Wood based panels such as particle board composite panels veneer faces bonded to reconstituted wood cores luan plywood and soft wood plywood all expand and contract with changes in moisture content and are not recommended as backing materials for ceramic tile.
Ceramic tile over luan plywood.
Never have that under tile.
Luan is a hardwood species containing extractives sugars.
The seams between vinyl tiles allow water from spills or mopping to soak into the luan which can result in swollen plywood and lumps that are visible on the surface of the vinyl floor.
The books says a minimum of 3 8 underlayment grade plywood no 1 4 then use ditra or a 1 4 cement backer board both are installed in a wet bed of thinset.
I installed 3 4 plywood over the 5 8 underlayment.
If you are looking for correct then the luan has to come up.
Ceramic tile stone and or cementitious backer unit installations utilizing any luan species will not be warranted by custom.
Luan plywood is an unacceptable and unapproved substrate to receive tile or cementitious backer units cbu.
I then installed 1 4 luan on top of this and was planning on attaching the ceramic tile to the luan.
Plywood in ceramic tile installations.