Above my kitchen is a full bath that has a tub on one side, two sinks and a toilet on the other. Starting a couple days ago it started dripping from the kitchen cabinets that are approximately beneath the tub. Since my 14-year old daughter was taking a shower, I figured that she just didn't close the curtain all the way. After letting things dry until the weekend, I decided to try to narrow down the leak. I filled and drained each sink several time with no problems. Then I flushed and flushed and flushed the toilet (6 times) with no problems. After waiting about an hour, I decided to turn on the tub and let it run free for 10 minutes. No problem. I used both the spout and shower head to fill and verified that it was a caulk or tile grout issue. So, I filled the tub half way and while it was draining, I flushed the toilet. Everything seemed to work OK, but the dripping came back. I had problems with the toilet years ago, when I discovered that it needed extension rings to match the height of the ceramic tile. These fixed it, but at that time, it was dripping in a different place, directly below the toilet. So, I figured that it must be the tub's p-trap. Cut a hole in the kitchen ceiling and found that all of the piping was completely dry. I can also get to the back of the tub from upstairs and everything was dry there too. In fact, I could see the entire 5' lateral run to the exterior wall, which was dry. The cabinets were one joist bay over, which I didn't open.
So, before I do more damage to the kitch ceiling, where might the leak be? If it is the toilet, the water is making an odd trip to get to the kitchen. I went ahead and cleaned all the pipes with lye to ensure that all of my daughter's hair was out of them, which made everything drain faster, but hasn't changed the leak. (No, I don't use lye that often and don't leave it in the pipes so that it can eat away the bottom of the pipes.) My fear is that there is some sort of decayed pipe where the lateral meets the stack, but that would be a royal pain to get to. I would pull the toilet, but I just don't know how that could be an issue since it isn't leaking at the toilet or underneath it.