A roof window for installation in a roof structure of a building, wherein the roof window comprises: - a frame arrangement (2, 20), wherein said frame arrangement (2, 20) comprises a top profile, a bottom profile and two side profiles (2c, 2d), and - an insulating glass unit that is supported by the frame arrangement, wherein the insulating glass unit comprises at least a first glass sheet, a third glass sheet, and a second intermediate glass sheet placed between the first glass sheet and the third glass sheet, wherein the first glass sheet is configured to face the exterior of the building when the roof window is installed in said roof structure, wherein a first insulating gap is provided between a major surface (3a1) of the first, outer glass sheet, and a major surface (3b2) of the second, intermediate glass sheet, and where said first insulating gap is sealed by means of a first edge seal, wherein a second insulating gap is provided between another major surface (3b1) of the second, intermediate glass sheet, and a major surface (3c2, 3d2) of the third glass sheet (3c, 3d), and where the second insulating gap is sealed by means of a second edge seal, said roof window being characterised in that a side portion of a major surface (3b2) of the second, intermediate glass sheet extends with a first distance past a side edge of the first glass sheet and past an outer side surface of the first edge seal that faces away from the first insulating gap, so as to provide an elongated utility space, such as a water drainage channel, which overlaps said side portion and is placed opposite to the outer side surface of the first edge seal. - a frame arrangement (2, 20), wherein said frame arrangement (2, 20) comprises a top profile, a bottom profile and two side profiles (2c, 2d), and - an insulating glass unit that is supported by the frame arrangement, wherein the insulating glass unit comprises at least a first glass sheet, a third glass sheet, and a second intermediate glass sheet placed between the first glass sheet and the third glass sheet, wherein the first glass sheet is configured to face the exterior of the building when the roof window is installed in said roof structure, wherein a first insulating gap is provided between a major surface (3a1) of the first, outer glass sheet, and a major surface (3b2) of the second, intermediate glass sheet, and where said first insulating gap is sealed by means of a first edge seal, wherein a second insulating gap is provided between another major surface (3b1) of the second, intermediate glass sheet, and a major surface (3c2, 3d2) of the third glass sheet (3c, 3d), and where the second insulating gap is sealed by means of a second edge seal, said roof window being characterised in that a side portion of a major surface (3b2) of the second, intermediate glass sheet extends with a first distance past a side edge of the first glass sheet and past an outer side surface of the first edge seal that faces away from the first insulating gap, so as to provide an elongated utility space, such as a water drainage channel, which overlaps said side portion and is placed opposite to the outer side surface of the first edge seal. wherein a first insulating gap is provided between a major surface (3a1) of the first, outer glass sheet, and a major surface (3b2) of the second, intermediate glass sheet, and where said first insulating gap is sealed by means of a first edge seal, wherein a second insulating gap is provided between another major surface (3b1) of the second, intermediate glass sheet, and a major surface (3c2, 3d2) of the third glass sheet (3c, 3d), and where the second insulating gap is sealed by means of a second edge seal, said roof window being characterised in that a side portion of a major surface (3b2) of the second, intermediate glass sheet extends with a first distance past a side edge of the first glass sheet and past an outer side surface of the first edge seal that faces away from the first insulating gap, so as to provide an elongated utility space, such as a water drainage channel, which overlaps said side portion and is placed opposite to the outer side surface of the first edge seal.