underlayment

I currently have a 3/4" subfloor that is pretty solid. The local mega home improvement dude tells me I could use 1/4" Hardibacker on top of that and my serface would be sufficient. Is this advise correct?

Please let me know what experice you have if you reply to this.

I have the same project underway and the 1/4" backer board installed with thin set and screws is the ticket.

Crowe and Qqq:

I would phone up some of the tiling contractors in the yellow pages. They would have a lot more experience with this than the guy working in a home center. Besides, it's not only the thickness of your subfloor, it's the size and spacing of your floor joists. Most homes now use 2X10 fir joists on 16 inch centers. I would see what you have and run that by some of the tiling contractors. The guy in the home center isn't the one who's actually installed tiles over these floors and found out the hard way which ones weren't strong enough.

...and the span of those floor joists affects how rigid the floor is too. Did the guy at the home center even ask about that?

Nestor provided a good piece of advice. The following are excerpts of requirements related to your question:

Interior Floors (Cement Backer Board Underlayment)
General: Subfloor - MINIMUM - 5/8" exterior-grade plywood on joists at 16" on center. Maximum variation in plywood surface shall not exceed 1/8" in 10'-0" from the required plane.

Specifically: The deflection of floor areas over which tile is to be applied shall not have a deflection greater than 1/360 of the span. Allowances are to be made for live load and impact as well as all dead load, including the weight of the tile and setting bed. Specifications for ceramic tile installations shall conform to local building codes, ordinances, trade practices, and climatic conditions.

No mention is made of a pretty solid subfloor or the minimum or maximum thickness of the CBU.