Sink leaking underneath (water appears under cabinet)

Use only checks that match the exact appliance or system and its official guide.

1. Check 1

   Remove items from the cabinet, dry the pooled water, place a tray or towel below the plumbing, and keep hands away from wet electrical devices while checking whether water continues with the faucet and drain unused.

   Expected result: Water continues to appear with the faucet unused, a supply hose or valve sprays or drips, water is near an electrical device, or the area stays dry until the sink is used.

   Next step: If water is active, near electricity, or cannot be stopped by a normally moving fixture valve, stop using the sink and arrange urgent plumbing or electrical help. Otherwise continue with a controlled source-tracing test.

   Why this matters

   Why: Continuous moisture with no use supports a pressurized supply or faucet leak. Water near electricity or an active spray raises immediate safety and damage risk. A dry cabinet until use makes a drain-side leak more likely.

2. Check 2

   With the faucet off, wrap a dry paper towel around the accessible shutoff valves, supply hose ends, faucet shanks, and connections below the basin; wait briefly and inspect which towel becomes wet first without loosening a connection.

   Expected result: Moisture begins at a shutoff handle or body, a hose or threaded end, the faucet shank, or no supply-side point becomes wet.

   Next step: Do not force or overtighten a corroded valve or pressurized fitting. Contain the drip, keep the cabinet dry, and arrange plumbing service for a confirmed supply-side leak or any connection that cannot be safely isolated.

   Why this matters

   Why: A wet valve body or handle supports a failed shutoff seal; a wet hose end or faucet shank supports a supply connection or hose leak. Dry supply points shift attention to the drain, trap, or water entering from above.

3. Check 3

   After drying the area, run a small, controlled amount of water into the sink while watching the drain body, tailpiece, P-trap joints, disposal or dishwasher connection if present, and wall pipe with a flashlight.

   Expected result: Water appears only during draining at a particular joint, the P-trap fills or drips, the wall connection becomes wet, or all visible drain joints stay dry.

   Next step: Stop using the sink if a drain joint leaks or the wall connection is wet. Do not dismantle a corroded or glued trap; arrange plumbing service and leave the area accessible for inspection.

   Why this matters

   Why: A use-dependent drip at a drain joint supports a waste-side seal or fitting leak. A wet wall connection or clear visible trap with continued water suggests a concealed or downstream defect. Dry visible joints shift attention to the flange, overflow, sink body, or a supply leak that was not reproduced.

4. Check 4

   With the drain closed and the cabinet dry, add a shallow amount of clean water to the basin and inspect beneath the drain flange and overflow area; do not rotate the drain body or cover its exterior with sealant.

   Expected result: Water appears under the flange while the basin is holding water, water emerges near an overflow opening, or the basin test stays dry until water is sent down the drain.

   Next step: Stop the basin test and arrange plumbing service for a failed flange or overflow seal, especially if the drain body moves or the sink surface is damaged. Continue only with a small drain-flow observation if no water is reaching the cabinet.

   Why this matters

   Why: A basin-only leak supports a drain-flange or overflow-seal problem rather than a supply leak. A leak only during drainage points more toward the waste piping, while a dry test does not rule out a leak at a joint under flow.

5. Check 5

   If the visible fittings are intact, inspect the sink underside, cabinet floor, nearby wall, dishwasher or disposal connections, and the countertop or faucet deck for an upper or adjacent water source while the sink is used normally but briefly.

   Expected result: Water starts above the drain, travels from the faucet base or sink rim, appears at an appliance connection, or cannot be traced to an accessible point.

   Next step: Stop using the affected appliance or sink when the source is active and photograph the trail. Have a plumber or appropriate appliance professional isolate the source; do not caulk over a wet seam or open concealed walls as a first response.

   Why this matters

   Why: A faucet-deck, sink-rim, appliance, or adjacent source can run into the cabinet and mimic a drain leak. An untraceable trail may involve concealed piping or water trapped beneath the sink rather than the lowest visible puddle.

6. Check 6

   After a confirmed repair, run a small amount of water through the sink and check every accessible supply and drain connection, then dry the cabinet and inspect the floor, stored contents, and wall for returning moisture or damage.

   Expected result: No new water appears and materials dry normally; a joint still drips; the cabinet floor is swollen or stained; or musty odor, mold, sewage, or contaminated water is present.

   Next step: Keep the sink out of service if moisture returns or contamination is present. Arrange follow-up plumbing and, when needed, water-damage or mold cleanup; do not paint, caulk, or close the cabinet while materials remain wet.

   Why this matters

   Why: Dry connections and a dry cabinet support a successful repair. A continuing drip means the leak source or seal remains unresolved. Swelling, mold, sewage, or persistent wet materials require building or cleanup attention in addition to plumbing work.