[kactuskid]

"You cannot install backer onto dimensional lumber. Any seasonal movement will be transfered into your backer board and cause all kinds of probs. Install 1/2" bc or better exterior grade plywood perpendicular to your joists every 6-8" oc with coated 1 1/4" deck screws after making sure you add a few mor screws into your plank subfloor. Then you can put 1/4" hardi overtop the ply set in unmodified thinset. If height is an issue, cut out the planking and replace with 3/4 bc or better ply and then 1/4" backer thinsetted and screwed, but that would be the absolute bare minimum floor structure for ceramic and will not do for stone. I also like to see a second layer of ply in the middle, and nothing less than 3/8" thick, but Hardi states 1/4" board over 3/4" ply is fine. If you run into probs, most they have to shell out is $10/sheet, so the second ply layer adds strength to the floor while cement board does not. Max span of those joists will be about 9.5' for stone and 12.5' for ceramic. If the spans are any longer, you will need to add structural support underneath or sister you joists.

Resecure the planks to the joists, screw your plywood only to the planks, not the joists to eliminate transfer of joist movement into your second layer of flooring. Use bc or better exterior ply (lesser grades have voids that can cause problems with supporting point loads), gapped 1/8" and between sheets and 1/4 around the perimeter to allow wall movement. Fill the 1/8" gap with cheap caulk to prevent thinset from filling the gaps and to allow for the minimal expansion/contraction you get with plywood. Spread unmodified thinset (cheap $5 bag stuff) with a 1/4" square notch trowel and set your backer board. Screw down every 6" oc with 1 1/4" backer screws, same gapping as the plywood, fill the seams with modified thinset and cover with alkali resistant mesh tape as you set your tile. Do not tile up to the perimeter or grout the perimeter either.As far as the specs for backer, you can set 1/4" backer over 3/4 ply on joists 16" oc, but that is scraping the bare minimum. As stated previously, with 3/4 ply, I recommend a second layer of 3/8" minimum as the ply gives the structural strength for deflection between joists and the backer does not. 1 1/4" ply is the minimum for direct to plywood subfloor tiling. "