I have an apartment that I rehabbed and I had the same softwood 3/4" t&g,
and I put ordinary
sheathing plywood over it (1/2"). This was before all the cement board
products were out. I put down ceramic tile with a high grade mastic (from
American Olean, I think). This was not cement based like thinset.
Not a single tile or a single grout joint has budged in 23 years.
I believe your contractor is just wanting to use cheap thinset and making
you pay for underlayment
that will tolerate the cheap adhesive.
It might be a good idea to simply change contractors at this point, and get
someone that knows adhesives.

bill

"Gord" wrote in message
oups.com...
The subfloor is 3/4" tongue and groove. I also put extra blocking
between the 10" joists underneath that, so there should be no problem
with stability. What I hope to do is attach additional plywood that
can get around the problem with adhesion for the tiles.