Flooring Tile - Subfloor requirements?

I'm preparing to install a bathroom floor with ceramic tile and electric radiant heat. I've gathered lots of info, including from this forum, but I'm still unsure of my plan - specifically the subfloor.

I've pulled up the old beat-up plywood, leaving the 3/4" boards (5.5" wide, diagonal across joists). Joists are spaced 16" O.C. Although this floor seems really solid, with no apparent movement, creaking, etc, I believe that I need a plywood layer before the backerboard (I've read threads here that say the boards are not an acceptable underlayment to backerboard - I'm not clear on why. And why is it OK to tile directly on 3/4" hardwood flooring, but not on backerboard on top of 3/4" wood subfloor?).

Question is, how thick plywood? I'm trying to minimize total floor thickness to stay close to the adjacent floor (it will likely be higher anyway with the heating cable). My tile book states "If the sufloor is at least 5/8" plywood, you can install backerboard in thin-set over the subfloor." I would think they mean 5/8" of total wood layer, but I'm not sure. Can I use 1/4" exterior grade plywood?

Plan then is unmodified thin-set, cement backerboard, then the heating cables. My plan is to attach the cables, spread just enough modified thinset to cover the cable, let it dry, then layer the same thin set and tile. Does this all sound right?

I very much appreciate the time that the experts put in to answering these threads. I welcome any specific pointers on any part of the process. Thank you.

Mike
Ithaca, NY

You need plywood under a backerboard, demensional lumber would leave voids and moves under the CBU.:)

All you need is 1/2 plywood, never use 1/4" wood of any kind where there is going to be tile.:)

Over the 1/2" ply. you can add 1/4" CBU, then your heating system, or a little more expensive way, is to Prime the plywood with SLC primer, install the heating matts, lay metal lath and pour SLC over the floor, then tile on that, CBU is easier to do for a beginner.:)

SLC=Self Leveling Compound.

Thanks for the help; I'll follow your advice and take the best option for a beginner. :) I just hope my guests won't fall over the threshhold when it's finished!

Mike

My tile book states "If the sufloor is at least 5/8" plywood, you can install backerboard in thin-set over the subfloor." I would think they mean 5/8" of total wood layer, but I'm not sure. The minimum is 5/8 subfloor, which would indicate t-g plywood used for subflooring, not square edge. You need a second layer of plywood if it's square edge plywood. What book you reading?
I believe that I need a plywood layer before the backerboard (I've read threads here that say the boards are not an acceptable underlayment to backerboard - I'm not clear on why. And why is it OK to tile directly on 3/4" hardwood flooring, but not on backerboard on top of 3/4" wood subfloor?). You always need a layer of ply to separate the backerboard from the dimensional lumber. Cement board is only a bonding surface and witout the plywood, the seasonal movement would transfer through the backer into the tile. Tile only moves by cracking. You cannot tile on dimensional lumber, that includes 3/4" hardwoods.
Can I use 1/4" exterior grade plywood? There are no 1/4" plys for under ceramic. Minimum thickness of any plywood layer overlayed is 3/8". Also, the plywood must be graded atleast cc plugged exterior rated. Never should you use nything stamped as cd or "sheathing rated"

The minimum is 5/8 subfloor, which would indicate t-g plywood used for subflooring, not square edge. You need a second layer of plywood if it's square edge plywood. What book you reading?

So is it possible to inlay tile in 3/4" hw so that it is flush with the hardwood floor? It would presumably work with 5/8", but no if we had to go thicker due to not having a t-g edge.

How thick would an SVC need to be?