Designing Kitchens and Bathrooms - Leaking tub

OK- Second floor bathroom, shower/tub unit with tiled walls above fiberglass tub, single control faucet (turn counter-clockwise to turn on towards hot). Walls on first floor (where I assume drain pipe runs) are getting wet. So, is the leak from the drain/drain pipe or the plumbing to the faucet, or the spout/shower diverter? Disconnected the faucet and backing plate, all seemed dry. Not sure what to suspect, have not pulled the spigut or the tub drain. What's the most likely failure in this set-up? BTW, caulking around tub and fixtures is fine.... Ideas on next steps?

Thanks,
Mike

Mike: Welcome to the forums! If your access to the control valve shows no leakage, I would guess the trap and/or overflow may have a loose connection. Do you have access to the bottom of the tub unit? If so, tighten all the connections from the overflow, drain and ptrap. You may even see some water damage at that point. Have you had the opportunity to observe the valve and shower action with the pipes energized and water flowing? Do you have any water damage to the ceiling in the room below, or is it just located in the wall?

Thanks Larry- maybe a bit of ceiling leakage on one side, crown molding on the other side of the dividing wall makes it tough to see. No real access to the bottom of the tub unit, indeed no access to the piping at all except thru the shower fittings. As it all backs to a closet on the other side, guess the easiest and cleanest approach is to cut an access panel (that should have been there from the start I guess) and I'll have clear view of most everything. Obviously I'm thinking out loud thus far and appreciate the questions and comments. In the event there is a fault in the drain pipe, does anyone make a flexible sleeve or insert to line the existing pipe without resorting to tearing out walls and pipe?

Your best bet is to do like you will do in the closet. Cut a hole under the tub and do the work, repairing the ceiling after you have it fixed. No sleeves, no magic. :(