The window system has a substantially rectangular frame structure having four corner sections (1b) and being adapted to be installed in a roof structure and a bracket arrangement comprising a set of bracket units (6b), each bracket unit including a base element (10b) mounted at each corner section of the frame structure. Each bracket unit furthermore comprises at least one supplemental element (20b,40b) adapted to be detachably connected to said base element. One supplemental element comprises a leg element (20b) adapted to be detachably connected to the base element, preferably rotatably and detachably connected with the base element.