One piece fiberglass bath tub leaking to floor below

I just noticed the ceiling in my dining room is wet from leak in the bath tub above. This is an 8 years old house with a one piece fiberglass tub/shower unit with Delta mixing valve. The plumbing pipes around the tub are probably PVC. I have checked around the unit for places it could have leaked. The overflow cover was a bit loose and the beveled washer was wet; a plexiglass holding rod doesn't have its ends sealed with caulk (but the fit is quite tight) and all sides are well caulked.

My son is the primary user of this bathroom and he assures me he has not filled the tub for a bath in the past 90 days.

I have bought a replacement beveled tub overflow washer that I'll install tomorrow and will also caulk the rod ends. I'm not sure this will fix the problem because the amount of water leaked below indicates that either the tub overflowed or there may be a leak behind/below the tub.

I'm looking for advice and ideas to find and fix the leak before replacing the drywall in the celing below.

Suggest if you are going to replace the drywall anyways, open it up and try to see where the water is coming from.

I'm letting the drywall dry out before testing again and hope I won't have to replace it.

The drain assembly could be loose; someone may have taken a shower(s) without closing the curtain or the door. The water supply lines could be leaking...the overflow being a little loose wouldn't be a problem unless the tub was actually overfilled; well, maybe if somebody was making waves

leak

Can't you make an access door on the other side of the wall to expose the drain assembly and faucet? From there you should be able to check and repair the waste and overflow and see if the caulking is bad around your faucet and from there you can see where water may be getting onto the floor and dripping to the ceiling below.

That may be your only leak if your son use the shower.

Unless your bathrooms are back to back I would by all means make an access and make it a permanent access.

Thank you all for your excellent ideas. They're helping me develop a plan of action. Here is what I'm doing:

1- For now, the shower/tub is out of use. The ceiling below has dried out.
2- I have replaced the sponge washer for the overflow and tightened the cover. 3- I have also replaced the gasket on the faucet cover to prevent any leaks from there.
4- I have caulked the acrylic rod ends to preclude any water leaking from there.
5- I have closed the drain and put some water in the tub and will check in the morning if any of it leaked and if the ceiling below gets wet. If it remains dry then I'll run some water through the drain and check if any water leak shows up below. If these two tests remain dry then I'll let my son use the shower and see the results.
6- If there is any leak then I'll make a permanent opening in the closet behind the shower and look for leaks and report back what I find.

Thank you again for sharing your expertise.

leak

Don't know what acrylic rod ends are, but it sounds like you have a plan.

I can't empathize enough that you should make an access door, especially when you have a closet there. Boy that would make me feel a lot easier to be able to SEE all the tell tail signs that are in there. Quite often there is more than one place that has leaked to some degree more or less. You could see if there was any wetness where water could have gotten on the floor from showers being taken. That is sometimes the last place a homeowner thinks about.

Once drywall has gotten wet, it is ruined and needs to be replaced.

My one-piece shower/tub drip. Toward end of shower, individual drips would appear on outside (in wall) of fiberglass at tub sill height. House is mid-1970s. There are noticeable cracks and one hole distant from leak. There are spider cracks in an area by shower floor, 2 feet below leak area.
After past plumbing challenges and home re-model, there is a plumbing access panel in adjacent closet. Also, a four-tube fluorescent fixture had been removed on ceiling below that replaced by a decorative ceiling panel (removable). I ensured it was removable to give plumbing access.
I tried taping the shower surface with tape to keep the leak areas dry, but it still leaked toward end of shower.
Consulting a fiberglass expert (sailboats), he said some fiberglass from that era was susceptible to water soaking through the surface! Big problem for sailboats.
I taped a large plastic to cover the whole shower wall by leak, no leak! He recommended FlexiSeal. It’s still curing for me so will report after next shower tomorrow.