A plywood subfloor under a shower also known as underlayment is vulnerable to moisture damage that can rot the wood.
Shower pan on a wood floor plywood thickness.
Water that gets into the mortar bed won t be able to drain out.
Over the membrane a mortar bed floor supports the tile.
No matter what the building inspector says it s not a good idea to install a pan liner and then a single sloped mortar bed over it.
When choosing your subfloor material take into consideration how the thickness will effect the overall heigh of the built up shower pan in relation to your shower curb.
Install the liner over a pre sloped bed as i explain here.
Where a rotten subfloor is left unattended the rot may extend to the floor.
After you have put down a double layer of plywood to provide the base for a custom walk in shower pan you need to cover the subfloor with a layer of tar paper in this case referred to as a vapor.
The key to a successful installation lies in the use of a chlorinated polyethylene cpe or pvc membrane tough but flexible plastics that form the pan of the enclosure and make the floor waterproof.
A piece of plywood can be purchased in 1 2 or 3 4 thickness and most cement backer boards will be 1 4 1 2 or 5 8 thick.