Screw fasteners for fibrous materials

A screw fastener particularly adapted to improve retention while minimizing mushrooming in fibrous workpieces. A single-threaded lower portion neighbours the pointed tip of the screw and a dual-threaded upper portion neighbours the screw head. The upper portion has a first upper thread that spirals in an opposite direction to the singular lower thread, and a second upper thread that spirals in a same direction as the singular lower thread. The lower thread includes a plurality of thread turns that share a same major thread diameter while axially spanning multiple areas of the lower portion of the shaft that vary in diameter, including a bulged area, a reduced upper area of lesser shaft diameter than the bulged area at a location between the bulged area and the head, and a downwardly tapered lower area at which the diameter of the shaft narrows from the bulged area toward the pointed tip.