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.
Ceramic tile over luan plywood.
It can also be used under hardwood laminate and carpet but this is much less common these days because subflooring is typically smooth enough for these materials many of which are installed with a different type of underlayment or pad.
I installed 3 4 plywood over the 5 8 underlayment.
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.
Never have that under tile.
I then installed 1 4 luan on top of this and was planning on attaching the ceramic tile to the luan.
Luan is a hardwood species containing extractives sugars.
Ceramic tile stone and or cementitious backer unit installations utilizing any luan species will not be warranted by custom.
I just finished prepping my 16 x 16 kitchen for ceramic tile.
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.
If you are looking for correct then the luan has to come up.