A fastener apparatus includes a frusto-hemispherical head portion having a flat top surface, a curved outer surface, a base portion, and a peripheral edge, with the flat top surface bearing a tool fitting. A downwardly-depending lip portion extends from the head peripheral edge and defines an annular recess in the base, the annular recess having a flat upper surface. The lip has a curved outer edge, a curved lower edge, and an inner edge having a curved lower portion and a vertical upper portion terminating in the annular recess flat upper surface. A shank portion is connected to the base and bears screw threads and a tip. An annular gasket portion is carried on the shank, and has a first uncompressed position contained radially within the annular recess, and a second compressed position extending radially beyond the lip outer edge. As the fastener is installed in a workpiece, the gasket expands to contact the lip inner edge and extend beneath the curved lower edge to form a seal between the fastener head and the workpiece. The fastener may include locking teeth terminating in the head base, adapted to resist loosening of the fastener from the workpiece. The curved outer surface of the fastener may include asymmetric ridges adapted to mechanically engage moving snow or ice to resist loosening of the fastener from the workpiece. The fastener head preferably has a width to height ratio of between 3:1 to 5:1.