A packed installation kit for use in the installation of a roof window system comprising two window units (11, 12) and configured to be built in side by side in an inclined roof structure such that the roof window system defines a common height and a combined width in the mounted condition, said installation kit comprising a number of installation assemblies (3, 4, 5, 6, 7, 8), each including one or more components, and optionally auxiliary equipment (96, 96A, 97, 98), characterised in that the components of a first set of installation assemblies (4, 5, 6, 7, 8) of the installation kit are accommodated in a first packaging, that the components of a second set of installation assemblies of the installation kit comprise two support beams (31, 32) accommodated side-by-side in a second packaging comprising a rectangular telescopic tube packaging having two parts (1031, 1032) of corrugated cardboard, a first part of the second packaging having a first length extending between an end portion and an opening, and a second part having a second length extending between an end portion and an opening, and the support beams (31, 32) having a length within a pre-defined range, that the total length of the second packaging as defined by the sum of the length of the first part and the length of the second part exceeds the length of the support beams (31, 32) such that an overlap between the second part and the first part is provided in a supply condition of the installation kit within said pre-defined range of lengths of the support beams (31, 32) of the second set of installation assemblies, and that means are provided to reinforce at least one of the end portions (10316, 10326), said reinforcing means being selected from the group comprising - provision of a plurality of cardboard layers at the end portion, - provision of a shock-absorbing element at the end portion, and - provision of an extended flap portion configured to overlap an exterior side of the second packaging. the components of a first set of installation assemblies (4, 5, 6, 7, 8) of the installation kit are accommodated in a first packaging, that the components of a second set of installation assemblies of the installation kit comprise two support beams (31, 32) accommodated side-by-side in a second packaging comprising a rectangular telescopic tube packaging having two parts (1031, 1032) of corrugated cardboard, a first part of the second packaging having a first length extending between an end portion and an opening, and a second part having a second length extending between an end portion and an opening, and the support beams (31, 32) having a length within a pre-defined range, that the total length of the second packaging as defined by the sum of the length of the first part and the length of the second part exceeds the length of the support beams (31, 32) such that an overlap between the second part and the first part is provided in a supply condition of the installation kit within said pre-defined range of lengths of the support beams (31, 32) of the second set of installation assemblies, and that means are provided to reinforce at least one of the end portions (10316, 10326), said reinforcing means being selected from the group comprising - provision of a plurality of cardboard layers at the end portion, - provision of a shock-absorbing element at the end portion, and - provision of an extended flap portion configured to overlap an exterior side of the second packaging. - provision of a plurality of cardboard layers at the end portion, - provision of a shock-absorbing element at the end portion, and - provision of an extended flap portion configured to overlap an exterior side of the second packaging.