When installing ceramic tile on plywood. I have noted that I need at least 1.25" of wood. I have a old 1X6 subfloor with 5/8 plywood underlayment on top of that. I meet the requirements of 1.25", IS THIS CORRECT? If so, what are the drawbacks of not using durock backerboard? The backer board will ad $400 to the project that I do not want to spend. If I lay the ceramic on the plywood I have noted to use a premium thin set mortar for plywood. Is there a posibility of the plywood warping or bowing up from the moisture of the mortar. Is there any applications that need to be applied to the plywood before the mortar? I want to do it right the first time.