Why Is There Water Under My Kitchen Sink?

Finding water under the kitchen sink is stressful — but the leak is almost always visible once you know where to look. Water under a sink comes from only three places: the supply lines (pressurized, leaks constantly), the drain assembly (leaks only when water runs down the drain), or the faucet/tap above (drips down through the mounting holes). This guide helps you trace it to the source.

First: protect the cabinet

Before diagnosing, put down old towels and empty the cabinet. If the cabinet floor is swollen, soft, or moldy, the leak has been going a while — dry everything with a fan and check for mold. A cabinet floor that stays wet rots, so finding the source quickly matters.

Quick diagnosis: when does it leak?

• Leaks constantly, even with everything off → pressurized supply line or shutoff valve. See Step 1.
• Leaks only when the faucet runs → faucet base, supply connection at the faucet, or sprayer hose. See Step 2.
• Leaks only when the drain is used → drain assembly: basket strainer, P-trap connections, or disposal. See Step 3.
• Leaks only when the dishwasher runs → dishwasher drain hose connection. See Step 3.

Step 1: Check the supply lines and shutoff valves

These are the pressurized lines — a leak here does not stop on its own.

1. Dry everything with a towel so new moisture is obvious.
2. Run your hand along each supply line (hot and cold) from the shutoff valve up to the faucet. Feel for moisture at every connection.
3. Check the shutoff valves themselves. The packing nut (under the handle) often weeps — tightening it a quarter-turn with a wrench usually stops it. If the valve body is corroded or the leak persists, the valve needs replacing ($10–$20, or a plumber if you are not comfortable).
4. Look at the line material. Braided stainless lines last 10–15 years; old plastic or corroded copper lines should be replaced proactively. A burst supply line floods the kitchen — this is the leak to take seriously.

If a supply connection is the source: turn off the valve, disconnect the line, replace the washer or the whole line, reconnect hand-tight plus a quarter-turn with a wrench.

Step 2: Check the faucet and sprayer from above

Water from the faucet area runs down through the mounting holes and appears “under the sink” even though the source is above.

1. Run the faucet and watch underneath with a flashlight. Drips falling from the faucet shanks or mounting nuts point upward.
2. Check the sprayer hose — pull it out and run water through it. A cracked sprayer hose leaks only when the sprayer is used, and the water runs back down into the cabinet.
3. Check the faucet base. If water pools around the faucet on the countertop and seeps down, the faucet’s O-rings or base gasket need replacing.

Step 3: Check the drain assembly

Drain leaks only appear when water flows down the drain — fill the sink and release it to test.

1. Basket strainer: the metal strainer where it meets the sink. If water drips from the locknut underneath, the plumber’s putty seal has failed. Fix: remove the strainer, roll a fresh rope of plumber’s putty, reset the strainer, and tighten.
2. P-trap connections: the curved trap under the drain has slip-joint nuts. Hand-tighten each nut — they loosen over time. If a nut is cracked or the washer inside is flattened, replace it ($3–$5 in parts).
3. Disposal connections: if you have a garbage disposal, check where the drain pipe meets it and where the dishwasher hose connects. These clamps loosen.
4. Dishwasher drain hose: the hose should loop up high before connecting. If the connection drips only during dishwasher cycles, tighten the clamp.

The paper towel trick

For elusive leaks: dry everything, then wrap dry paper towels around each connection and joint. Run water (faucet, then drain, then dishwasher). Check the towels — the wet one marks your leak. This turns an invisible seep into an obvious answer.

What this costs

Mistakes to avoid

• Ignoring a supply-line leak. Drain leaks are messy; supply leaks are destructive — they run 24/7 at full pressure. Prioritize accordingly.
• Overtightening plastic slip nuts. Hand-tight plus a small nudge is enough. Cranking cracks them.
• Using the wrong sealant. Plumber’s putty for the basket strainer (not silicone — it makes future removal miserable); Teflon tape for threaded supply connections.
• Not replacing old supply lines. If they are 15+ years old, discolored, or stiff, replace them now — on your schedule, not at 2 AM.
• Forgetting the dishwasher hose. It is the forgotten connection that only leaks during dishwasher cycles, making it hard to catch.

Frequently asked questions

Is water under the sink an emergency? A supply-line leak is — turn off the shutoff valve immediately. A drain leak can wait a day or two but should not wait a week; standing water rots the cabinet and grows mold.

Why does it only leak sometimes? Intermittent leaks are almost always drain-side (only when water flows) or temperature-related (a connection that weeps when pipes expand with hot water). The paper towel trick catches these.

Can I just put a bucket under it for now? As a stopgap for a drain drip, yes — but a supply leak needs the valve shut off, not a bucket. And do not let “temporary” become permanent; cabinet damage compounds.

How do I know if there is mold? Musty smell, black or green staining on the cabinet floor or back wall, or wood that stays damp. Small surface mold can be cleaned with a mold-killing cleaner; extensive mold inside the cabinet structure needs professional remediation.

When to call a pro

• A supply line bursts or a shutoff valve will not close — water is flowing and you cannot stop it. Know where your home’s main water shutoff is before this happens.
• The leak is inside the wall behind the cabinet — opening the wall and repairing supply pipe is plumber work.
• The cabinet floor is rotted through — a carpenter or cabinet shop may need to rebuild it.
• You find extensive mold — professional remediation is safer than DIY for large growth.
• The faucet itself needs replacing and the mounting nuts are corroded solid — a plumber has the tools (and patience) for seized fittings.