Finally, someone else HAS the problem I HAD. My fiberglass tub/shower enclosure had that spongy floor feeling since day one in this house, the house was two years old and they did not use this bath. I had access to the bottom of the tub floor from two separate wall, one a bedroom wall, the other the wall between the bath/toilet and the room with the sinks and mirror. I removed the baseboards within each room, then cut a section of drywall about 1/2" less in height than the baseboard out, say about two feet long, so I could re-use it. I found that the tub floor was not sitting on anything as it should have been. I placed heavy duty contractors garbage bags underneath the tub floor perpendicular to the length of the tub, with the opening to the back wall of the tub, the bedroom side. Then I got me a piece of clear tubing that would just slip onto the plastic nozzle of that "Great Foam" stuff, about three feet long. I placed one end of the tubing into a garbage bag and blew the Great Foam into it as I pulled the can back slowly. It worked, the foam spread out until it reached the floor of the tub then spread sideways. I wasn't trying to fill the entire void, just enough to add strength. I did, I think :whistling2:, four of the bags like this and when they had finished spreading and solidified, I had the support I needed. The it was just a little drywall repair, replace the baseboard and things have been fine ever since. Good Luck, David