How to Fix a Leaking Pipe Under the Sink
Most under-sink leaks are at the slip-joint connections of the P-trap. 90% are 5-minute fixes.
TL;DR
Under-sink leaks come from one of three places: P-trap slip joints (most common), supply hose connections, or the drain tail-piece. Identify the source by drying everything and watching where water reappears. Most are tightened or rewashed in under 15 minutes.
What’s in this guide
- Find the leak source
- Tighten or replace washers
- Replace failed parts
- Test
You opened the cabinet under your sink and found wet wood, a small puddle, or a steady drip. Don’t panic — most under-sink leaks are simple to fix in under 30 minutes. Here’s the diagnosis order.
Tools & Materials You’ll Need
Tools
- Channel-lock pliers
- Adjustable wrench
- Bucket
- Towels
- Flashlight
Materials
- Replacement P-trap washers ($2)
- Replacement P-trap assembly ($8) — if the old one is corroded
- New braided stainless supply line ($12) — if the leak is at the supply
Step-by-Step Instructions
Dry Everything and Identify Source
Wipe down all pipes and fittings under the sink. Run water and watch carefully. The leak source is wherever water reappears first. Check three places in order: P-trap slip joints, supply hose connections, drain tail-piece.
Tighten Slip-Joint Connections
P-trap connections are slip-joint nuts that hand-tighten plus 1/4 turn with channel-locks. If the leak is from a slip-joint, simply tighten further (don’t over-tighten — plastic threads strip). 50% of P-trap leaks are fixed by snug.
Replace P-Trap Washers
Place bucket below. Loosen slip-joint nuts on both ends of the P-trap. Disassemble. The white plastic or rubber washers come out — note orientation (beveled side typically faces the slip-joint nut). Replace with new washers ($2 for a pack), reassemble, hand-tight + 1/4 turn.
Replace the Whole P-Trap (If Corroded)
For older metal P-traps with visible corrosion or pinholes, replace the entire trap with a PVC trap kit ($8). Universal, easy to install, lasts decades.
Address Supply Line Leaks
If the leak is at a supply line connection (top of valve or top of faucet), tighten gently. If still leaking, the rubber washer in the connection has failed. Replace the entire supply line with new braided stainless ($12) — easier than just replacing the washer.
When you fix an under-sink leak, also inspect the supply lines and shutoff valves. Old shutoff valves that haven’t been turned in 10+ years often seize and won’t close in an emergency. While you have water off, exercise the shutoffs (turn off, on, off, on). If they don’t turn smoothly, plan to replace them — $40 each, including labor, while we’re already there.
Don’t over-tighten plastic slip joints
Cracking the slip-joint nut means buying a new P-trap. Hand tight + 1/4 turn is the rule.
Real Scenarios from Our LA Service Calls
$80 leak that became $4,200 in damage
Homeowner ignored a slow drip under their kitchen sink for “a few weeks.” When we opened the cabinet, the entire base was rotted, the cabinet was structurally compromised, and water had wicked into the subfloor. Total: $4,200 to repair the cabinet base, replace flooring, and re-install everything. The original leak: a $3 washer.
When to Call a Plumber Instead
DIY isn’t always the right call. Bring in a licensed plumber if any of these apply:
- Leak is from inside the wall (not visible at fittings)
- You’ve replaced washers and P-trap and the leak persists
- Visible water damage to cabinet, subfloor, or wall
- Sewer smell coming from under the sink (broken seal, gas leaking)
Frequently Asked Questions
How long does an under-sink P-trap last?
PVC: 30+ years. Chrome-finish brass: 10–15 years (corrodes from inside). Original cast brass (pre-1970): 50+ years if not damaged.
Can I use teflon tape on slip-joint connections?
No — slip joints rely on the rubber washer for sealing, not threads. Tape on slip-joint threads can prevent proper compression and cause leaks.
Why does my P-trap leak only when running the disposal?
Disposals push water through more aggressively, creating brief pressure spikes. Slow leaks from marginal seals only show up under pressure.
My drain pipe is metal but my P-trap is plastic — is that OK?
Yes — slip-joint connections handle dissimilar materials fine. Plastic-to-metal is common and normal.
Should I use silicone caulk on P-trap joints?
No — slip joints need to be removable for future cleaning. Silicone makes them permanent.
Need professional help in Los Angeles?
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