The First Mud Layer of Tile Shower Pans
Without a slope to the waterproof membrane, water pools over the liner of tile shower pans and just stays there. Over time that's the start of mold and mildew completely filling the shower base. Not a good thing.
That's why you need a layer of mortar for the waterproof membrane as base.
The pre-slope layer is installed above the felt layer and the mesh wire on the floor. It's installed to slope from the top of the drain base lip, up toward the shower walls. The slope must be 1/4 inch per foot. So, for a four foot shower, the mortar base will slope up to a minimum of 1 inch high on the shower walls. Some pros suggest that the drain should not be set on the subfloor, The idea is that the drain base should be installed 1 1/4 inches above the subfloor. Then mortar is packed under the drain base. Then the mortar is sloped to the wall from the lip of the drain base.
That means the first mortar layer would be about 1 1/4 inches thick under the drain base, sloping to about 2 1/4 inches thick at the walls for a four foot shower.
Why so thick under the drain base? Simple. Just to put enough mass in the mortar layer at the thinnest point so it won't crumble and come apart.
Here's how the mud is installed...
To see how to pack the deck mud of tile shower pans... Click Here!!!