What are the risks if we do not install cement board first, if any?
Not putting an additional subflooring is almost a guaranteed failure. ATC and TCA are tile code guide lines that state that a minimum of 1 1/4 subflooring is needed to do tile. There are other circumstances ( like deflection) that come into play , but properly prepped subflooring is the best bet against failure. It doesnt need to be a cement board ( and there is a proper installation method) .There are other ways , depending or your exact situation that can be used to properly do tile. Any questions you can e mail me through my avatar and check my qualifications there. GL
if your subfloor is a solid peice with no seams in that area and the floor itself is solid you should be ok but if there is movement your tiles and or grout could crack if it wouldnt make the tiles too high you better off using the cement board
