Plumbing in the Home/support under integral apron cast iron tub
I pulled and relocated a full apron 5 ft alcove cast iron tub.
It is a 1951 American Standard, original to the house.
I have set the tub with a ledger board along the 5 foot wall. In the new installation, the cast iron basin is a little over and inch off the floor deck at the low end and has no feet. There are some tabs on the sides at the middle that are about 1 1/2" off the deck and "feet" at the back about 3/4" above the floor deck.
There was no indication under the original installation that the "feet" under the basin were supported with shims. I did not look specifically for marks on the floor boards. I assumed the tub was fully supported by the ledger along the 5 foot wall. There is a possibility the "feet" at the back end of the tub were on the 1x8 subfloor because the front drain was not over any subfloor and could have been actually below the level of the floor bringing the back "feet" into contact.
Looking at installation instructions on new cast iron tubs, that are not a match for what I have ... there is no match for what I have, I see that the space between the little feet on the bottom and the floor is supposed to be shimmed and they do not call for a ledger board at all.
All that said, are these side tabs and "feet" supposed to have something fitting under them?
How would these points be shimmed in a retrofit where the apron blocks all access?
Over the years, I have installed dozens of these tubs, both Kohler and American Standard.
A full on cast-iron tub is very strong and rigid and does not require a bottom support. It is supported by the ledger board along the wall and the base of the skirt sitting on the floor. The actual slope for drainage is built into the tub itself. That's all that is needed.
Acrylic and/or fiberglass tubs do require "bedding" to prevent flex and cracking of the gelcoat. It also makes them quieter. Cast-iron tubs have so much mass this is never an issue.
It is also very common to have a 12 in. square cutout in the subfloor at the drain end for access to the drain shoe and the waste and overflow mechanism and piping joints from below or behind the tub.