On the kitchen ceiling in our kitchen, I had noticed in the last year or two that the paint was bubbling in a spot but I wasn't sure what could have caused it.

This spot has slowly but progressively gotten worse to where now the paint has really bubbled and cracked and some has come off. We were standing in the kitchen one time when it was raining heavily outside (and had been all day) and noticed water running down the wall next to this spot where it was coming through the ceiling. There was some spackle exposed by the bubbling paint and it was wet.

The kicker is, this is on the first floor of our 2 story house. I can't find anywhere water has been leaking on the second floor directly above this spot (which is a bathroom). The bathroom has a sink and toilet against the wall directly above where this is leaking and a plumbing vent that exits the roof above there. The only thing I can figure out is that water is running down the plumbing vent and dripping down to the spot where we are seeing the bubbling paint.

However- It only seems to happen when it's been raining for an extended period of time. I have checked it almost every time it has been raining and rarely has there been any moisture. We rarely use the upstairs bathroom so I am not led to believe anything is leaking up there. I went on the roof to this particular vent and everything appears to be in good shape, I can't even see where it would be leaking. I'm not sure what to do.

 

It could take quite a while for rain to leak and come out in a certain location.
Is there an attic you can look in ?

 

No...there is space above the second floor, with a tiny access panel in one of the closets near the bathroom (like the access panel might be big enough to stick your head through), but it's not really accessible at all. I'm not even sure why the access panel is there.

 

I have seen situations like that several times. Worst was a very nice 3 story home and the ceiling light fixture in the kitchen on the 1st floor side wing of the house would fill with water during heavy rains. Turned out to be wind blown water coming in the ridge vent on the main 3 story part of the house. The water ran down a truss in the attic, down inside the wall cavity, then across the sheetrock out into the single story part of the house until it found the hole for the light fixture.

You're just going to have to play investigator. You may find that the leak only exists when it rains with wind from a particular direction. Or, it may be a small leak that doesn't make it all the way to the kitchen with shorter periods of rain but a long duration heavy rain allows time for the water to make it somewhere visible.

 

The rubber roof boot on the vent pipe is often the culprit.