Where do you get your 1" T&G plywood?

He doesn't. He means 1 -1/4 subfloor. I did that once. The stuff is a btich.

Yeah, 1" doesn't exist.
Wondering why he would tear out the existing 1x subfloor and replace with 1⅛" plywood. Why not just cap a layer of ½" plywood to the existing subfloor if you're worried about support? That turns it into an 1¼" subfloor. Plenty of added strength without having to charge the customer for all that extra labor and material.

Well, if you really want to get technical. It is 1 1/8" unlike the older days, 1 1/4". There are plenty of 1" plywood and its available in specialty plywood houses, if not special layup or order maybe necessary

I just dislike stacking up too many layers of floors. Once I am that far, how much more is it to remove a few pieces of 1x 6 sub floor (We discovered that more then few pieces are usually split from nails or rot). Its a half day's work, one day including some additional framing such as blocking and replacing hacked up framing from plumbers who like to remove parts of joist for the toilet flange. That's the reason I do it that way. My clients are more then happy to pay it since its normally under 1000 bucks for a small bath when its all said and done. Again, I dislike installing cast iron tubs, especially the deep and larger one over 1x6 sub floor. We try our best to do nice work so if it cost more and delay the job a couple days, most people are happy to do it.

PL down ½" plywood and you've got a subfloor every bit as supportive for a fraction of the time and materials. Work smarter, not harder and save your customer an unnecessary added expense. It's ok Look, we all learn from old guys with decades of experience. It's how I learned most of what I know. No one knows it all, certainly not me, so my ears are always open. I listen and evaluate and sometimes I learn something new even after 50 years of doing this stuff.

We used the Schuler (sp) when we built in our basement bathroom and put electric heating in it as well. Fantastic system, just use good materials.