Hi,
I had rough-sawn 1x6 subfloor in my kitchen. I ripped pieces of 3/4 plywood to fit between the joists and screwed them from below into the subfloor planks. This step stiffened the system remarkably; each plank is tied into the rest, and they cannot flex. I filled the gaps between planks with foam insulation, to prepare for the leveling. I used floor leveler over the sealed planks (sealed with plasterweld). To save height, I used Ditra isolation membrane under the 12x12 marble tiles. (didn't use backer board).
This extremely unconventional treatment has worked well two-plus years in; no cracked tile.
The Lowe's guy is a moran, lauan is incompatible with thinset.
Casey

A Lowes employee suggested using 1/4 in. luan underlayment between the subfloor and the HardieBacker.

Not only no, but HELL NO!

Luan is instant death to a tile floor, even under backerboard. The guy WAS right about one thing, though-- you can't go directly over planking with the backerboard. You need a layer of plywood. Being that the planking has given you the strength in the subfloor, all you should need is 3/8" plywood, and then the cement baord, or if that's too much elevation, use Ditra instead, which is only 1/8" thick.

I'm going to type that again, but in caps. This is a question that leads people to my site on searches on a constant basis--

YOU CAN NOT HAVE LUAN IN A SUBFLOOR UNDERNEATH CERAMIC OR STONE TILE. Period.

Casey-- it's more than just being incompatible with thinset, although that's one of the reasons. But if that were the case, here, it would be fine, because you don't WANT it to stick in this case. It's there to bed the backerboard, not bond it. The biggest problem is that it can literally compress to half its thickness, causing bigtime voids under the tile, and ultimately failure in the floor. I've seen this happen within as little as 6 months. It was the very first time I ever used Ditra as an underlayment-- replacing a floor that was only 6 months old. I pulled up pieces of luan from under that floor where by the cabinet toe kick, it was full thickness (1/4"), and not a foot away, it had compressed to a shy 1/8". This is no exaggeration.

We put 3/4" ply over our diagonal 1x6 boards and then Ditra and the floor is nice and tight. It hasn't been in very long, but you could even feel how nice and firm it was under foot just walking across it during construction.

Thanks all for your inputs,

I suspected that may be the case but didn't know the reason for not using luan. I am going to go the route of using 3/8in. plywood and Ditra as bill vincent suggested. Should I use construction adhesive between the existing subfloor and the plywood or will screwing it down be sufficient? TYIA

Ed

Just screw it down.

Luan is just a fancy name for lousy plywood with no structural rating.