Under-Sink Leak Repair: Fix the Most Common Household Leak Yourself
The cabinet under your kitchen or bathroom sink is the most likely place in your house to find a leak. Almost all under-sink leaks are fixable in under an hour with basic tools. Here's how to find the source and fix it.
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Finding the source: supply line vs drain
Before you touch anything, identify which type of leak you have — they have completely different fixes.
- Supply line leak: Water present even when the faucet is off. You'll see dripping or pooling under the sink at all times. Source: the braided metal or plastic lines running from the shutoff valves up to the faucet.
- Drain leak: Water only appears when the sink runs. Source: the P-trap, drain basket, or drain line running from the sink to the wall. If you run the faucet and see water dripping from the curved plastic or metal pipe under the sink, it's a drain leak.
- Faucet base leak: Water at the deck of the sink around the faucet base when running. Usually a worn O-ring or failed putty under the faucet body. Requires pulling the faucet to repair.
The dry test: Dry everything out completely. Put paper towels under the sink. Run the faucet for 30 seconds. Check where the paper towels get wet — that's your leak location.
Fix by leak type
Supply line leak
Parts needed: Replacement braided supply line ($10–20, match the length). Steps: Turn off the shutoff valve under the sink. Place a towel under the connections. Use adjustable pliers to unscrew the supply line from both the shutoff valve and the faucet inlet. Thread the new line hand-tight, then snug — don't overtighten. Turn the valve back on and check for drips.
P-trap leak (slip joint)
P-trap is the curved pipe that holds water to block sewer gas. Leaks at the slip-joint connections are common — the plastic washers inside harden and crack over time. Unscrew the slip-joint nuts by hand (they're plastic), replace the washers or the whole P-trap assembly ($10–25), and reassemble hand-tight. Run water and check. Don't overtighten plastic slip joints or you'll crack them.
Drain basket leak
Water dripping from where the drain basket meets the sink basin means the plumber's putty has dried out or the basket is loose. Turn off the water, remove items from under the sink, unscrew the locknut holding the basket from below (you may need a drain wrench — $10), remove the old putty, apply fresh plumber's putty around the drain opening, press the basket in, and tighten the locknut.
Garbage disposal connection leak
If water leaks from where the drain connects to the disposal, the gasket between the disposal drain and the P-trap has failed. Disconnect the drain elbow, replace the gasket ($5–10), and reconnect. If water leaks from the disposal body itself (sides or bottom), the internal seals have failed — replacement is more cost-effective than repair for most disposals.
Tools and parts list
- Adjustable pliers or channel-lock pliers (for supply line connections)
- Bucket and towels (there's always residual water)
- Replacement braided supply line — match the existing length and connection type
- P-trap replacement kit (PVC, 1-1/4 inch for bathroom sinks, 1-1/2 inch for kitchen sinks)
- Plumber's putty (for drain basket)
- Teflon tape (for threaded connections)
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