I have a leaking shower pan. The pan was made by placing fiberglass matte over the subfloor and coating with two component resin. It is maybe 1/32" thick. The building inspector says this is the standard construction in our area. It is flat on the floor, with no slope to the drain. The tile base is on toop of the fibergalss and slopes to the drain. The pan leaked at two corners, probably due to settling or poor installation and the drain doesn't appear compatible. The drain is an Oatey Casper PVC tile drain which is designed to clamp on a flexible liner. There is no gasket or other soft material to clamp it to the fibergalss sheet, and so it cannot be a reliable seal. With a flexible membrane, the water that gets through the tile joints and cracks can run into four little weep holes around the clamping flange. There is no obvious way to make this work on a fiberglass sheet. Furthermore it seems the pan needs to be sloped to the weep holes so that the water doesn't stay trapped.