A window installation system, comprising: a roof window including a window frame having a top frame member, a bottom frame member defining an inner width (wi) of the window frame, and opposing first and second side frame members, defining a central opening, each of the frame members (300, 200, 100) having an inner face including an interior portion (301, 201, 101) and an exterior portion (202, 102) facing the central opening, and a sash having a top sash member, a bottom sash member and opposing first and second side sash members, together encasing a pane and connected to the window frame in a mounted condition by means of a set of hinges, the bottom frame member having a pre-defined width (wbf) and the side frame member a pre-defined width (wsf), and a kit of parts for protecting the window frame in a window installation process bringing the roof window to the mounted condition, in which the kit of parts comprises at least one protection member forming a protection kit, characterized in that said at least one protection member is adapted to be positioned on the bottom frame member to cover the pre-defined width (wbf) of the bottom frame member, in that said bottom protection member has a base portion with an interior longitudinal edge and an opposed exterior longitudinal edge, each extending between one end edge and another end edge and defining a pre-defined length (lb), the base portion of the bottom protection member having a pre-defined outer width between the interior longitudinal edge and the exterior longitudinal edge, and a pre-defined inner width, said pre-defined inner width being smaller than the pre-defined outer width and smaller than or substantially equal to the pre-defined width (wbf) of the bottom frame member, in that the pre-defined inner width is defined as a distance between at least one interior engagement flange extending from the base portion at the interior longitudinal edge and at least one exterior engagement flange extending from the base portion at the exterior longitudinal edge, in that the bottom protection member is provided with at least one reinforcement section (115, 116; 118) in connection with the base portion, and in that during the window installation process bringing the roof window to the mounted position, the protection kit is configured to support the sash by at least the top sash member being in contact with the protection kit and to engage sash hinge parts with corresponding frame hinge parts by allowing the protection kit to guide the sash. a roof window including a window frame having a top frame member, a bottom frame member defining an inner width (wi) of the window frame, and opposing first and second side frame members, defining a central opening, each of the frame members (300, 200, 100) having an inner face including an interior portion (301, 201, 101) and an exterior portion (202, 102) facing the central opening, and a sash having a top sash member, a bottom sash member and opposing first and second side sash members, together encasing a pane and connected to the window frame in a mounted condition by means of a set of hinges, the bottom frame member having a pre-defined width (wbf) and the side frame member a pre-defined width (wsf), and a kit of parts for protecting the window frame in a window installation process bringing the roof window to the mounted condition, in which the kit of parts comprises at least one protection member forming a protection kit, characterized in that said at least one protection member is adapted to be positioned on the bottom frame member to cover the pre-defined width (wbf) of the bottom frame member, in that said bottom protection member has a base portion with an interior longitudinal edge and an opposed exterior longitudinal edge, each extending between one end edge and another end edge and defining a pre-defined length (lb), the base portion of the bottom protection member having a pre-defined outer width between the interior longitudinal edge and the exterior longitudinal edge, and a pre-defined inner width, said pre-defined inner width being smaller than the pre-defined outer width and smaller than or substantially equal to the pre-defined width (wbf) of the bottom frame member, in that the pre-defined inner width is defined as a distance between at least one interior engagement flange extending from the base portion at the interior longitudinal edge and at least one exterior engagement flange extending from the base portion at the exterior longitudinal edge, in that the bottom protection member is provided with at least one reinforcement section (115, 116; 118) in connection with the base portion, and in that during the window installation process bringing the roof window to the mounted position, the protection kit is configured to support the sash by at least the top sash member being in contact with the protection kit and to engage sash hinge parts with corresponding frame hinge parts by allowing the protection kit to guide the sash.