Introduction
A leak under the sink is a problem that demands immediate attention — left unaddressed even for a short time, it can cause water damage to cabinets, subfloors, and in NYC apartments, to the unit below yours. The good news is that most under-sink leaks come from a limited number of sources: loose compression fittings, worn-out washers, cracked P-trap connections, or a failed supply line. Identifying the exact source of the leak is the first step, and the repair is usually well within DIY territory once you know what you are dealing with.
Step-by-Step Instructions
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Step 1 Identify the Leak Source
Clear out everything from under the sink and lay down dry paper towels or newspaper across the cabinet floor. Run the water and watch carefully to trace where the drip originates. The three main areas to check are: the supply line connections (where water comes in from the wall), the drain connections (the P-trap assembly below the drain), and the faucet base or sprayer hose above. Wipe all connections dry with a paper towel, then run the water and feel each joint with your fingers. Water travels along pipes, so trace the moisture upward to find the true origin.
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Step 2 Fix Supply Line Leaks
If the leak is at a supply line connection, try tightening the compression nut with an adjustable wrench — give it a quarter turn clockwise. Do not over-tighten, as this can crack the fitting or strip the threads. If tightening does not stop the leak, turn off the water supply, disconnect the line, and inspect the washer or ferrule inside the fitting. Replace any worn or misshapen washers. If the supply line itself (the flexible braided hose connecting the wall valve to the faucet) is leaking along its length, replace the entire line — they are inexpensive and available at any hardware store.
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Step 3 Fix P-Trap and Drain Leaks
The P-trap is the curved pipe section that holds water to block sewer gases. If it is leaking, hand-tighten the slip-joint nuts at each end of the trap. If hand-tightening does not help, use slip-joint pliers to gently snug them — these are plastic on many sinks and crack easily if over-tightened. Inspect the nylon washers inside each slip joint — they flatten and harden over time, losing their seal. Replace them with new washers of the same size. If the P-trap itself is cracked (common with old chrome or plastic traps), replace the entire trap assembly.
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Step 4 Address Faucet Base Leaks
If water is seeping from the base of the faucet and dripping down through the mounting hole, the O-rings or cartridge inside the faucet body are likely worn. This requires disassembling the faucet from above — remove the handle, unscrew the retaining nut, and pull out the cartridge or stem. Replace the O-rings or the entire cartridge. While you have the faucet apart, inspect the mounting gasket between the faucet base plate and the sink surface — a deteriorated gasket lets water seep through the mounting holes.
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Step 5 Test and Monitor
After making repairs, run water at full pressure for several minutes while watching every connection. Then let the water sit (fill the sink and drain it) to test the drain side. Dry all connections completely with a paper towel and check again after 30 minutes. Place a dry paper towel under the repaired area and check it the next day — slow leaks may not be immediately visible. In NYC apartments, even a small persistent drip can cause cumulative damage to the unit below, so it is worth being thorough with post-repair checks.
When to Call a Professional
Call a plumber if the leak is coming from inside the wall (behind the shut-off valves), if the shut-off valves themselves are leaking or will not close, or if the pipes are badly corroded and crumble when you try to work on them — a common issue in older NYC buildings with original galvanized plumbing. Also call a pro if you have already caused water damage to the unit below, as your building management will likely require a licensed plumber's assessment. In co-ops and condos, you may be liable for damage to neighboring units caused by plumbing failures in your apartment.
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