Mysterious water stain on ceiling

My uncle's home has about a two foot long by three inch (what looks like)water stain on my family room ceiling. Directly above the stain is a double sink vanity. The sink traps and cabinetry appear to be bone dry, even with water running. The stain is recent and appeared quickly (over a couple hours).

How can I diagnose this?

House is pretty new - 14 years.

Sounds like it is either in the wall or the horizontal run, unless it could be leaking around the vent pipe. Did you have a lot of rain recently? The flashing around the vent pipe that likely goes up through the roof above the vanity could be the source. Unless you can isolate it to that area (can you get into the attic to see if any of the insulation is wet, or along the pipe? can you see through to outside?), then you'll have to tear out some drywall (hopefull, since it is easier to patch than plaster). Depends on if the ceiling is textured whether it is easier to do it behind the vanity or in the ceiling.

If you can rule out the flashing through the roof, then it's probably tear out wall or ceiling (or both, if the ceiling gets truely soaked, it will need to be replaced). Note, though, that water can flow a long ways...could be the tub or toilet as well. Is it a walk-in shower? Did one of the kids leave the doors or shower curtain open and spill a bunch of water on the floor?

dealing with water stains...

I have been told that the best way to deal with a leak affecting your ceiling is to drill a small hole in the ceiling and let the water drain out and catch it in a bucket.

This way the water stain won't spread all over your ceiling.

Of course, you still have to diagnose the problem, but this can minimize the ceiling damage.

If it rained quite a bit just prior to the water stain appearing, then I would concentrate my efforts on the exterior of the house. But if it did not rain, then please consider the following:

If the stain dries out and doesn't come back, I wouldn't worry about it--probably just an overflowed toilet or shower curtain that was not doing its job. But if the ceiling stays wet, I'd definitely poke a hole in it and let any water drain out that may have accumulated. Then I'd have someone go upstairs and flush the toilet a few times, then wait a few minutes and run the water in the bathtub, sink, etc.. I'd also check the plumbing behind the shower access panel. Also, check to see if any tiles are loose in your bathtub/shower, or if any grout or caulk is missing. If it is missing grout or caulk, pour water on that area to see if it is the source of the leak. Also, just because it is dry below your p-trap in the sink doesn't rule out the sink drain as a culprit. See if the tailpipe is loose that screws into the fitting against the wall. The drain could be leaking behind the cabinet or in the wall.

If there's a large leak, either in one of the drain pipes, valves, or supply lines, it will be easy to troubleshoot. For small leaks, this method may not work, since the drywall can soak up a lot of water before it becomes saturated and lets the water flow out. Also, leaks in copper pipes that are in the wall or ceiling are difficult to troubleshoot. In these cases, you may have to cut out a large section of drywall--but that should be your very last resort, since you'll obviously have to repair all the drywall you cut out.

Eric