Abstract: For shielding a window area (2) in a wall or an inclined roof surface (3) a roller screen, e.g. an insect net (1), is provided which comprises a spring-biassed roller bar (9) at the upper edge of the opening (2) and a bottom bar (13) at the lower edge of a flexible screen web (1). The bottom bar (13) has guide members (17) slidable in guide rails (11, 12) having at the bottom a locking device for arresting the bottom bar (13). At either end of the bottom bar, the guide members include a guide member (17) and a subjacent roll (18) pivotally journalled in the bottom bar (13) so that, from a first position in which it is in axial alignment with the guide member, the roll may be swung to a locking position outside the path of movement of the guide member (17). The rolls (18) are functionally connected and biassed so that the first position and the locking position are stable switch-over positions.