I can't answer about the plywood (what's your joist size and spacing?) but whatever Hardibacker you put down, you *have* to use thinset and screws. I don't know that Liquid Nails is a proper substitute for thinset - but it's probably more $$$. Just don't use screws alone. My installer did and I've had nothing but trouble since about 6 months after install. I had the floor done 26 months ago. The 50sf bathroom floor my brother and I put down a month earlier following the Hardibacker directions is fine. The almost 500sf I had the pro do has had about 60sf replaced 9 months after install, and another 50-60sf a year after that (2 months ago). We'll see how long this next "fix" lasts - it may end up in court.

This salesman is a moron. yes, you DO have to use both screws (or nails) and thinset. The thinset isn't to bond the panels, but rather cushion them and fill the paper thin void that would otherwise exist. This of course, means that Liquid Nailz or any other construction adhesive would defeat the purpose in a multitude of ways, actually making it worse than using nothing at all between the panels.

As for your subfloor, so long as your joists are 16" on center spacing, you should be fine to go over the subfloor you have now. If they're 19 or 24" on center, yes, you'll have to add another 1/2" plywood first.

Bill - The last "fix" was the same as 2008's - put more screws in, reset the (cleaned up) tiles using Flexbond this time. I know I should have taken your advice and insisted on tearing it all out and doing it right, but when he started it seemed like a small area (when he left the first day I found about 10sf more that just pulled up, made him come back the next day and reset that too). But I really didn't want the hassle and my lawyer said to "work it out" rather than suing.

Now I'm wondering if there's anything I can do in the basement to stiffen the floor so we can make this job last and won't have to move the appliances, tear out the tile, and move out of the house for 2 weeks to fix it.

Can I add more plywood b/t the joist underneath, screw the panels up into the existing subfloor so no banging from hammers? Will that help at all? Or cover the joists (and insulation) with sheets of plywood spanning the joists? Might not be able to do that everywhere since ductwork and plumbing hangs below in some areas. Ironically, the worst (heaviest traffic) place in the kitchen where all the tile to date has come loose is right near the center of the house where we have lally columns and doubled-up 2x10s, so that *should* be stiffer than the middle of the room (13 ft span, blocked about halfway). The foyer tile has just failed all over - I only have a small corner by the stairs and in the coat closet that hasn't already been reset. Thanks.

If I'm not mistaken, I already told you what needed to be done. Your lawyer be damned. Your joisting and subfloor could be just as solid as could be. If the installation ABOVE the subfloor isn't right, it's not going to last. End of story. As to whether or not you go after the guy or not, that's your call.

Your tile store salesman sounds like the same one I encountered years ago. He said just nail 1/2" plywood over my existing 3/4" OSB subfloor and that would be good enough. It was....for about a year. then the grout started cracking and I fought it the whole time i lived in that place. I'll never do that again. there is NO substitute for doing it right, even if it means tearing it all up and starting over. If you don't, you will be fighting it from now on.

I just used a leveling compound on my kitchen floor and it is perfectly level, do I still need to use the thinset between the subfloor and the hardibacker? What does taping the edges mean and what kind do I use and how? Any advice given would be greatly appreaciated.

Thanks,
Jeff

Yes...you must still use the thinset...Its not about being level.