The majority of under-sink pipe leaks come from one of three sources: a loose slip joint nut where the drain pipe connects to another pipe segment, a worn or compressed rubber gasket inside one of those connections, or a cracked P-trap. All three are fixable by hand or with a pair of slip-joint pliers in under 20 minutes.
Finding the Source
Dry everything under the sink thoroughly with a towel. Place a sheet of dry paper towel under the pipes. Run water in the sink for 30 seconds and watch where the moisture first appears on the paper towel, this tells you which connection is leaking without guessing. Feel each connection with dry fingers while the water runs to identify dripping versus condensation.
Fixing a Loose Slip Joint Nut
The white plastic nuts that connect sections of drain pipe together are slip joint nuts. They can loosen over time from vibration and temperature changes. Tighten them firmly by hand first. If the leak persists, use slip-joint pliers for a quarter turn more, do not over-tighten plastic nuts as they crack. If tightening does not stop the leak, the gasket inside has compressed and needs replacement.
Replacing a Drain Gasket
Turn off the water supply to the sink using the shutoff valves underneath. Place a bucket under the P-trap, the curved section of pipe. Unscrew the slip joint nuts by hand or with pliers. Slide the pipe sections apart and remove the old gasket, it is the rubber ring visible inside the connection point. Take it to a hardware store to match the size, or bring the pipe section. New gaskets cost $1 to $3. Press the new gasket into the connection, reassemble, and tighten. Run water and check for leaks.
When It Is the P-Trap
A cracked P-trap, visible as a physical crack in the curved plastic section, needs full replacement. P-trap kits cost $8 to $15 at any hardware store and come with new gaskets included. Replacement takes the same process as gasket replacement with one additional step of disconnecting the P-trap from the wall drain outlet. Mark the positions before disassembly. Slip-joint pliers are available on Amazon if you do not have them. For the full home maintenance framework, the home maintenance checklist covers annual plumbing checks. The beginner home tool kit guide covers what tools are worth having on hand. The Broke Mom Home Reset ($17) covers the full home management system.
