Abstract: A mechanical fastener for securing a flexible sheet of an elastomeric material to the upper surface of a roof via the use of a plurality of such fasteners, each of which include upper and lower retaining members, wherein the latter are anchored to the roofing surface. Both of the retaining members are formed with a series of annular concentric ridges and grooves which cooperate with each other to form a series of annular gripping zones with the elastomeric sheet when the upper retainer is drawn into a clamping relationship with the botton retainer by a threaded fastener. The fastener is engaged with an internally threaded central boss formed on the lower retainer. A sharp bottom edge of the fastener cuts an aperture in the elastomeric sheet prior to engagement with the threaded boss.