Dear Estrom,
First things first. Stand in the middle of the target floor and jump. If you feel any give at all, you need to brace up the joists from underneath. Second, I would use at least 1/2" plywood as an underlayment. Then install the durok cement board. Tape all joints as you would drywall, except use the tile mortar to cover the tape. The slightest vibration will crack ceramic tile and grout. In a 90 year old home, in my experience, a few cost me alot, this seems the way to go.
Good Luck,
Len

Iceman, thank you for your response....

What's your take on this, we're putting in all new cabinets as well... should we tile, then put in cabinets OVER the tile? Or should we put in the cabinets then tile around them?

Dear Estrom,
I would go through the small expense of tiling under the cabinets if only to facilitate leveling the base cabinets,waterproofing under the cabinets, i/e/,spills,leaks,etc. I've done it both ways and sometimes it is a royal pain to get the base cabinets leveled without tile under them. Especially in an older home.
Good Luck,
Len

Len,

Thanks for your responses. We're very fortunate that even though the house is 90 years old, the kitchen floor is still quite solid (no movement when we bounce.

So, based on your recommendations, we've decided to remove all the old flooring down to the subfloor, put down durok/cementboard (taped joints, of course) and then put in the new cabinets. This is a real challenge!

We're redoing the kitchen in phases (one wall every six months or so) so we can only do 90% of the floor right now. Frightening.

Eric

OK... since you've been so helpful, can I trouble you for one more question?

As part of the kitchen remodel, we're going to use 12x12 ceramic tiles for the counter top, too.

I'm going to have to build the countertop. I was planning on using eithe 1/4 in or 1/2 in plywood with cementboard on top of that, followed by the tile... does that sound about right?

Eric

Dear Estrom,
Since you won't be standing on your countertop, I think 1/4" plywood would suffice. But, when you put down the plywood, caulk the seams and glue the plywood to reduce movement and seal the countertop tile.
Len

Just thinking: is there any time ever when anybody might stand on the countertop - maybe to change a light bulb or paint or catch a spider on the ceiling or...? Better safe than sorry.

Dear R,
Good point.
Len

Well, while it is unlikely, I can't say never. Let me make sure I understand what everyone is saying:

Use 1/2 in plywood AND cement board for the countertop then tile over THAT.

Or

Just use 1/2 in plywood?