I have a newly constructed townhouse with a shower on the 3rd floor master which has a fiberglass shower base. As soon as I used the shower, I noticed that the floor squeaks and the drain cover does not sit quite flat/level. Seems to me that there is a problem under the floor that should be addressed before it get worse over time. I am a 'shower person' of many years and I have never been a shower where the floor squeaks. Also be aware that I weigh just over 100 lbs, so the problem is not requiring much pressure to be noticed. There is also a corner deep tub which has a tiled deck and the wall is a bit bowed as well as the tile not applied too well because of it. Don't know if these 2 problems would be related, but guessing they are. Customer service from my builder is willing to look at problems like this, so I would like to have your opinion of possible reasons for this and how it should be fixed (not patched!! ). Thank you for your advise and help.

Not enough support for the shower allowing the floor to move up and down on the pipe. Another possibliity is that the shower base to was not supported between it and the wood floor underneath the shower base allowing the base to sink a bit when you step in and rise when you step out. Many builders just put the base in place without following the instructions that came with it.

Many fiberglass or acrylic shower bases naturally squeek when 1st installed and should go away within a few days. If not then perhaps it was installed incorrectly

I'm a contractor and I can tell you that the reason its squeaks is for some reason and there can be a few, all the weight of the tub is not supported by the ledger board its fastened to. Also we set ours in concrete so there is no movement.
As far as fixing it your going to have to get behind the tub to maybe spray some great stuff foam, (use the yellow and red can )

Sometimes when they are installed the rough opening may be a little tight and the squeak can come from the tub rubbing slightly against the stud walls

Hope this helps , NICHOLS TRIM

I agree with glavine, but will go a step more the bow in the wall should n't be there they may have used what is called green board and missed a screw or two while installing. Backa board is the best for wet locations, the stall floor should have had a dry mix placed on top of a rubber membrain to support the stall liner without this being installed the stall will squeak.