We are renovating a bathroom and the plumber installed our 48x60 fiberglass shower base (approx 80 lbs) . This bathroom is on the 2nd floor and has a new plywood subfloor. I did not realize until after they installed it that they did not follow the manufacturers directions, which were to use a leveling compound to level the entire floor, then adhere the shower base using construction adhesive (applying the adhesive to 4 "blocks" which are under the base). Instead, they used joint compound - which they placed in "globs" on the subfloor and then set the shower base on. They did level the shower base, using shims to get it level in all directions. They stated they would pour liquid concrete under the base the following day to fill any additional voids that were left. However, the following day they said that the concrete isn't necessary anymore. This change in plans made me question the entire process and here I am panicing about our new $1000 base.

I called the manufacturer and also the plumbing superintendent who both said they should follow the installation directions. Our contractor thinks that it's okay if they don't use leveling compound over the WHOLE floor, but said he would have used soft sand cement as a leveling compound and set it in that.

I spoke with the owner of the company about my concerns and they said they only use cement if it's installed on slabs / basement floors because of the high water content. He said the high water content of cement is not good for wood subfloors. He also said that they have never used a leveling compound across the entire floor and that he would be worried that setting the pan just on that would leave voids. I argued that if the entire floor is level, it actually decreases any chances of voids. We compromised and agreed that we would see if the joint compound hardened in 3 days. If I'm unhappy at that time, they will pull it up and reinstall if that's what we want them to do.

My concerns are:

1. Will joint compound actually harden approprately?
2. Will the joint compound soften if somehow exposed to water / moisture?
3. Should they pull up the shower base and reinstall?
4. If they should reinstall, what is the appropriate method and material?
5. Is there a greater risk of damaging the base at this point if we try to reinstall it?

Thank you for any guidance!

The shower base is a Fleurco brand, purchased from our local tile shop.