Fix a Leaking Sink Base — Diagnose and Repair

Most sink base leaks come from one of three places: the drain assembly, supply line connections, or the faucet mounting nuts. The fix path depends on where the water is actually coming from.

Step 1: Identify the Source

1. Dry everything under the sink completely with towels
2. Run water for 30 seconds and watch where water appears first
3. If it drips from the drain tailpiece (the vertical pipe below the drain): it's the drain seal
4. If it comes from where metal braided lines connect to shutoff valves: it's supply line connections
5. If it pools around the faucet base on the countertop: it's the faucet mounting or base seal

Fix for Drain Assembly Leak (Most Common)

1. Tighten the large plastic locknut on the underside of the sink that holds the drain in place — hand-tighten firmly, then 1/4 turn with slip-joint pliers
2. If still leaking: remove the locknut, clean old putty from the drain flange, apply fresh plumber's putty around the flange, and reinstall
3. Alternative to putty: use a silicone gasket designed for sink drains

Fix for Supply Line Connections

1. Turn off the water at the shutoff valves under the sink (turn clockwise)
2. Tighten the compression nuts where the braided lines connect to the valves — use an adjustable wrench, not pliers
3. Turn water back on and check — if still leaking, the supply lines may need replacing

Fix for Faucet Base Leak

1. Tighten the mounting nuts under the sink that hold the faucet in place
2. If leak persists: turn off water, remove faucet, clean old sealant, apply silicone caulk around the base, and reinstall

When to Call a Plumber

If the P-trap (the curved pipe) is cracked, corroded, or the leak is coming from inside the wall, call a professional. DIY fixes apply to connections and seals only.

Pro tip: Place a shallow pan or towel under the leak while you work so you can see if your fix stopped it immediately when you test with water.