My story: Bought and just rehabbed a 1980's home. Noticed a small stain on the ceiling under the master bath before starting the reno. Figured it was probably an old stain as the ceiling seemed dry to the touch. Painted the entire house, including the ceiling. Didn't use primer, just sprayed on ceiling paint. Moved into house, started using the shower in the master bath right above the old ceiling stain. Few days later, noticed the stain returned. Thought there might be a leak in the shower drain, since it seemed to pop up around the same time I started showering. Called out a plumber, had the drain (which did seem to be in pretty deteriorated shape) replaced from above, through the shower pan. Also had a plug put into the shower pan to ensure the pan itself wasn't leaking, which it wasn't. But I'm noticing now that the stain area, which is consistently about 15x2 inches, seems to be a little bit damp/soft. But maybe it's just my imagination. Probably need to get a moisture meter to find out. The stain *does not* seem to be growing/expanding. It's been the exact same size, shape, and color for many days even after continuous showering and use from the bathroom above. I'm pretty sure it's the same size it was before I painted over it.

There's been no rain in my area for months, so no roof leak. No stains or evidence of water damage anywhere else except this stained area.

Here's my question, for you experienced pipe guys: if this was an active pipe leak of any kind, I'd see more evidence of a problem, right? I'm positive it's not a supply line leak, but would a drain pipe leak be more gradual and cause the stain to maybe stay moist but also stay the same size? My theory (and hope) is that I simply didn't prime the spot on the ceiling before I painted, so the stain returned.

It's all very speculative and annoying. Any help or advice is appreciated.

 

Prime it with BIN pigmented shellac and then repaint it. If it reappears, you didn't fix the leak.

Generally if you suspect plumbing is dripping you want to open the ceiling up by cutting a hole.

 

As X said, you need to prime the the stain with an oil or shellac based stain before painting or you could simply be seeing the old stain bleeding through the paint.

 

Yep, my plan was to prime and then repaint and see if it returns. I guess I was looking for some consolation that if this was indeed a pipe leak (of any kind) that I'd be seeing more evidence of damage progression, versus a stain that doesn't seem to be growing in size.

 

I think the consolation is that since you're not seeing it grow and you didn't seal the stain when you painted that the likelihood is the stain just bled through the paint.

 

I agree, if it was an active leak the size and darkness of the stain would grow. The drywall would also start to get soft. Most any oil base primer will hide mild water stains but the real bad ones need a pigmented shellac like Zinnser's BIN.