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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, ...?

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  • Is this a single family home? Where (exactly) is the AC unit, and its condensate drain line? Find out where that fan exhausts to (and like that other question), inspect the louver to see that it's closing right. If it just dumps into the attic... well, that's another problem. - Shot in the dark: if you have a bathtub on the 2nd floor; fill it to the over flow and see what happens.
    – Mazura
    Commented Aug 17, 2016 at 0:45
  • @Mazura made a few edits. Stupid question: how to track the fan exhaust line if there's no exterior vent visible? I'd rule out the bath tub, as there was a separate issue half a year ago with a leaky diverter, that caused a different large wet spot downstairs, which is now dry.
    – zeeMonkeez
    Commented Aug 17, 2016 at 0:53
  • It could come out the roof somewhere. If you're not comfortable walking on the joists of your attic or your roof, you might need to call someone. The 'trail' begins in the bathroom; it must be traced from there. Pictures might help.
    – Mazura
    Commented Aug 17, 2016 at 1:03
  • Was it also very humid, or just hot?
    – Tester101
    Commented Aug 17, 2016 at 10:43

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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.

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