On Jul 9, 4:15*pm, wrote:
Dear Readers,
I'm renovating my bathroom: I removed the old cast-iron bathtub which
was sitting directly on the wood subfloor. *I’m also removing the
adjacent floor tile and its 1/4” plywood base, down to the subfloor as
well. *I plan to put down Durock and lay floor tile above that.
Should I also put the Durock below the new cast-iron bathtub? *Other
than a minor increase in elevation of the tub, is there any reason why
I shouldn’t put Durock beneath the tub? *All opinions appreicated.

There's no real reason to have the cement backer board under the
typical tub. I usually just run it under the front edge a couple or
three inches so the tub is resting on it, and the rest of the tub is
supported by the wood blocking attached to the studs.

In your case, since the tub used to sit on the floor, it's probably
simplest to just complete the floor with the cement backer board.
It's a good place to use up some of the larger scrap pieces.

What is the existing subfloor like? Diagonal shiplap boards are
usually uneven and the cement backer board may fracture over a hump.
It's generally a good idea to set the backer board in thinset when
you're putting it down to make up for any discrepancies and fill in
the voids so there's no movement.

R