Under-Sink or Supply-Line Leak?

A wet cabinet floor, a swollen base panel, a slow drip you only notice when you grab the dish soap. Under-sink leaks are easy to ignore, and that’s the problem: water sitting in a cabinet ruins the particleboard and can work its way into the subfloor.

The good news is that most under-sink leaks come from a short list of parts, and most are fixed in one visit. Here’s how to stop the water, figure out where it’s coming from, and know when to call.

Same-day service available. Mon–Fri 7am–5pm, emergency calls taken. Sonoma County homeowners only. CA License #1112228.

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Step 1: Stop the Water

• Close the shutoff valves under the sink. These are the small valves (often called angle stops) where the supply lines come out of the wall. Turn them clockwise until they stop. Hand-tight is enough.
• If a valve won’t turn, or starts leaking when you turn it, don’t force it. Older valves can crack or start dripping at the handle. Shut off the water at your main instead.
• Know where your main is. On most homes it’s where the water line enters the house, often near the front hose bib or in the garage. On a private well, it’s usually near the pressure tank.
• Protect the cabinet. Take everything out, put down a towel and a shallow pan, and leave the doors open so it can dry.

Step 2: Find Where It’s Coming From

Dry everything with paper towels, then turn the water back on (if it’s safe to) and watch. A dry paper towel wrapped around each connection shows you fast which one is wet.

Quick rule of thumb: if it drips all the time, even when nothing is running, it’s on the supply side (the lines under pressure). If it only drips while water is going down the drain, it’s on the drain side.

Supply lines

The flexible lines that run from the shutoff valves up to the faucet. Look for drips at the nuts on either end, rust or corrosion at the fittings, kinks, or a braided line that looks swollen or frayed. A bulging supply line can burst, so if you see one, leave the valve off and call.

Shutoff valve (angle stop)

Water around the handle or where the valve meets the pipe. Many homes in Cotati, Rohnert Park, Petaluma and Santa Rosa still have their original shutoff valves, and with our hard water they get crusty and stop sealing.

Faucet base or faucet body

Water on top of the counter around the faucet that runs down through the mounting hole, or a drip from the underside of the faucet itself. Usually a worn cartridge or O-ring inside the faucet, or loose mounting hardware.

P-trap and drain connections

The curved pipe under the sink and the fittings around it. Leaks here only show up while water is draining. Common causes are a loose slip nut, a worn washer, or a cracked or corroded trap.

Disposal, dishwasher and sink strainer (kitchen)

A garbage disposal that leaks from the bottom has usually failed inside and needs replacing. Drips at the dishwasher hose or the sink strainer basket are often a loose connection or a worn seal.

What You Can Safely Do, and When to Call

Fine to try yourself: hand-snug a loose plastic slip nut on the P-trap, wipe everything dry, and keep a pan under it until it’s fixed.

Call us when:

• A shutoff valve won’t close, or leaks when you turn it
• A supply line is bulging, rusted or kinked
• The water seems to be coming from inside the wall behind the cabinet
• The cabinet floor is soft or stained, or the leak keeps coming back
• The faucet itself is leaking and you’re thinking about replacing it

If the water is coming from the wall or the floor rather than the parts under the sink, that’s a bigger leak. See our leak repair page.

What We Do

• Find the actual source, not just the wet spot
• Replace worn supply lines and old shutoff valves as needed, usually with new quarter-turn valves that are easy to close next time
• Reset or replace the P-trap and drain connections
• Repair the faucet, or replace the faucet, sink or disposal if that’s the better fix
• Run the water, check every connection, and make sure it stays dry before we leave

You get the price before any work starts. If we see other worn valves or lines nearby, we’ll point them out and you decide whether to do them now.

Under-Sink Leaks Across Sonoma County

We’re based in Cotati and work for homeowners. Serving Sonoma County. We fix kitchen, bathroom and laundry sink leaks in Cotati, Rohnert Park, Petaluma, Santa Rosa and the towns around them. Not sure if we come to you? See our service areas.

Under-Sink Leak FAQ

How do I stop a leak under my sink?

Turn the shutoff valves under the sink clockwise until they stop. If a valve won’t turn or starts leaking, shut off the water at your main instead. Then dry the cabinet, put a pan under the leak, and call or text (707) 596-3095.

Why does my sink only leak when I run the water?

If it only drips while water is going down the drain, the leak is usually on the drain side: a loose slip nut, a worn washer, or a cracked P-trap. If it drips all the time, it’s more likely a supply line, shutoff valve or the faucet.

When should supply lines be replaced?

Replace them if they’re rusted, kinked, bulging or frayed, or if they’re the original lines in an older home. Many homeowners have them replaced whenever a faucet or toilet gets replaced, so the new fixture isn’t hooked up to an old weak spot.

Should I replace the shutoff valve too?

If the valve is old, crusty, hard to turn, or doesn’t fully stop the water, yes. A shutoff that doesn’t work is no help in an emergency. We’ll tell you honestly whether yours needs it.

Do you offer same-day under-sink leak repair?

Yes, same-day service is available, depending on the schedule. Our regular hours are Monday through Friday, 7am to 5pm, and we take emergency calls. Call or text (707) 596-3095 and we’ll tell you when we can be there.

More answers on our FAQ page.

Other Ways We Help Around the House

• Tankless Water Heaters
• Water Heater Repair & Replacement
• Drain & Water Line Repair
• Fixture Replacement
• Leak Repair

See all plumbing services.

What Neighbors Say

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