A fastener adapted for attaching roofing insulation to a sheet metal base has a shank of a nominal diameter terminating on the one end in a head and at the opposite end in a point and has a threaded portion on only a part of the shank, the threaded portion tapering inwardly in diameter in the direction of the point so as to originate and terminate in shank portions of the nominal diameter and the threaded portion having a plurality of longitudinal helical grooves formed therein.