I do maintenance and small remodel for a woman who had her upstairs bath tiled. I then put the claw foot tub back in place, plumbed it up again - there has been no cracking. This was a smallish tub, I've definitely seen claw foot tubs bigger.

The sub floor is OK but not extraordinary. Who knows? Come to think of it, another client has a bath floor with cracks just from the weight of human footfalls, but this is cheap tile, looks like the cheap variety anyway, and the floor feels extra flexy.

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1986 300SDL, 362K
1984 300D, 138K