A roof window system configured for being mounted in an opening in a roof structure of a building, said roof window system including a ventilation unit configured for being mounted adjacent to a roof window and adapted for providing ventilation of the interior of the building in which the roof window is mounted, where said roof window comprises at least one frame defining a frame plane and including a pane mounted in said frame, the frame comprising a top frame member intended for being located highest in the mounted state when seen in the direction of inclination of the roof structure, a bottom frame member opposite the top frame member, and two side frame members extending between the top frame member and the bottom frame member, said frame members together form a window structure delimiting a frame opening and each having an interior side intended for facing the interior in the mounted state, an exterior side intended for facing the exterior, an inner side facing the frame opening and an outer side facing away from the frame opening, where the ventilation unit is configured for being arranged primarily adjacent to or near the outer side of a frame member, and where said ventilation unit comprises a ventilator and a housing with a top side facing the exterior of the building and being prepared for accommodation of said ventilator, said housing comprising a lower side configured for being arranged adjacent to or at the exterior side of said frame member, said housing constitutes a separate module mounted adjacent to the roof window, the top side is positioned substantially in the plane of the frame plane, characterized in that the housing comprises a longitudinal channel, said longitudinal channel is adapted to be mounted onto the window frame, in parallel to the longitudinal edge of the top frame member, and the housing comprises a drainage channel extending from the longitudinal channel to the ventilator, such that it forms a positive angle with a bottom side of the longitudinal channel, defined in an opposite direction of the top side. where said roof window comprises at least one frame defining a frame plane and including a pane mounted in said frame, the frame comprising a top frame member intended for being located highest in the mounted state when seen in the direction of inclination of the roof structure, a bottom frame member opposite the top frame member, and two side frame members extending between the top frame member and the bottom frame member, said frame members together form a window structure delimiting a frame opening and each having an interior side intended for facing the interior in the mounted state, an exterior side intended for facing the exterior, an inner side facing the frame opening and an outer side facing away from the frame opening, where the ventilation unit is configured for being arranged primarily adjacent to or near the outer side of a frame member, and where said ventilation unit comprises a ventilator and a housing with a top side facing the exterior of the building and being prepared for accommodation of said ventilator, said housing comprising a lower side configured for being arranged adjacent to or at the exterior side of said frame member, said housing constitutes a separate module mounted adjacent to the roof window, the top side is positioned substantially in the plane of the frame plane, characterized in that the housing comprises a longitudinal channel, said longitudinal channel is adapted to be mounted onto the window frame, in parallel to the longitudinal edge of the top frame member, and the housing comprises a drainage channel extending from the longitudinal channel to the ventilator, such that it forms a positive angle with a bottom side of the longitudinal channel, defined in an opposite direction of the top side.