A roof window system comprising at least two window units (11, 12) configured to be built in side by side, separated by a spacing, in a roof structure of a roof, in particular comprising an inclined roof surface, each window unit (11, 12) comprising a top member (111, 121), two side members (112, 113, 122, 123), and a bottom member (114, 124), the side members defining a common height and depth of the roof window system, said roof window system furthermore comprising a stabilizing assembly positioned in the spacing between neighbouring window units (11, 12) in the mounted condition of the roof window system, wherein the stabilizing assembly has a generally longitudinal extension and comprises a set of stabilizing rails (6; 6b; 1006; 1006b; 2006; 2006b), each stabilizing rail includes a strengthening profile, at least one of the stabilizing rails (6, 6b; 1006; 2006) of said set includes an insulating profile (61; 1061; 2061) fastened to the strengthening profile (62; 1062; 2062) of said at least one stabilizing rail, the strengthening profile (62; 1062; 2062) of each stabilizing rail (6; 6b; 1006; 1006b; 2006; 2006b) comprises means for connection to the side member (113, 122) of a respective one window unit (11, 12) of neighbouring window units (11, 12), wherein the strengthening profile comprises a connection flange with connection means (623, 624, 625, 626) for the connection of the stabilizing rail (6; 1006; 2006) to the window unit, and the insulating profile (61; 1061; 2061) of the at least one stabilizing rail provided with an insulating profile has such a thickness (tk, tb) and such a depth (dk, db) that the insulating profile or insulating profiles substantially span/s the gap and the depth of the spacing between facing side members (113, 122) of neighbouring window units (11, 12) in the mounted condition of the roof window system, the stabilizing rail (6; 1006; 2006) has a length substantially corresponding to the common height of the roof window system, characterised in that the insulating profile has a length corresponding to the full length of the stabilizing rail (6; 1006; 2006) and the strengthening profile has a length which is shorter than the full length of the stabilizing rail, preferably leaving an end portion at each end of the stabilizing rail in which the insulating profile protrudes beyond opposing ends of the strengthening profile, the length of said end portions being preferably about 10 to 60 mm, more preferably about 40 mm. the stabilizing assembly has a generally longitudinal extension and comprises a set of stabilizing rails (6; 6b; 1006; 1006b; 2006; 2006b), each stabilizing rail includes a strengthening profile, at least one of the stabilizing rails (6, 6b; 1006; 2006) of said set includes an insulating profile (61; 1061; 2061) fastened to the strengthening profile (62; 1062; 2062) of said at least one stabilizing rail, the strengthening profile (62; 1062; 2062) of each stabilizing rail (6; 6b; 1006; 1006b; 2006; 2006b) comprises means for connection to the side member (113, 122) of a respective one window unit (11, 12) of neighbouring window units (11, 12), wherein the strengthening profile comprises a connection flange with connection means (623, 624, 625, 626) for the connection of the stabilizing rail (6; 1006; 2006) to the window unit, and the insulating profile (61; 1061; 2061) of the at least one stabilizing rail provided with an insulating profile has such a thickness (tk, tb) and such a depth (dk, db) that the insulating profile or insulating profiles substantially span/s the gap and the depth of the spacing between facing side members (113, 122) of neighbouring window units (11, 12) in the mounted condition of the roof window system, the stabilizing rail (6; 1006; 2006) has a length substantially corresponding to the common height of the roof window system, characterised in that the insulating profile has a length corresponding to the full length of the stabilizing rail (6; 1006; 2006) and the strengthening profile has a length which is shorter than the full length of the stabilizing rail, preferably leaving an end portion at each end of the stabilizing rail in which the insulating profile protrudes beyond opposing ends of the strengthening profile, the length of said end portions being preferably about 10 to 60 mm, more preferably about 40 mm.