Over the membrane a mortar bed floor supports the tile.
Shower pan on a wood floor plywood thickness.
A mortared shower pan allows you to custom fit a shower enclosure.
Where a rotten subfloor is left unattended the rot may extend to the floor.
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.
Install the liner over a pre sloped bed as i explain here.
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.
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.
A plywood subfloor under a shower also known as underlayment is vulnerable to moisture damage that can rot the wood.
I used an unbonded mortar bed so i have a tar.
Reinforce the shower pan.