Deckdrain is a geocomposite drainage layer comprising a high performance extruded, flexible cuspated HDPE core of various thicknesses and profile heights, with a nonwoven geotextile filter thermally bonded to one side. The geotextile has a flap that extends beyond the core on one edge. The product is practically impermeable one side. It is used as an engineered drainage layer behind retaining structures, on roof decks and in subsurface works. Deckdrain is also used as a void former to create drainage paths within concrete structures such as tunnel linings or under concrete paving.