Open your ceiling and expect it to be open awhile. Start with a 12/12 hole and get a ‘jacuzzi access panel’ to cover the hole.

Inspection from below is your best best. I had a similar problem, it turned out that a plastic drain elbow had split on its upper side, it only leaked when a lot of water was dumped down the drain at once.

I think we identified the source of the leak but we don't know where exactly is the leak.

When upstairs neighbor runs kitchen faucet for at least 4 minutes, we notice water dripping from the ceiling.

Plumber is coming on Tuesday. Where could the leak be occurring for it to be traveling down the ceiling? Could it be plastic elbow as suggested by previous poster?

Thanks

The only truism about water is that it runs downhill. And because of capillary action, it can cling to a pipe and run downhill a LONG way before it drops. If you are lucky, the hole will show the root issue. But it could need to be enlarged all the way back to the source. This is an in person judgment call.

thank you @minardi trying to understand what are all the points of failure in a kitchen drain pipe. i will dig some kitchen draining diagrams from the internet.

There is water under the sink when it is being used. Two more days for the plumber but want to run any tests that I can, as a rookie.

Any pointers?

Plug the sink, add a couple of gallons of water, get ready to look for leaks, pull the plug.

Have a container ready to catch the drips.

In your first post you stated: I confirmed (with their tenants a couple of times and with the owner personally) that there is no leak either under the kitchen sink or on the floor.

In a later post you said: There is water under the sink when it is being used.

Did the story about the leak change?

@mike_home yes, it changed :) unfortunately. we did two tests on two different days and running kitchen sink resulted in water dripping from our ceiling.

we have plumber scheduled for next week but in the mean time (to be precise tonight) i will be visiting upstairs unit to see if it is something real obvious that is the actual source and which can be fixed easily. if not, we will just have to sit tight until plumber arrives.

any suggestions you have?

There is water IN the upstairs neighbors cabinet? Their under sink plumbing needs to be tightened/ sealed or might be shot. If the leak is between floors it could be anything depending on the age of the building systems.

Thanks @hallett.

There is water in the upstairs neighbors cabinet AND the water is dripping through our ceiling.

I will confirm this later tonight.

If you can see the source of the water dripping then putting a basin under the sink would be a tempoary solution. If not perhaps putting a towel under the sink to soak up the water.

Update: There is water dripping from the pipe (highlighted in blue) below. What is that line used for? Also, what is the purpose of that white washer that I pointed in blue?

Looks like a drain, white washer is a connector.

While waiting for the plumber, give a twist…usually, lefty loosely, righty tighty..

Put a pan/bowl under the drip.

Good work!

Thanks A Mat. What exactly do you mean by drain? Those other two pipes are supply lines going to faucet. So that drain pipe is coming from faucet?

Drain is coming from the sink.

We need a wider view for context.

I never imagined that it would be this difficult to see from underneath the sink. You need long hands, flexible body, good knees :-)

I will try to get a wider view this evening.

The picture you are seeing are from the faucet hole. So, it cannot be kitchen sink drain pipe, correct? Could it be the sprayer?

I will upload a better picture this evening

100% could be the sprayer. 95% are plastic junk.

hello all. if we experienced this issue (water dripping from plaster ceiling) on and off for the last few months whenever upstairs neighbor used faucet,

do I need to worry about 1) structural integrity 2) mold. Am I overthinking?

whom can I approach to rule out these two issues - handyman?