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P-Trap Leak: Only Drips When You Drain — Which Is Why It Hides

What the trap does

That U-bend under the sink is not a design quirk — it holds a standing plug of water that seals sewer gas out of your house. Every drain has one. It also happens to be assembled from the loosest connections in your plumbing: slip joints with plastic washers, hand-tightened, taken apart every time someone retrieves a ring.

Which makes it the most commonly disturbed, most casually reassembled joint under the sink — and a steady producer of slow leaks.

Why trap leaks fool people

Three reasons this leak runs longer than it should:

1. It only leaks under flow. Check the pipes at random and they are dry. The leak exists for the ninety seconds a day the sink drains.
2. It often leaks under volume only. A trickle passes; a full basin released at once finds the bad washer. Rinse your hands for a week: dry. Drain the pasta pot: drip.
3. The evidence lands where nobody looks — the back half of a cabinet floor behind the cleaning supplies.

The confirm-it test

1. Towel-dry the trap, slip nuts, and tailpiece.
2. Paper towels flat under the whole assembly.
3. Fill the basin high. Release all at once.
4. Watch the joints and check the towels.
5. Repeat with hot water — heat expands a marginal joint.

Wet during drain-down: trap side. Wet at other times: faucet or supply side — different problem, more urgent, because supply is pressurized.

Fixing it

Most trap leaks are one of these, in order of likelihood:

• Loose slip nut — hand-snug plus at most a quarter turn with pliers. Over-tightening cracks plastic nuts and deforms washers; it is the most common way a "fix" creates the next leak.
• Worn or misseated washer — the beveled washers cost pennies; note their orientation (bevel toward the joint) when replacing.
• Cracked trap body — hairline cracks in old PVC or, on chrome-plated metal traps, corrosion from the inside out. Metal traps that show green/white crust or flake when touched are done: replace with a PVC kit, a few dollars and fifteen minutes.
• Misalignment — if the trap only seals when forced into position, the tailpiece and waste arm are out of line; correcting the geometry beats forever re-tightening the symptom.

No shut-off needed for trap work — just don't run the faucet while it's apart.

The part people skip: the cabinet

The repair takes fifteen minutes. Then comes the question that actually matters: how long was it dripping before you caught it?

Empty the cabinet completely and inspect the floor, especially the back corners: swelling, delamination, dark staining, mold spotting on the underside (lift the front edge if it's loose), and the wall behind the cabinet. Gray water plus enclosed space plus Houston humidity is the standard recipe — cabinet interiors are among the most common places we find established mold behind a "minor" leak.

Looks rough, smells musty, or the particleboard has gone soft? Get a reading before restocking the cabinet: (346) 385-3496, a photo is enough to tell you which way it goes.

Prevention

• After any trap disassembly (retrieving jewelry, clearing a clog), run the full-basin test before closing the cabinet
• Replace washers whenever the trap is opened — they are pennies, and reused washers are the top cause of the next leak
• Include one dry-fingered touch of the trap in your twice-yearly walkthrough
• A water alarm on the cabinet floor covers the trap, the supply lines, and the faucet base all at once