Your structural screw won't set because the driver bit has rounded off in the head — the T-15 recess has wallowed out and the bit spins instead of drives. This is the most common failure on structural screw connections and it costs you time on a connection you thought was done. T-15 is the most common star drive size for structural and framing screws, and impact driver heat accelerates bit wear more than standard drivers. 2-inch T-15 star drive bit in red. The 2-inch length gives you enough reach for structural screws driven at an angle or into ledger connections without the bit bottoming out before the screw seats. T-15 is the most common driver size for structural screws, ledger screws, and composite deck fasteners — this one size covers the majority of structural fastening jobs on a fra Stocked at Crofton Lumber and Akron Lumber.