The pre-slope sits on top of the subfloor and it is the start of your shower pans moisture management. It will set the proper negative slope for each layer that will sit able it. This slope will encourage water that travels from the shower walls and onto the shower floor to lead directly to the shower drain. The slope will further deter water from pooling in any area of the shower pan.
Why Install Pre-Slope
There are only two schools of thought as to whether you should install a pre-slope in your shower pan. The first is the belief that a pre-slope is not needed since the top layer of mortar mix will be sloped and the tile adhesive, grout, and tile will deter any significant amount of water from absorbing down to the flat water resistant barrier. The second understands that water absorption will occur at all top levels, therefore, when water does reach the water resistant barrier, it should be encouraged to lead downward to the shower drain. The question is, with a perfectly installed shower pan, how much water absorption will be allowed? A shower pan should be built with the intent to mitigate the risk of failure, therefore, a pre-slope might seem redundant, it remains to be an integral step in building a shower pan.
The most common method for constructing a pre-slope is with a mixed cement and sand deck mud that has a consistency that will allow it to be balled up in the palm in your hand. Using a trowel, a 2×4 wood piece, or even a level, you can pack the deck mud up to the desired height all around the shower pan. A wet consistency will not provide you with the ability to properly pack and smooth to a desired negative slope. An alternative option would be a pre-manufactured product that can be cut to the exact size of your shower pan and come with an accurate negative slope that leads to the shower drain.
You will need to check with your state and local building codes to determine the exact requirements for constructing a pre-slope in your shower pan. The rule of thumb is to measure the distance from the shower drain to the surrounding shower walls and corners, including the nearest high point on the shower curb, and for each foot, you will need to have a 1/4 inch negative slope. This will discourage moisture from pooling at the edges of your shower pan and encourage it to drain downward to the shower drain.