First, do some investigating.
I am sure others will add suggestions to my list.
I would check:
1) Go up to the tub and wiggle the overflow cover and drain fitting. Neither should be loose. Tighten the two screws on the overflow cover if it is loose, the gasket behind the tub may be loose and leaking. If the drain is loose, try to turn it tighter, there is a gasket under the tub at the drain.
2) Turn the water to the tub on and let it run, use the hot water. After about a 1/2 hour, go check for leaks where your saw them. If there are leaks present, you either have a leak from a tub valve running into the wall, a leak from the tub spout, or a drain line leak (which could be at the drain fitting at the tub). Or you may have more than one of the above, it could be a combination of leaks.
3) Close the tub drain and fill the tub about half full with water and let it set that way for a couple of hours, then check for leaks below where you saw them. If you see leakage, then the tub drain fitting may be leaking at the gasket under the tub.
4) Fill the tub to the overflow and leave the water running so water is continually running into the overflow (make sure not to actually overflow the tub and flood the floor). After a few minutes, turn the water off and open the tub drain. Being as water rises to its own level, while the water is draining out of the tub, water will rise up into the overflow pipe. Go down and check for leaks where you found them. If you see leakage, the overflow gasket and / or the overflow pipe (this may be a two piece section with a slip joint in it) is leaking (provided you have already checked for leakage in the drain line as shown above).
5) If none of the above cause leakage to come out, try this. Look around the tub at the tub wall and tub floor joints. Check for any evidence indicating where water could go through. If your child splashed water out of the tub, and the tub / wall or tub / floor was not properly sealed / constructed during construction, the water would puddle and leak down into the ceiling area, where you would see it show up.
6) If you can, place a small level on the head of the tub and then along the sides of the tub. The level should be indicating level. If either (or both) indicates a slope toward the inside corner, water could be puddling at the corner, going under the grout, and leaking out and over the tub corner behind the tile and the wall. To check this you will need to turn the tub on and (using you hand) direct water back toward the wall where it will puddle. After a while, go check for the leak.
7) If none of these show, I would call a plumber and explain what you’ve already checked. This may help the plumber save some time in finding what is leaking.
NOTE: when making the above checks, don’t just wait the amount of time I stated, check for the leak frequently. If you have a leak, you will not want the water to be pouring out and you did not check it until after 2 hours. If you find leakage, immediately turn off whatever is on and drain whatever has water in it.
I recommend using hot water when making these tests as hot water makes things expand and if there is a leak, it will likely leak more with hot water than with just using regular cold water.
[Edited by Jerry Peck on 20 December 2000]