket locknut — the large plastic or metal nut that threads onto the basket from below, sandwiching the sink between the basket rim and the mounting hardware.
2. Insert a pair of slip-joint pliers into the basket strainer from above (spread the pliers handles and let the tips brace against the strainer cross-bars) to prevent the basket from spinning.
3. Use a second pair of pliers or a pipe wrench to loosen the locknut counter-clockwise. It may be very tight.
4. Once the locknut is off, push the basket up through the drain hole from below and lift it out.
5. Scrape all the old putty or silicone from the drain opening in the sink with a plastic scraper. Clean the area with isopropyl alcohol.

Installing the new drain basket:

1. Roll a rope of plumber’s putty about 3/4 inch thick and press it around the underside rim of the new basket.
2. Press the basket firmly down into the drain opening from above.
3. From below, slide on the rubber gasket, then the cardboard friction ring, then thread on the locknut by hand.
4. Hold the basket from spinning (use the same crossed-pliers trick), then tighten the locknut firmly. The putty will squeeze out around the basket rim above.
5. Remove the excess putty from around the rim with a damp cloth.
6. Run water and check for leaks before reassembling anything else under the sink.

Step 3: Replace Worn Supply Lines

Flexible supply lines carry hot and cold water from the angle stop valves to the faucet. The ribbed plastic or vinyl lines that come with many faucets have a lifespan of 7 to 10 years. Braided stainless steel lines last much longer.

Shutting off the water:

Locate the two angle stop valves under the sink — one for hot, one for cold. Turn each clockwise until it stops. If the valves are stuck (which is common on older installations), use an adjustable wrench — but do not force them past their hard stop, as they can crack. If a valve will not close fully, shut off the main house water supply instead.

Removing old supply lines:

1. Hold the angle stop valve body steady with an adjustable wrench.
2. Use a second wrench to loosen the compression nut at the bottom of the supply line.
3. Disconnect the top of the supply line from the faucet the same way.
4. Note the thread size — most kitchen supply lines are 3/8-inch compression at the valve and 1/2-inch or 7/16-inch at the faucet end. Bring the old line to the hardware store to match it.

Installing new supply lines:

1. Thread the valve end nut onto the angle stop hand-tight, then snug it with a wrench — one-quarter turn past hand-tight is enough.
2. Connect the faucet end the same way.
3. Turn the angle stops back on slowly and check all connections immediately for drips. The braided covering on the new line will hide a slow seep — feel along the entire length and check the floor.

Step 4: Fix P-Trap Slip-Joint Leaks

The p-trap is the curved pipe under the sink that retains a water seal to block sewer gases. Slip-joint connections use large plastic nuts with tapered rubber washers — these washers are the most common failure point.

When to replace vs. tighten:

Try tightening the slip-joint nut first — hand-tight plus a quarter-turn with pliers. If it still drips after being snugged, the washer inside is compromised and the fitting needs to come apart.

Removing the p-trap:

1. Place a bucket under the trap before loosening anything — it will be full of water.
2. Loosen the slip-joint nut at the drain tailpiece (coming down from the sink basket) and the nut at the wall stub-out.
3. Slide the p-trap free.

Inspecting and replacing washers:

Slide each plastic nut off the pipe ends and shake out the rubber washer inside. If the washer is cracked, flattened, or has a distinct compression groove worn into it, it needs replacement. Hardware stores sell universal slip-joint washer assortment packs — bring the old washer to match the diameter.

Replacing the entire p-trap: If the plastic is yellowed, brittle, or has visible cracks, replace the entire assembly. A complete 1-1/2-inch p-trap kit costs about $8 and takes 10 minutes to install. The new fittings include fresh washers and are much less likely to leak.

When reassembling, hand-tighten each nut until snug, then add a quarter-turn with slip-joint pliers. Do not overtighten — over-compressed washers deform and leak.

Step 5: Fix the Garbage Disposal Connection

There are three places a garbage disposal can leak:

Top mount (at the sink flange): The disposal mounts to the sink drain via a mounting ring assembly. If the plumber’s putty seal between the sink flange and the sink surface fails, water leaks between the flange and the sink. The fix is the same as replacing the drain basket gasket — remove the flange, scrape the old putty, apply fresh putty, and reinstall.

Side connection (dishwasher drain hose or drain outlet): A loose hose clamp where the dishwasher drain hose connects to the disposal inlet nozzle will cause a drip. Tighten the hose clamp with a screwdriver or replace the section of hose if it is cracked.

Bottom drain (internal seal failure): As noted in the FAQ, a leak from the very bottom of the disposal housing — from the center drain port, not a side fitting — means the internal motor seal has failed. This is not repairable. A disposal showing this leak type should be replaced.

Replacing a disposal: Replacement is straightforward — new disposals from the same brand (particularly InSinkErator) often drop straight onto the existing mounting ring without any modifications. Remove the old unit by twisting the mounting ring a quarter-turn, disconnect the wiring (or plug), and reverse the process with the new unit.

Step 6: Check the Faucet Base

Sometimes what looks like a drain or supply line leak is actually the faucet body leaking where it seats on the sink deck. Water wicking down the faucet shank and dripping from the underside of the sink looks identical to a supply line drip.

Dry everything, then run water and watch the bottom of the faucet deck plate. If water appears there before it appears at the supply line connections, the O-ring gasket under the faucet base has failed.

The repair is to remove the faucet, clean the sink surface, and either replace the rubber gasket (if your faucet has a replaceable base gasket) or apply a bead of clear silicone under the base plate during reinstallation.

When to Call a Plumber

DIY repair is appropriate for every situation described above. Call a plumber if:

• The angle stop valves will not close and you cannot isolate the water
• There is a leak at a soldered copper connection in the supply lines
• The drain stub-out in the wall is leaking (indicates a failed trap arm connection inside the wall)
• You find active mold or rotted wood inside the cabinet — this needs professional mold remediation before you close the cabinet back up

Related Reading

• How to Replace a Kitchen Faucet in an Afternoon
• How to Unclog a Kitchen Drain Without Chemicals
• How to Install a Garbage Disposal: Step-by-Step Guide