We have a flat roof with modified bitumen roof material. The roofer used rigid insulation sheathing (pink foam sheets) to building the pitch back up. This was applied over the plywood. When you walk on the roof it is soft underfoot. I am concerned over time this will create dips and hold water. Everything I read about the insulation sheathing is for wall assemblies. Can anyone tell me if this is a recommended installation for a flat roof? Or should other material have been used.