Abstract: A system, flashing apparatus, and associated methods for protecting vertical butt joints in installed siding having locking profiles. The lower profiled edge of the siding is modified proximate the ends during manufacture or in a factory setting to create a modified section configured to mate with a unique piece of factory-produced or pre-made flashing of similar or matching profile. The flashing piece fits with the ends of the adjacent siding boards in a horizontal row and aligns with the profile of the locking edges of vertically adjacent siding boards so the system sheds water effectively and prevents water from penetrating behind the siding. In addition, the factory-modified siding and the pre-manufactured flashing pieces may be supplied as a ready-to-install system that simplifies the siding installation process and shortens the time and cost of installation.