A method for mounting a roof window in a roof structure comprising a roofing material, said roof window comprising a frame, a sash carrying a pane element, and said frame comprising a top frame member, two side frame members (22, 23) and a bottom frame member, said frame members together defining a frame opening and a frame plane and each extending in a length direction and each having an outer side facing away from the frame opening, where said method comprises the steps of A) Arranging the roof window in an opening in the roof structure, B) Arranging a plurality of flashing members so that they cover a joint between the frame of the roof window and the roof structure with a top flashing member extending along the top frame member, a bottom flashing member extending along the bottom frame member, and at least two side flashing members (1012, 1013) extending along the side frame members (22, 23), each flashing member being arranged with an inner section adjacent to a frame member (21, 22, 23, 24) and an outer section extending from the inner section away from the frame, and during step B) a flange of at least one side flashing member (1012, 1013) is inserted into a flashing reception groove in the outer side of at least one side frame member (22, 23), said flashing reception groove extending in the length direction of the side frame member, characterised in that three or more side flashing members (1012, 1013) are inserted into the flashing reception groove in the outer side of each side frame member (22, 23), that each side flashing member is arranged so that it overlaps with another side flashing member, that the inner section of each side flashing member (1012, 1013) comprises a gutter (1012g, 1013g) extending in the length direction, and that said gutter has the same depth over the entire extend of the side flashing member and the same depth in all side flashing members, so that when the side flashing members are mounted, a continuous gutter extending along length of the side frame member is formed. A) Arranging the roof window in an opening in the roof structure, B) Arranging a plurality of flashing members so that they cover a joint between the frame of the roof window and the roof structure with a top flashing member extending along the top frame member, a bottom flashing member extending along the bottom frame member, and at least two side flashing members (1012, 1013) extending along the side frame members (22, 23), each flashing member being arranged with an inner section adjacent to a frame member (21, 22, 23, 24) and an outer section extending from the inner section away from the frame, and during step B) a flange of at least one side flashing member (1012, 1013) is inserted into a flashing reception groove in the outer side of at least one side frame member (22, 23), said flashing reception groove extending in the length direction of the side frame member, characterised in that three or more side flashing members (1012, 1013) are inserted into the flashing reception groove in the outer side of each side frame member (22, 23), that each side flashing member is arranged so that it overlaps with another side flashing member, that the inner section of each side flashing member (1012, 1013) comprises a gutter (1012g, 1013g) extending in the length direction, and that said gutter has the same depth over the entire extend of the side flashing member and the same depth in all side flashing members, so that when the side flashing members are mounted, a continuous gutter extending along length of the side frame member is formed.