A puddle in the cabinet under your sink is a common leak — and usually a fixable one. Shut off the little valve under there, and let’s figure out what’s leaking.
Written bySteven Lockhart·Reviewed byMiguel Lopez, Owner & Licensed Plumber, New Pro Plumbing (CSLB #974600)·Last reviewedAugust 2026 · 7 min readQuick Answer
A leaking pipe under a sink usually comes from the supply-line connections, the P-trap, or the drain slip joints. First, shut off the small angle-stop valves under the sink to stop the water. Many under-sink leaks are simple fixes — a worn washer, a loose connection, or a corroded trap. New Pro repairs them fast across West LA.
The cabinet under a sink holds several things that can leak: the flexible supply lines and their connections, the shut-off (angle-stop) valves, the P-trap that curves below the drain, and the slip-joint connections of the drain assembly. A leak from any of them ends up as the same puddle — so the job is identifying which one, and why.
Leaking pipe under sink — Leaking pipe under sink repair is the diagnosis and repair of a leak in the supply lines, shut-off valves, P-trap, or drain connections beneath a sink, after isolating the water at the under-sink valves.
Most under-sink leaks are minor and quickly fixed, but because they’re often slow and hidden behind stored items, they can rot a cabinet before you notice — which is when people call an emergency plumber.
A leak only when water runs is usually the drain or trap; a constant drip is usually a supply line or valve.
If the shut-off valve itself fails or won’t close, treat it like a burst and shut off the main.
Stop the water, clear the cabinet, and let us pinpoint it.
Close the small valves on the supply lines under the sink.
Remove stored items so nothing else gets water-damaged.
Dry it, then run water to see where the leak reappears.
Constant drip versus only-when-running points to the source.
We identify and repair the exact failure.
We flag any water damage or mold to address.
Water beyond the cabinet? If it’s spread to the floor or a ceiling below, the leak may be bigger than the trap — see ceiling leaks or, on a slab, slab leaks.
Cabinet soaked or the valve won’t close? Call and we’ll sort it fast.
Get a Free QuoteIt’s easy to shrug off a little water under the sink, especially when a quick tighten seems to stop it. But the cabinet under a sink is one of the worst places for a slow leak to hide: it’s enclosed, it’s dark, and it’s usually packed with stored items that soak up water and conceal the puddle. By the time the smell or the warped cabinet floor gives it away, the leak has often been quietly working for weeks.
That’s why the goal is to fix the cause, not the symptom. Tightening a corroded slip nut might stop the drip for a day, but a failing part will leak again, and each cycle adds to the water damage. Replacing the worn washer, trap, or valve properly ends it. The U.S. EPA’s WaterSense program points out that a startling share of household water is lost to small, fixable leaks exactly like these — which is why even a minor under-sink drip is worth sealing correctly rather than living with.
None of this is dramatic work, but it’s still done right. We operate under a license you can verify with the CSLB (#974600), and for the rare under-sink issue that turns into something larger — a failed valve feeding a bigger repair — we handle any permitting through the Los Angeles Department of Building & Safety. Mostly, though, this is the satisfying kind of call: a small, fast fix that saves a cabinet.
Some under-sink leaks are a two-minute fix; others mean a failing part.
| Cause | Fix | Note |
|---|---|---|
| Loose connection | Re-seat and tighten | Quick fix |
| Worn washer / gasket | Replace the seal | Inexpensive |
| Corroded trap / valve | Replace the part | Prevents recurrence |
Whatever it is, we fix the cause rather than just tightening it — flat-rate, like 24/7 emergency plumbing should be.
Under-sink leaks are usually among the more affordable plumbing repairs, since the parts — traps, washers, supply lines, angle stops — are inexpensive and accessible. Cost mainly depends on which part failed and whether corrosion has spread. We diagnose the exact source and quote one flat rate, so a simple job stays a simple price.
No hourly clock, even on small jobs — see how our emergency plumbing service prices flat-rate.
A homeowner finds the cabinet under the kitchen sink damp and the stored cleaners sitting in water.
New Pro finds a corroded P-trap slip joint, replaces the trap assembly, and checks the supply connections.
The leak's gone, the new trap won't corrode for years, and the homeowner catches the damp cabinet before it rotted through.
The Bottom Line
A leak under the sink is one of the most common and most fixable plumbing problems — but a slow one hidden behind the dish soap can quietly ruin a cabinet. Shut off the little valves, clear the space, and get the exact source fixed rather than just snugging a nut and hoping.
The usual culprits are the P-trap (the curved pipe below the drain), the slip-joint connections of the drain assembly, the flexible supply lines and their fittings, or the shut-off valves. Leaks happen when a connection loosens, a washer or gasket wears out, or a metal trap or valve corrodes through. Identifying which of these is leaking — and whether it’s a loose fit or a failed part — determines whether it’s a quick reseat or a part replacement.
Turn off the two small angle-stop valves on the supply lines under the sink — one for hot, one for cold — by turning them clockwise. That isolates the water to that sink without shutting off your whole house. Then clear the cabinet of stored items, dry it out, and place a container under the leak. If the angle-stop valve itself is the leak or won’t close, shut off your main water valve instead and call us.
A single small leak usually isn’t an emergency, but it shouldn’t be ignored. Because these leaks are often slow and hidden behind stored items, they can go unnoticed long enough to rot the cabinet base, damage the flooring, and grow mold in the enclosed space. Catching and fixing it early keeps a minor plumbing repair from turning into a cabinet-and-floor repair, so it’s worth addressing sooner rather than later.
Some under-sink leaks are genuinely simple — hand-tightening a loose slip nut or replacing a cheap washer can solve a minor one. But if the trap or valve is corroded, the threads are stripped, or the leak returns after tightening, the part needs proper replacement, and forcing a corroded fitting can make it worse. If a quick tighten doesn’t hold, it’s worth having us replace the failed component so it’s sealed correctly and won’t leak again.
Timing is the tell. A supply-line or valve leak drips constantly, because those lines are always under pressure, even when the faucet is off. A drain or P-trap leak only appears when you actually run water down the sink, because the drain only carries water then. Noticing whether the leak is constant or only-when-running helps us — and you — narrow down which part is failing before we even open the cabinet.
Written by the New Pro Plumbing content team and reviewed for technical accuracy by Miguel Lopez, owner and California-licensed plumber (CSLB #974600). Guidance follows U.S. EPA, CSLB, and LADBS sources where cited. Any figures reflect New Pro Plumbing's flat-rate practice and West LA conditions, not a quote — we confirm pricing in writing before any work begins.
Last reviewed: August 2026.
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