OK, this one has been driving me nuts for a while now.

I have a shower/tub on the second floor that leaks down on to the first floor. It only happens with showers, not baths. It takes about 5-10 minutes from the start of the shower for the leak to appear, but when it does, it is pretty steady and yields about 1 to 1.5 cups of water.

There is a handy access to the tub's plumbing in an adjacent room, so I can clearly inspect in the inflow and the drain. No leaks there, on the pipes, wall, or anywhere else I can see. Bone dry.

The bathroom floor is dry after a shower.

The grout around the shower and tub is in good condition, as is the seal where the tub meets the floor.

I have removed part of the ceiling under the bathroom on the first floor. The leak comes from a spot in the floor a few inches away from the corner of the tub. There are no pipes there whatsoever-- just the base floor and the layers of bathroom tiles about it (again, these tiles are dry on top.)

My best guess is somehow water is getting in through the tiles in the shower, and the lip of the tub behind the tiles is gathering the water behind the tiles and dumping it on this one spot. Just a hunch, though. Again, there are no obvious places for water to penetrate, nor any evidence of water moving behind the tiles when I inspect from the other side. But I can't think of any other way for that much water to appear between the tiles and the floorboards. Any ideas?

  • 3
    Can you post some photos?
    – Yehuda_NYC
    Oct 20, 2015 at 12:45
  • 1
    You say it happens with showers but not baths. Does the shower head blast the wall somewhere? Most showers are designed so that the water lands in the tub if unobstructed. A cup of water seems like a lot if it's coming from mere overspray. Also - tile, grout, thinset, and backer board are NOT waterproof - these installs require a waterproof membrane behind the tile which redirects seepage back into the tub. I'm not saying this is your problem, you have plenty of things to try.
    – jqning
    Oct 20, 2015 at 15:14

5 Answers 5


Although it could be the grouting my money is on the seal being blown as the culprit. A seal can look like it's in great condition but not have good contact with the tub or the wall. My method for detecting this is pretty low-tech - I get a butter knife and see if I can pull the sealant strip away from the surface, or if I can easily slide it under. My guess is that you will find that the sealant is not attached to the tub.

You could also try pouring water on the edge of the tub so that it hits the seal but not the tiling, if you get a leak then you know it's the sealant, if not keep moving the water up until you get water dripping.


We had an issue with an upstairs tub that was driving us nutty. And it happened only during showers. After many false starts on diagnosing this - it finally turned out that there was an almost unnoticeable crack in some of the 2" tile grout lines, right where the shower spray impinged against the wall. So - we de-grouted the offending and neighboring tiles, found a matching color grout, and did the repair. Since then, we've talked to other homeowners who have had a similar "mystery" bathtub-water-leaking-through-the-ceiling experience; and who had the same root cause. Inspect the suspected grout carefully. If possible, to confirm, run a test with an aggressive shower user and stand downstairs at the leak zone. You may get confirmation of the problem.


I had two similar problems where there was only a leak during showers. To further diagnose where the problem was coming from, I used a shower head with a hose attachment and ran it to the toilet. Then ran water to test if the problem was in the water source.

Turned out, for one of them leaked with this set up, which allowed me to identify the issue as a poorly sealed spigot; when the shower was running the back pressure of the sealed off spigot was enough to cause a slow leak that then ran back along the pipe into the wall.

The other issue turned out to be identical to DA01's.


I believe we can only guess as to the specific issue. But I had a similar problem in a house once. In fact, it wasn't even all showers that the problem would surface...only certain people showering.

In our case, it turned out that one of our roommates just couldn't figure out how to use a shower curtain combined with the angle they set the shower head too. What was happening is that water was getting out of the tub onto the floor, where it was finding its way between the tile floor and the tub where there was some failing caulk. The water was then trickling down to the subfloor into the basement.

In our case, pulling out all the old caulk and re-selling helped, along with a bit of roommate nagging.


I had a similar incident, almost exactly as you are describing. What I discovered was that with the shower on, the rate of water flowing through the drain was much higher then when the tub was draining, and the drain line was backing up.

This wouldn't normally be an issue, but a seal was missing around the drain and water was flowing out of the drain pipe onto the ceiling of the room below.

I would look at something having to do with a higher flow rate through the pipes as potentially causing the issue.

Also, just because you are seeing water damage near the corner of the tub does not mean that is the origin of the water. Water can flow along pipes, walls, studs and wires to reach the point where you are seeing the apparent damage. The source could be very far away.

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