A waterproofing flashing and resulting juncture, comprising in a first embodiment a generally horizontally positioned planar member which is positioned in sandwiched engagement between the upper peripheral edge of a shower pan or shower bench sidewall (or equivalent wallboard) and a lower peripheral edge of adjacent wallboard (or equivalent shower bench, shelf or ledge), and a vertically extending member attached to a distal edge of the horizontal member extending vertically upwardly behind the wallboard (or equivalent shower bench, shelf or ledge). An alternative form of the invention comprises a T-shaped flashing member comprised of a mid-section to which are connected first and second attachment members or wings, the wings adapted to connect the flashing to the room-facing side of the wallboard and/or shower module sections. A further version of the invention comprises a waterproof juncture comprised of a water-impermeable membrane applied over at least the upper peripheral edge of a wallboard or shower pan sidewall and a portion of an adjacent wall-forming member which is coplanar with the wall studs, which in turn will form a waterproof covering over the seam between the upper peripheral edge of the wallboard or shower pan sidewall (or equivalent shower bench, shelf or ledge) and the adjacent wall-forming member.