An attachment collar (10) for sealing attachment of a roof-penetrating building structure in a roof, in particular a frame (1) of a skylight, to an exterior weather-shielding membrane (7) in an underroof structure, comprises mutually connected attachment members (11 - 14), each being provided with an interior edge flange (16) for fastening to a side face of the roof-penetrating building structure and an exterior side edge (20) in abutment against said underroof membrane (7). The attachment members (11 - 14) are made as flexible, substantially nonelastic lengths of material, and side attachment members (12, 14) intended for positioning transversely to battens (9) in the roof structure are manufactured with material surplus, e.g. by pleating or folding, in a direction parallel with said interior edge flange and exterior side edge (16, 20).