Check this out on the John Bridge forum.

I'm pretty sure Spruce isn't a hardwood but it's still possible that it could move enough with expansion and contraction to cause adhesion issues over the long term. Ditra, tile backer or some type of lath should be used to isolate the movement of the wood. I think I'd choose whatever method ends up adding the least height and it's probably a good idea to add some more fasteners from the wood to the joists.

I would definitely check with tile installers. We put tile in the kitchen and bath of the second floor and, in no time, had cracks in the tile. We ended up taking out all the tile and using sheet vinyl.

Like Jeoffoy said...Go to WWW. John Bridge dot dom

Dimensional lumber is not an appropriate substrate. Even Ditra, the king of the uncoupling membranes, requires OSB or plywood under it.

Here is a link that might be useful: Ditra installation instructions

The TCNA guidelines require at least an added 1/2" of CC grade plywood or better (easier to find BC or AB)

No, using a 1/2" layer of ceement board doesn't count. You'd still need the plywood.

Over your plywood, you can use any of a number of approved tiling surfaces such as ditra, greenskin, or one of the noble products