I've got a leak under the kitchen sink but can't figure out where it's coming from. I used the sink all day, hot and cold water, the sprayer attachment, and there didn't appear to be a leak. Left the sink alone all night, then checked in the morning, and there was a pool of water on the cabinet floor. What could cause this and how do I fix it?

Usually water is directly under neath the leaking connection. It can be just about any joint, drain or pressurized, under the sink. So first, look directly above the wet spot and tell us what's there...

It could be a very small leak from the hot or cold line that connects to the faucet from underneath. Take a piece of paper towel and try to dry off ONE supply hose at a time then look to see if you have water on the paper. If so that hose line might be the problem. Water tends to make a small water line along the length of the supply line. Also it will ALMOST always be the connection area where the leak is coming from.

If no water is found while doing the paper dry trick then try this.

Plug the sink drain, fill half full. Have the sink base doors open. Remove the drain plug and watch from underneath the sink as the water drains through the pipes. If the leak is their it will start to flow out of one of the p-trap fittings.

If this does not bring about a leak after doing then a few times... then it must be the connection lines. THey are under pressure so just sticking around and watching what is going on onder the sink SHOULD reveal something.

Good luck.

Tonight try wrapping the pipes with toilet paper abd check periodically. Wrap the drain and trap too because that is what I suspect is slowly dripping during the night.

Do you have a dishwasher? Did you set it to run after you left the kitchen?

I'll try wrapping the pipes tonight and see what happens. What is making me crazy is it only happens at night. All day long I use the sink normally and there is no leak. But in the morning I find a small puddle. I haven't been using the dishwasher at night so that's definitely not it. If it was even a small leak, wouldn't there be some evidence of it during the day? Why only over night?

What's above kitchen?

There's nothing above the kitchen.

Just a side thought - when you say it is happening at night

How cold are the pipes getting at night - are they contracting slightly and opening up a small leak?

It has been a bit cold here at night, but not freezing. If that's the problem--the pipes contracting and opening up a small leak--what's the fix?

Im no expert here, but I had a similar problem when I lived in a rented cottage when I was young, and the sealant around the joins of the pipes was degrading and allowing a very small leak to appear

I don't know if this is the problem, but it just sounds similar to something I have seen before

Sometimes, suggest for fault finding is to strip all the plumbing down and then resinstall it, so all the joints are resealed etc

Another thought... Are you connected to the town water system? If so their might be a chance that your pressure regulator is not working properly. The town water is anywhere from 80 to 120 psi. The pressur eregulator reduces that to 40 - 60 psi whne iu comes into your house. If the regulaot goes out the weakest valves in your house, be it any number of faucets, the toilet, etc will discharge water now and then as the house water pressure builds up. It wants to release and these valver tend to be the erea for this release. Most homes use a thermal expansion tank to aid in this sudden high pressure. So just something to think about.

But I HOPE that the paper wrapped around the pipe reveals something. :)

I would not worry about pipes shrinking at night and causing a leak. Let us know what gets wet on the TP by the AM.

You can also put newspaper near the area and a paper towell where you see the most water. The drip usually shows up quickly on the towell.

Okay, so the toilet paper trick seems to have done it: the paper was wet around the hot water connection. I guess this means I need to take that apart and reconnect it? Maybe switch out the connector just to be safe? Still, why would this only happen at night?

Pressure/temperature variations.

The compression fittings can crack. Inspect them for cracks.

Before doing too much work try just tightening the nut just a hair, if it moves it may seal. Why only at night, only a guess if you are on city water, pressure goes up a bit at night when the usage is down, only a guess.