We get called by building owners to help figure out the source of damaging water leaks that appear on ceilings or walls. When dealing with dripping water leaks the first step in triage is to ask if it happens after a rain or if it happens only when it’s below freezing. The second question is typically how long it has been happening for, if there any renovations in the area and how long the client has been experiencing the issue. Turns out this leak was a bit different and completely unexpected.
I was at the house in the summer time and the client talked about water coming through the ceiling drywall under this plumbing stack:

I suggested to the client that he have a roofer check for step flashing along the wall and that perhaps there was water getting around the cap through shingles.
Given I was there for an energy audit and there was no water coming through the ceiling at the time, I chalked it up to a flashing issues because the client couldn’t say definitively that is was a winter only issue; they felt the leak was taking place year round. I asked them to take notes on when the leak happened more intensely.
Nine months later, and the client had the roof replaced yet the leak was still happening this January, so I went over to investigate. It was above zero and raining last week, but on inspection day it was -11C and very cold. The client said the leak was dripping under both conditions. I checked on the work outside:
Uphill from the leak was the natural gas furnace and hot water heater and there was plenty of condensation happening. Was this my water source? It couldn’t be discounted.

The furnace and hot water heater were co-vented through this chimney and a good amount of condensation and ice buildup was taking place here. Was this the source of my water leak?

I’ve never seen a rain cap on a plumbing stack, but that was the roofer’s idea. Again, like the chimney up-hill, a lot of condensation happening and dripping to the roof deck around the base forming ice. Was this the source of my water leak?

I’m not sure why there’s clear silicone, but I think the roofer had been warned about the problem and was taking no chances. The ice on the shingles was from the condensation taking place under the rain cap on the plumbing stack above.
Inside, the offending area finally had the drywall cut out to see what was going on. The back of the drywall had a two areas of distinct water marks showing there was more than one leak but both were under the plumbing stack:

With the ceiling drywall cut out to expose the floor joist cavity, we could see the 3″ black ABS pipe that goes form the wall cavity below on a 45 up and through the plywood sub-floor of the knee-wall above. In the bottom lower right hand corner, you can see a well worn hole through the drywall where years of constant dripping etched a hole through the drywall.
For the denouement, check out the video below!