Likely cause: a leak where the drain seals to the sink

Why this happens

Delta says that when there is not enough silicone or putty between the flange and the sink, water leaks out of the sink into the drain even with the stopper fully closed. Delta adds that this water seeps into the drain through the overflow holes in the drain body, so the cabinet floor stays dry. Pfister describes a related leak under the sink, where the sink and the pop-up drain meet, caused by a washer that is missing, deformed or pinched, or by uneven putty. Pfister notes that many sinks, especially ceramic ones, have rough and uneven surfaces at that joint.

How to fix it

1. Has a chemical drain cleaner gone down this sink? Then do not open the drain. Family Handyman says leftover caustic chemicals are a danger to anyone who takes the trap apart. Call a plumber and say what you used. If not, open the stopper and let the sink empty. Clear the cabinet and put a bucket under the P-trap, as This Old House advises. Then close both stops under the sink so the faucet cannot be run while the drain is open. If a stop will not turn, do not force it. Leave the faucet off instead.
2. Confirm it with Delta's test. On a lift rod drain, remove the nut that holds the horizontal rod, pull the rod back and lift the stopper out. Then slide the rod back in and put the nut back on so that opening is closed. Pfister says to re-install the ball rod to keep water from leaking out there. On a push-to-close drain, take the stopper out from above. Open the stops again, plug the drain opening with a rubber stopper that seals the opening only and does not cover the outside of the flange, then fill the sink.
3. Read the result. If the level holds and nothing drips under the sink, the flange seal is fine and the pop-up stopper is what leaks. Pull the rubber stopper, let the sink empty and close both stops again before you open the drain to put the pop-up stopper back. Once the rod nut is snug, open the stops. Then clean the stopper and check its rubber seal with the hair and buildup result and the worn stopper seal result. If yours is a push-to-close stopper, use the push-to-close result instead. If the level still drops, or water still shows under the sink, the flange seal is leaking. Let the sink empty and close both stops again before you take the drain apart. If a stop will not turn, do not force it. Leave the faucet off instead.
4. For a drip under the sink, look at the washer above the large nut. Pfister says it should sit as an even ring, not pinched or pushed to one side. If it is pinched or off to one side, loosen the nut, re-center the washer and retighten with even pressure. Makers differ on how tight this nut goes, so follow the instructions for your drain, and keep in mind that This Old House warns over-tightening can crack the sink. If the flange above is set in putty, plan to reseal it too: Oatey says to reapply fresh putty whenever a fixture is disassembled or adjusted.
5. If you re-centered the washer, check your work: open the stops, plug the drain with the rubber stopper again, fill the sink and watch the joint. Pfister tests for leaks the same way. Then let the sink empty and close both stops again. If the joint now stays dry and the level holds, skip ahead to the step that puts the stopper back. If it still leaks, or the washer was never the problem, the flange needs resealing. Choose the sealant first. Delta says to read the warning labels on plumber's putty to make sure it will not stain marble or attack plastic sinks or rubber parts. Oatey says regular oil-based putty may stain porous materials such as granite and marble, and that its putty is not made for ABS plastic, where it says to use 100% silicone sealant. Oatey also says to review the fixture maker's instructions first, because some fixtures include a gasket or require silicone.
6. Now reseal the flange the way Delta describes. First make sure the sink is empty and both stops are closed. Then unscrew the large nut under the sink, pull the flange straight up through the sink (pry gently if it sticks), clean off the old sealant, apply a fresh, even bead of silicone or putty, then reinstall the flange and tighten the nut. The cleaning step is Oatey's: it says old putty can get in the way, and the surfaces should be clean and dry before fresh putty goes on. Bob Vila's walkthrough loosens the P-trap nuts first to make room. Drains come apart in different ways: Kohler's guide shows a flange that unscrews from the drain body. If yours will not come free, do not force it.
7. Do not rotate the flange or the drain body once it is tight. Delta says that can break the new seal. Kohler's installation guide also says to be careful not to rotate the drain while you tighten the nut.
8. With the stops still closed, put the stopper back the way it came out and snug the rod nut. If you loosened the P-trap, line it up again and tighten its nuts, as Bob Vila's walkthrough does. If you used silicone, wait before you test: Oatey says silicone has to cure according to its own instructions before it gets wet, while putty needs no cure time and can be tested right away. Then open the stops, fill the sink and check both the water level and the cabinet floor. Let the sink drain and check the trap nuts for drips too.
9. If the flange, drain body or gaskets are corroded or damaged, or the leak comes back, the next step is a whole new pop-up drain assembly, fitted by the installation sheet packed with it. You can do that yourself or hand it to a plumber. This Old House rates that job easy to moderate.

When to stop. Call a plumber if the large nut is rusted in place, if the flange or drain will not come free without force, if you have a pedestal sink, or if the leak continues after a reseal. Bob Vila warns that too much pressure can damage a porcelain sink. This Old House says a pedestal sink may have to come off the wall for this work, and a master plumber quoted by Family Handyman says a rusted pop-up has to be cut out with a hacksaw.

If you need the part

Only needed if the drain is corroded or damaged, or resealing does not stop the leak. Match the old drain before you order. Bob Vila says to check the product information to make sure the new assembly is compatible with your sink, and Pfister notes that its Push and Seal drain must go in a sink with an overflow, so check whether yours has one.

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