Here's a short video Brian sent me a couple years ago which I forgot to put on the site ... until today. It clearly demonstrates the kind of movement which can be expected in any type of backeboard if mortar isn't used in its installation.
The cement board in this video only has screws fastening it to the subfloor with no mortar underneath.

5 Second Video Clip showing up and down movement when stepped on.

Here are the side view photos showing the spaces under the cement board allowing it to move each time it's stepped on.

Gee Harry, I had hair when I sent you that one.

You use to have hair???


You also sent me a couple a good photos which I'm going to find so I can post here.

For anyone not sure of what they are watching in the video, the cement panels are moving up and down each time they are stepped on. The voids under the cement board are allowing movement. If the panels had been set into a bed of mortar as required (even cheap unmodified mortar), there would be no voids causing this problem.

It was so long ago that I sent you that video, that your site was called Upper Canada Tile Setters.

I should point out that this was a job I was asked to do, where the contractor already had the board down. I told him the board was installed incorrectly, but he said he has been doing it that way for 20 years. I told him I have been fixing them for 20 years also. :rolleyes:

You may also notice the 1/4 lunan plywood under the cement board too. This is really a bad idea, and should not ever be done. Though repairing this type of install, does keep us busy.

Wow,this is a great site Harry. The only bad thing is I keep reading and learning and not working on the bath remo that is awaiting and my wife keeps giving me "the look" because it's still not finished.My question is how much thin set (mortar bed)do I need for the CBU's to be in? Let me guess,just the right amount?? Seriously,just enough to fill the voids and completely level it, before I screw it down?

If the dips in the floor are too bad then maybe smooth things out a bit a day before the cbu goes in.
The photos above are more to the extreme of what we normally see. I usually use a 1/4" x 3/8" trowel to set the cement board into and basically the panels with follow the contours of the existing surface.