A door drop stop safety system which is adapted to be installed on existing building overhead doors which employ a central axle; slab doors, bifold doors, and rollup doors which contains structure for raising and lowering the the door including a rewind box including an inertia reel and a length of strap. The inertia reel allowing normal motion, but is adapted to lock as soon as a predetermined rate of rotation is reached due to the door beginning to free fall. The system includes structure means for mounting the rewind box on an existing door, including slab doors, bifold doors, and rollup doors.