lagman wrote:
I noticed a water bubble on my bedroom ceiling, so I went in the attic
to investigate. It turns out a pipe that vents to the roof had become
disconnected at the first joint, so I just re-connected it. I assume
when it rained hard enough, rain water was leaking down the pipe and
on to the ceiling (we just had a hard rain).

But that got me thinking.. Even if the pipe didn't become
disconnected, should rain water still be leaking down the pipe?

Thanks,
Dan

Why not? The pipe is typically square cut. It doesn't take much water to
stain a ceiling.