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Well, while it is unlikely, I can't say never. Let me make sure I understand what everyone is saying:

Use 1/2 in plywood AND cement board for the countertop then tile over THAT.

Or

Just use 1/2 in plywood?

Dear Estrom,
Use 3'4" marine plywood and tile. This grade of plywood is of a higher density than the regular CDX grade and the glue is better.
Len
P.S. I must have been tired when I said use 1/4" underlayment for a countertop. Sorry

For either the floor or the counter, cementboard/hardibacker is the preferred underlayment for tile. The purpose of the underlayment is to provide a supporting surface that is similar to the nature of the top floor layer. Doing so provides consistent support that bends as much (and as little) as the floor surface and that absorbs moisture as much as the surface. Any inconsistency can lead to shifting or cracking or expansion over the long term. Thus, I would use plywood under wood floors (wood under wood), but not under tile. Same for countertops, which get less weight but more moisture.

Cementboard provides a firmer, stronger surface that helps prevent cracks in the tile/grout, and to which the tile adhesive grips better. Plywood can bend more than tile (if only a bit), which can crack tile over time. The top layer of the plywood can also separate from the rest of the board, taking the tile and adhesive with it. (Usually, you will screw the backerboard into the plywood, so the same problem does not exist in the plywood-cementboard connection.) Cementboard, on the other hand, will hold the adhesive as strong as (if not stronger than) the tile, itself. Again, your home will not explode if you use plywood under tile, but it would be better to use cementboard ot hardibacker board: something that is more rigid and solid like tile is.

My thoughts on tiling under the cabinets: don't bother doing it; instead, install the cabinets and then lay the tile. Again, your home won't explode if you do it the other way, and it is done all the time, but it wastes time, materials and money, especially if you use nice, expensive tile. The tile is there to serve as a beautifying layer that also protects the floor from foot traffic and such. You do not need to protect or beautify under the cabinets because nobody will see or walk under them. You also install kickplates at the floor level of cabinets that will cover any visible seam at the edge of the tile where it meets the cabinet.

Also, even if you want to elevate the cabinets to the level of the tile, tile costs more per square foot than plywood or other material that you could use to level the cabinets witht he floor. No need to spend the money on a beautifying floor surface when you do not need to do so. Tile also is not a smooth, level surface. There is a chance that a cabinet will line up right along a grout line, which will make the cabinet uneven.

Finally, you should always work from top down (with gravity) when remodeling so as to prevent damage to lower layers while working on higher layers. Although tile can most likely resist the wear of installing kitchen cabinets on top of them, there is a chance that you might scratch the tile or something like that while you are in the "construction mode" (rougher hands and feet) than you would not be in during daily use. Flooring should typically go in as the final stage of a remodel so that the subfloor gets the construction wear and tear and spillage, not the finished floor material.

[This message has been edited by Lawrence (edited January 17, 2002).]

I agree with all of that except the tiling. Tile the whole floor first. Who knows what the future of that room holds and who knows how long the tiles will be attainable. The leveling will be much easier as well. Lastly it makes the pattern on the tile around the cabinet base look much more consistant and natural. My opinion of course.

I am soon to begin the same type of project withe the same issues. My floor, however, does give. How would I go about bracing the floorjoists? Or is there a product that is more forgiving, that still keeps the look of ceramic tile?

If you aren't sure how much support you have in the area where you are laying the tiles.Goto a site and find a deflection test.It takes the floor joist itself plus the spacing and the amount of distance from the two load bearing walls and if the deflection is greater than 360 you are good to go,under 360 and you need to beef up the floor.Hope it helps!