Ceramic and porcelain tile cannot bend, flex, or shift.
Complicating matters, groutthe material that fills tile seamscannot flex or shift.
Tile needs a rock-solid base and one that will not transmit cracks into the tile.
The Spruce / Ana Cadena
Attaching the tile directly to the concrete is the third and least preferable method.
Acting as a buffer layer, it is flexible and does not mimic the actions of the concrete.
Because subfloors can move and crack, they transmit the same to the tile above.
The Spruce / Ana Cadena
Uncoupling material breaks this chain of transmission.
Uncoupling membranes are valuable if you anticipate any movement or cracking from the concrete floor.
Generally, it is well worth the cost and the time to purchase and install anuncoupling membrane.
Uncoupling membranes are not perfect, though.
Benefits of Uncoupling Membranes
Uncoupling membranes do not eliminate the need to install control joints.
Perimeter movement joints and joints within the tile field may still be needed.
Tile laid on top of membranes should be 7/32-inch thick or a thickness specified by the membrane manufacturer.
Also be sure to check that the floor is level and fix any low spots.
In short, installing cement board between concrete and tile is possible.
But generally, it is not worth the effort and it may even result in a poor tile installation.
Most importantly, cement board is not considered to be an effective uncoupling material.
But that only describes concrete in its perfect, unchanged state.
Concrete responds poorly to foundation shifts.
Groundwater pushing up from below can crack it.
Tree roots routinely burrow under concrete slabs, then lift and crack them.
The best mode of thought is to assume that your concrete will crack at some point in its lifespan.
Cracks and gaps can be filled with Portland cement-based fillers.
Painted concrete can be made porous with sandblasting or other hard abrasive actions.
All of the movement in concrete is transferred to the tile.
Cracks in concrete immediately becomecracks in the tile.
That said, installing tile directly on concrete is not guaranteed to crack.
Stable concrete can act as a substrate for many years.
