Under-Sink Leak Repair

Common Symptoms

• Water pooling on the cabinet floor under the sink
• Wet or warped cabinet floor or back wall
• Dripping from drain pipes or connections when sink is used
• Musty or mildew smell inside the cabinet

Most Likely Causes

1. 1

   Loose P-Trap Slip Joints

   The P-trap is the curved section of drain pipe that retains water to block sewer gases. It connects via slip-joint nuts and plastic or rubber washers. Over time, vibration and thermal cycling loosen these joints, allowing water to drip at the connections whenever the sink drains.

2. 2

   Worn Drain Gaskets

   The slip-joint connections inside the P-trap and drain arm rely on rubber or plastic washers (gaskets) to seal. These harden and crack with age, especially in sinks with hot water use. Even if slip-joint nuts are tight, a cracked washer will leak and must be replaced.

3. 3

   Leaking Supply Line

   The braided stainless supply lines connecting the shutoff valves to the faucet body can develop pinhole leaks at the fittings or along the hose itself, especially on older plastic-core supply lines. A leaking supply line drips even when the sink isn't in use — the key distinguishing sign.

4. 4

   Deteriorated Putty Around Basket Strainer

   The basket strainer (the metal drain fitting in the sink basin) is sealed to the sink with plumber's putty. When this putty dries out and cracks, water leaks from the bottom of the sink flange around the outside of the drain body, dripping down onto the pipes below.

Quick DIY Checks

1. 1Step 1 — Identify the leak source precisely: clear out the cabinet and thoroughly dry all pipes and surfaces with a towel. Place a dry paper towel or newspaper under the pipes. Run water in the sink for 30 seconds and watch carefully — note exactly where the first drop appears. Check both the drain side (P-trap, drain arm, strainer) and supply side (the two braided lines running from the shutoff valves up to the faucet).
2. 2Step 2 — Tighten loose slip-joint nuts (drain leaks): if the leak is at a P-trap connection, try hand-tightening the slip-joint nut first — often that's all it takes. If hand-tight isn't enough, use slip-joint pliers for an additional 1/4 turn. Do not overtighten plastic nuts — they crack easily. If tightening stops the drip, you're done. If the nut is already fully tight, the washer inside needs replacing.
3. 3Step 3 — Replace worn washers: unscrew the slip-joint nut by hand (or with pliers if stuck), slide it back, and remove the old washer. Take it to the hardware store for an exact match, or buy a P-trap repair kit that includes multiple washer sizes. Seat the new washer squarely inside the nut, reassemble, and snug the nut by hand then 1/4 turn with pliers.

Repair vs Replace

Recommended Tools & Parts

• Plumb Craft P-Trap Repair Kit

  Universal P-trap repair kit with slip-joint washers in multiple sizes for 1-1/4 inch and 1-1/2 inch drain connections. Includes 10+ washers, slip-joint nuts, and gaskets to cover most kitchen and bathroom sink drain repairs.

  $5–$10

  Buy on Amazon →

• BrassCraft 20-inch Braided Stainless Supply Line (3/8 OD x 1/2 FIP)

  Flexible braided stainless steel supply line for faucet connections. 20-inch length covers most under-sink installations. Stainless mesh over polymer core — much more durable and kink-resistant than older plastic hose supply lines.

  $8–$15

  Buy on Amazon →

• Oatey Plumber's Putty

  Standard plumber's putty for sealing basket strainers, sink drains, and faucet bases to the sink surface. Stays workable, does not shrink or crack, and is easy to remove for future repairs. Not for use on marble, granite, or porous stone (use silicone sealant instead on stone sinks).

  $4–$8

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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