Diagnosing a possible drain leak


A few months ago I noticed a small water stain appearing where a plaster crown molding meets the ceiling. Recently I noticed a dripping noise after I shut off a sink in a bathroom that's more or less above the stain. I might not have noticed it sooner because there's usually a lot of noise from the bathroom vent fan.
Before I call a plumber or have the ceiling or wall ripped open, I'd like to find out if the drip actually indicates a leak, and not just normal water draining through the drain pipe. If I run the sink for 10 seconds, then shut it off, the dripping starts about 15 seconds later, and continues for about another 15 seconds, at a rate of about 3 drips per second. I can hear it from the bathroom as well as from the downstairs room. The plumbing is about 20 years old, in a house that's much older.
Are there any tricks to find out what's going on?
Jimmy
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snipped-for-privacy@mailinator.com wrote:

If the 20 yr old plumbing was done in PVC, dripping noises are common and may not indicate a leak.
A stud finder can be useful in locating wet plaster since it appears denser than the surrounding dry plaster. Try it.
An Ohmmeter might be used with a pair of probes to detect areas of lower than normal resistance. Takes some experimenting.
Touching the suspect area with the back of the hand can sometimes show up moist spots from the feel of it.
Jim
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snipped-for-privacy@mailinator.com wrote:

The water stain indicates a leak.
Whether it's actually the drips hitting the ceiling or inside the drain is of little consequence; the leak needs fixing.
As another poster said, investigate for dampness in the plaster and the ceiling area. If it is drain rather than supply, not using the suspect sink for a while should allow it to dry completely whereas a supply line leak would continue.
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The thing is the dark area on the ceiling is so small and slow-growing that I'm not even sure it's a leak. It's maybe 6 inches long and a quarter inch wide, along the edge where the ceiling meets the molding.

How long should I avoid using the sink to wait for it to dry?
I think the drain pipes are PVC, but if they're copper, does that mean a dripping noise definitely indicates a leak?
Jimmy
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snipped-for-privacy@mailinator.com wrote:

What else do you think it might be? Even if it's mold/mildew that would be indicative it's damp.
Get up there and see if the spot is damp to touch, slightly softened if actually plaster, etc., ...
The drip may or may not be inside the pipe; there's no way to tell from here as I can neither see nor hear it to judge.
If it's actually the water dripping on the upper side of the ceiling it may be possible to tell that by being close and listening carefully--the old mechanic's trick of the screwdriver touching the wall and using it as a stethoscope might help locate it if so.
Drips inside drains can occur w/ either Cu or PVC. If it's been a long time in the house and never noticed it at all until recently, it's likely a new symptom.
How long before the area dries out if damp would depend on many factors but I'd think a week would make a difference that should be discernible at most if it's significant at all...
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