Abstract: Apparatus and associated methods relate to a flexible membrane defining pathways for receiving a flexible conduit, and further defining attachment regions between the pathways formed with overhanging walls. In an illustrative example, the membrane may be vacuum-formed plastic sheet material. In some embodiments the flexible conduits may conduct electricity or heat transfer fluids, for example. In various implementations, the overhanging walls in the attachment regions may advantageously provide improved vertical attachment strength to a rigid planar substrate placed over a mortar filled into the attachment regions.