Leaking Pipe Under the Sink: Diagnose and Fix It

Common Symptoms

• Water pooling or wet spots in the cabinet under the kitchen or bathroom sink
• Musty or mildew smell from under the sink cabinet
• Cabinet floor is stained, swollen, or warped from repeated moisture exposure
• Visible dripping from the P-trap, drain arm, or supply line connections
• Water only appears under the sink when water is run (drain leak) or constantly (supply line leak)

Most Likely Causes

1. 1

   Loose or Failed P-Trap Slip-Joint Connections (Most Common)

   The P-trap's slip-joint connections rely on friction and a rubber compression washer to seal — they do not use glue or solder. Over time, the rubber washer degrades, or the plastic slip nut loosens from vibration and temperature cycling. A loose slip joint drips at the joint itself every time water runs through the drain. This is the most common source of under-sink leaks and the easiest to fix — tighten the nut by hand plus a quarter turn with channel-lock pliers.

2. 2

   Cracked or Misaligned P-Trap

   PVC P-traps can crack if they were overtightened during installation or a previous repair, if something heavy was placed against them in the cabinet, or if the drain arm and tailpiece are misaligned, putting side stress on the trap. A hairline crack in the P-trap body or in the slip nut itself allows water to seep out continuously during drain use. A cracked P-trap must be replaced — it cannot be reliably patched.

3. 3

   Failed or Corroded Supply Line Fitting

   Supply lines (the braided stainless steel or ribbed plastic hoses connecting the shut-off valves to the faucet) have compression fittings at each end. Over 10–20 years, the rubber seals inside these fittings harden and crack, or the fitting itself corrodes. A supply line leak drips even when the faucet is off (because supply lines are always under pressure), whereas a drain leak only appears when water is running. Leaking supply lines should be replaced immediately — the braided hose itself can also fail catastrophically.

4. 4

   Worn Strainer Basket or Drain Flange Gasket

   In kitchen sinks, the sink basket strainer (the chrome fitting that sits in the sink drain opening) has a rubber gasket between the strainer body and the underside of the sink basin. This gasket dries and cracks over years, allowing water to seep around the drain opening rather than through it. The leak drips from the strainer locknut area at the very top of the drain tailpiece, not from the P-trap. A failed strainer basket gasket requires replacing the strainer body and gasket, which takes 30–60 minutes.

Quick DIY Checks

1. 1Step 1 — dry the cabinet and isolate the source: this is the most critical step and is almost always skipped, leading to the wrong repair. Remove everything from under the sink. Dry the entire cabinet floor, the P-trap, the drain arm, and all supply lines completely with paper towels. Then run water in the sink for 30 seconds and watch carefully — look for drips forming at a specific joint, not just where water has pooled. For a supply line leak, turn off the faucet and check whether dripping continues — a supply line leak drips even with the faucet off. A drain leak stops immediately when you stop running water. Mark the exact location with a piece of tape before reaching in to fix anything.
2. 2Step 2 — tighten or replace the P-trap slip-joint nuts: locate the two slip-joint connections on the P-trap — one where the trap connects to the drain tailpiece (vertical pipe from the sink) and one where it connects to the drain arm (horizontal pipe going into the wall). Hand-tighten each slip-joint nut clockwise, then snug it an additional quarter-turn with channel-lock pliers. Do not overtighten — plastic slip nuts crack if overtightened. Run the faucet for 30 seconds and recheck. If the dripping has stopped, the repair is complete. If the nut is already tight but still drips, the rubber compression washer inside the fitting has failed — replacement washers cost $1–$2, or replace the entire P-trap for $5–$12. If the P-trap body itself is cracked, replace the trap.
3. 3Step 3 — replace a cracked or damaged P-trap: shut off the drain (no need to shut off the water supply for a drain-only repair). Place a bucket under the P-trap. Unscrew both slip-joint nuts counterclockwise by hand — they are usually hand-tight plus a quarter turn. Lower the P-trap into the bucket (it holds water as a sewer gas trap). Take the old P-trap to the hardware store to match the size — kitchen sinks typically use 1-1/2-inch traps and bathroom sinks use 1-1/4-inch traps. Slide the new slip nuts onto each pipe end, insert the P-trap, and hand-tighten both nuts. Snug with pliers — no more. Run water and check for leaks. This repair takes 10 minutes and costs $5–$15. For a complete parts kit see the [Kitchen Sink Drain Clogged](/fixes/kitchen-sink-drain-clogged) guide.

Repair vs Replace

Recommended Tools & Parts

• Keeney 1-1/2 in. PVC P-Trap Kit with Slip Joints

  Complete P-trap replacement kit for kitchen sinks (1-1/2 inch) including the J-bend, extension tube, and both slip-joint nuts with compression washers. Fits standard drain tailpieces. Also available in 1-1/4 inch for bathroom sink use. Includes all replacement washers needed. PVC construction resists grease and corrosion.

  $8–$15

  Buy on Amazon →

• Channellock 430 Slip-Joint Pliers 10-inch

  10-inch slip-joint pliers for tightening and removing P-trap slip-joint nuts and supply line connections. The angled jaw grips plastic slip nuts without cracking them. An essential under-sink plumbing tool — also useful for tightening supply line fittings and removing stuck drain strainer locknuts.

  $15–$22

  Buy on Amazon →

• BrassCraft 12 in. Braided Stainless Steel Supply Line

  Braided stainless steel supply line with 3/8 in. compression × 1/2 in. female thread ends — the standard size for most residential faucets. Rated to 1,000 PSI burst pressure. 12-inch length fits most under-sink applications; also available in 9-inch, 16-inch, and 20-inch. Replace supply lines every 10–15 years before they fail.

  $6–$12

  Buy on Amazon →

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

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