What to put under my bathroom tile??

So I have researched this and have gotten all sorts of feedback but still not sure. I am installing tile in my bathroom. The existing floor is two sheets of 5/8" plywood stacked. The floors is solid. I did have to piece in a small section near the tub. I have heard that since it is > 1" then I could either use thin set or some type of matting? Other say backerboard? The floor area is rather small, maybe 3'-4' across.

Were are looking at 4x4 tiles if that matters.

Thanks.

In such a small area I'd use a piece of 3/8" cement board with the manufacturer's recomended bonding agent and grout.

I have used 1/4" Hardibacker in situations similar to yours, with good results.

Industry standards do have one approved method to tile over TWO layers of plywood, PROPERLY installed. Now, there are some fairly stringent rules on how that must be done, then you must use a (pretty expensive) highly modified thinset to make it all work. Most people prefer another approved method. Your choices are:
- cbu (1/4" is fine on a floor) that is applied onto thinset and fastened with the specified fasteners (hot-dipped galvanized roofing nails or special cbu screws) at the proper spacing, then applying reinforcment mesh tape on the seams.
- an antifracture membrane (such as Ditra from www.schluter.com)

Note, those two layers of ply are probably fine, but if any layer has a 'D' face, or is not made with exposure I or exterior rated glue, it shouldn't be underneath tile. Plus, the ply MUST run across the joists, not parallel with the joists. Then, regardless of the strength of the subflooring (this keeps the deflection down BETWEEN the joists), if the joists themselves aren't strong enough, the tile job could fail. This is determined by the total length UNDERNEATH the floor between supports (not the room size), the joist spacing, the joist depth, and wood type. If the deflection rating of the joists is stronger than L/360, then you should be fine. Check out www.johnbridge.com for tiling help.