Calculate Proper Slope
Like we already discussed, the main thing people worry about when building a concrete shower pan is getting the slope right. Obviously you need water to flow toward the drain, not away from it, so the rule of thumb is to have at least 1/8 inch of rise for every foot of run. Huh? Just figure out which corner is furthest from the drain and measure the distance. This tells you your “feet” measurement (the run). Now calculate the rise by multiplying the by 1/8″.
Ex: If it is 3 feet from the center of the drain to the furthest corner, then your minimum rise is 3/8″, but you’d be safer if you went even higher.
Now that you’ve determined rise, you must add it to the height of the shower drain to get the final height.