The short answer
A kitchen sink strainer can leak at the top flange, the large gasket and locknut below the bowl, or the tailpiece connection. Dry the assembly and test these seals separately before buying parts.
Find the first wet point
Empty the cabinet, place a tray below the drain, and dry the strainer body, locknut, and tailpiece. Then run two tests.
- Close the basket and hold a shallow amount of water in the sink. If moisture appears beneath the flange before release, suspect the flange seal or sink surface.
- Release the water and watch the locknut, body threads, tailpiece washer, and P-trap. Water that starts only during flow points lower in the assembly.
A paper towel wrapped around each dry joint can show where moisture begins. Follow the highest wet point; water tracks downward.
| Leak point | Check before replacement |
|---|---|
| Flange at sink | Cracked sink, movement, wrong or failed sealant |
| Large lower gasket | Centering, damage, reversed washer stack |
| Locknut/body | Corrosion, crossed threads, loose assembly |
| Tailpiece | Flanged washer orientation and nut alignment |
| P-trap | Slip washer, pipe alignment, cracked tubing |
Oatey’s guidance shows that strainer and tailpiece designs use different washers and connection styles. Match the installed parts and sink material rather than assuming a universal washer stack.
Repair boundary
Reseating the flange requires removing old sealant and following the strainer maker’s instructions for putty, silicone, or a supplied gasket. Do not use plumber’s putty on materials whose manufacturer prohibits it. Support the strainer so it does not rotate while the locknut is tightened.
Stop for a cracked sink, badly corroded metal, seized parts, or cabinet damage that hides the leak path. Test with a full bowl and normal drain flow before returning items to the cabinet.