2. A Failing Sink Strainer or Loose Sink Flange
The sink strainer is the metal assembly fitted into the drain hole at the bottom of your sink basin. It relies on a thick ring of plumber’s putty or a specialized rubber gasket sandwiched between the metal flange and the sink surface to form a completely watertight seal.
When you wash heavy pots and pans or fill the sink with heavy water loads, the structural bond between the sink and the strainer can flex. Over years of use, plumber’s putty dries out, crumbles, and washes away. When this happens, water escapes around the top perimeter of the drain opening, dripping down the exterior of the tailpiece pipe and collecting quietly on the cabinet floor below.
To test for a compromised sink strainer:
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Plug the sink drain with a stopper and fill the basin with a few inches of water.
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Examine the underside of the sink basin using a flashlight.
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Check the area where the metal drain assembly meets the underside of the sink bowl for active droplets or moisture lines.
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If water leaks while the sink is plugged, the upper seal is failing. Disassemble the strainer locknut under the sink, clean off the old dried putty, apply a fresh bead of plumber’s putty beneath the top flange, and firmly tighten the assembly back into place.
3. Leaking Pressurized Water Supply Lines or Shut-Off Valves
Unlike drain pipe leaks that only produce water when you actively run the sink, pressurized supply lines carry continuous water pressure. A leak in a hot or cold supply line, or at the shut-off valves mounted to the wall, will leak continuously—even when the faucet is turned completely off.
These lines are typically made of braided stainless steel or flexible plastic connected to copper or PEX tubing. Common failure points include deteriorating rubber washers inside the compression fittings, corroded valve stems, or tiny pinhole leaks caused by friction against the cabinet wall.
To check your supply lines safely:
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Carefully inspect the angle stop shut-off valves coming out of the back wall or cabinet floor.
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Run a dry tissue along the length of both the hot and cold braided supply hoses, paying close attention to the metal compression nuts at both top and bottom connections.
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Look for green or white mineral crust (calcium deposits) around the valve stems, which indicates a slow, ongoing pinhole leak that evaporates before creating large puddles.
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If a compression nut is damp, use two wrenches (one to hold the valve body stable and one to turn the nut) to gently tighten the connection a quarter-turn. If the hose itself is corroding or cracked, turn off the main water valve and replace the supply hose entirely.
