2nd story shower/bath leaking only when shower is used. Bath faucet does not leak to lower floor ceiling as the shower only does. Any idea how to fix problem? Desperate to fix as 1st floor ceiling is leaking & rotting.

Most likely one of two things. Your pipe going up to the shower pipe from the mixer is leaking or you have a terrible caulk around the seam where the wall meets the tub. Wrap a wet towel around the shower head allowing most of it to drape to the tub, turn the shower on, if the leak comes back it is not the caulking. Remove the escutcheon plate from the mixing valve, handle to, turn the water on and look inside with a flash light. You should be able to see but a wad of toilet paper rubbed around should come back dry. If its wet inside you can try to tighten the pipe that the shower head connects to but there are two other connections inside the wall that are not reachable without making some holes.

Thank you for your reply.
There are no visible signs of leaking in the bathroom, only from the ceiling. The showe/tub unit is a single prefabricated unit.

So you are saying that I should take off the shower head & the shower head neck; then look down inside the wall from there to tighten the pipe. If this doesn't work then I will have to cut from the ceiling & investigate from there?

No not quite but did you perform the test with the towel? We need the water running but no chnace of the caulking getting wet. If we assume or miss a steep the chances of a fix go down.

Okay. I assume the mixer is where the hot & cold knobs are, right? If so, then my problem is... how do I get to those if they are behind the wall (shower/tub enclosure is a single prefabed unit).

There is no water leaking from the shower head... at least back into the tub from the outside of the prefab enclosure.

You help is VERY appreciated!

My two thoughts both include water leaking inside the wall where the mixing valve is located and the pipe to the shower head. The handle comes off and then the plate )escutcheon) allowing you to look inside for water. When you get that far you'll see what I am talking about.