I have a problem similar to this: Downstairs half bathroom, ceiling exhaust fan dripping, first noticed during very hot weather. However, the amount of water makes me think it is not condensation.
Also, the downstairs doesn't have AC, just the upstairs, so the temperature difference would be smaller.
The dripping is not constant, but so far I haven't found a pattern. I'm not sure where the fan vents to, probably the soffit.
From attic crawl space I cannot see along the joists between which the fan sits (hidden by chimney, which itself is dry), but the neighboring joists & ceiling appear dry. The fan housing itself can't be removed without being destroyed, so investigation around the fan will involve removal of drywall.
There is an upstairs bathroom nearby, but neither the sink nor the toilet appear leaky.
Single family home ('78), the second floor was built on in the nineties. AC unit sits in attic, pipes run in exterior wall far away from issue.
What would be the best line to investigate, and should we get a plumber, handyman, …?
Best Answer
Turns out this was not a plumbing issue, even though all signs pointed to it: related to bathroom use (even though with long delay), traces of moisture in hardwood floors tracking from upstairs bathroom wall to leak, 'bathroom' smell.
In fact someone installed the drip pan under the AC in the attic at an angle, so condensate leaked, soaked the insulation, tracked along second floor ceiling to return air register, along that vent down the wall onto the subfloor... There it found a convenient knot, dripped on straps, and found its way to the fan.
Fortunately we haven't used the AC too much this year, so the joists are fine and hopefully the hardwood floors will dry, too.
Lesson learned: you may want to think x is your problem, when really it's y.