Do I really need thinset under my hardibacker?

I am going to be installing a tile floor in a small bathroom. I have never installed a tile floor before, so this will be my first attempt. I was going to put down hardibacker which would be just nailed or screwed to the subloor. Now someone has told me that I should put down thinset under the hardibacker. Do I really need to do this?

 

only if you don't want your floor to crack

 

Haha...no I don't want it to crack. I guess I just don't understand why it will crack if I don't use the thinset.

 

Cement board is not a structural component of a tile isntallation. It will flex under load if not properly supported and this verticle movement will result in cracked tiles and grout. There will be small voids between the plywood base and the cement board. ALso, every screw or nail will result in tear out under the backerboard creating even more voids. This is where the thinset comes in. When you comb your thinset on the floor, keep your trowel lines all in on direction and the screws or nails will compress the thinset distributing it evenly under the backer and filling in all the voids. THis gives the backer the uniform support required underneath. Think of it this way, the thinset holds the backer up and the screws or nails hold it down. As long as you are not using a backer from Custom building products (wonderboard, fiberock) then you can use an unmodified thinset under the backer which costs about $8/bag, mixed with water and spread using a 1/4"x1/4" square notch trowel held at a 45 deg angle to the floor. Using a 1/2" backer is no better than a 1/4" backer. Only go to the 1/2" if you need the extra height offered by it. However, what is your current subfloor? Let us have a chance to critique it now so you are not faced with problems later. WHile you are at it, maybe you can give us some extra structural information, such as the size of your joists, their spacing, and how long they span when measured from underneatht he room, from support to support.