Rip it out, its not that hard. Then thinset, cement board, and tile.

Evenness has nothing to do with weather or not you lay cement board. You need to take the movement out of the wood. Tile doesnt move, when it moves they break, or the grout cracks and crumbles.

 

You also need to know how thick the plywood. CBU adds no structural strength.

 

20 inch tile is large format, and requires a much stiffer floor (measured by the L/D ratio) than smaller tile to prevent cracking of the tile. For a detailed discussion about floor stiffness and how to determine how stiff your floor is, see www.schluter.com

Schluter makes ditramat, which is a bond breaker commonly used to reduce the potential for tile cracking. Regardless whether you use their product or not, you absolutely need to determine the stiffness of your floor, which is a function of the size of joists, the span of the joists, the spacing of the joists, the load on the joists, and the type of subfloor you have. Floor stiffness is not affected by installation of cement backer board, so if your floor is not stiff enough for 20 inch tile, you need to develop a plan to increase the stiffness, else you should consider using a smaller format tile. The penalty for installing a large format tile on a floor which is not stiff enough is cracked tile, cracked grout, or both.

 

I know the right way is to take it out and lay a cement board down...

Well then?

 

Thank you all for your response, I am going to rip it out and lay cement board down.

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On a concrete floor you can .