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.
Shower pan on a wood floor plywood thickness.
Install the liner over a pre sloped bed as i explain here.
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.
Reinforce the shower pan.
Water that gets into the mortar bed won t be able to drain out.
A plywood subfloor under a shower also known as underlayment is vulnerable to moisture damage that can rot the wood.
Over the membrane a mortar bed floor supports the tile.
Where a rotten subfloor is left unattended the rot may extend to the floor.
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.
I used an unbonded mortar bed so i have a tar.