The pan naturally bows in a concave fashion with the drain area being the lowest point due to the solid surface on the top of the pan and waffled structure on the bottom. This is beneficial because the drain portion of the pan is forced to sit tight to the drain support block after the perimeter screws are installed.
What if the pan is bowing the opposite way?
This would be a very rare occurrence, but the answer is to simply add weight (like a bag of thin-set) to the drain area of the pan while continuing your work. Leave weight on pan for 24 hrs before tiling. You can also screw through the pan around the perimeter of the drain recess. See next FAQ for details.
