Indented Hex Washer 3/8" Across The Flats F/T Self Piercing Scew. Concrete screws cut their own threads when used in concrete, block or brick. Carbide bits are recommended for use in masonry. Flat head screws are used when protrusion of the fastener above the mating surface is unacceptable. Typically 100 degree flat or for fastening softer materials, 82 degrees flat.