In some jobs lately, we are convincing clients to waterproof their plywood subfloors and underlayments. Now that could be done with 1/4 inch cement board followed by trowel on membranes, or it could be done with Ditra. Could it also be done with Kerdi? In other words, would we get the same effect as, say, Hardibacker, along with the waterproofing effect of the Kerdi? After all, if CBU does not add any real structural integrity but does help with bonding, couldn't Kerdi accomplish the same result? While saving 1/8" over Ditra and 1/4" plus over backerboard in height? I know it wouldn't have the 'deflection assistance' of Ditra, but if that's not the issue, if there's adequate deflection support, why not use just the Kerdi?
The only advantage I can think of is that CBU is nailed/screwed down to floor, but if tile bonds well to kerdi walls, I can't see why it would not bond well to floors even better, cuz then gravity is on your side.
Go ahead and tell me it's a dumb idea
but remember to tell me why, too!
