I am replacing a second floor tub with an Americast Princeton tub. The subfloor is about 5/8" out of level in the 30" width of the tub. It slopes down on the apron side. The old tub was not level and water ran out from showers and did some minor subfloor damage at the front corner of the apron by the drain. I cut out and replaced a section of the plywood subfloor to make sure I had a solid subfloor.
To get this new tub installed correctly, I suppose the apron will need to be shimmed up about 5/8". My question is: it seems that most of the tub weight (water and/or person) will be on the on the sides of the tub where it rests on the stringer and on the shims under the apron rather than on the plastic honeycomb bottom transferring the weight directly to the subfloor. The instructions say that the that the base does not need to contact the floor for there to be a proper installation. But this doesn't seem to be nearly as stable as placing some piles of mortar under the honey comb base. I have read about putting some plastic over the mortar to keep it from directly bonding to the tub.
I have hired a plumber who has helped me in the past. He says that he does not use a motar with Americast tubs. However, in this case with the need to use 5/8" of shim, would it make for a better installation to use mortar to take up the space between the subfloor and the tub so to establish weight transfer contact with the subfloor? I am concern about the weight deflecting the tub. Porcelain doesn't flex like steel.