The seal is fitted in an aperture (7) of the roller shutter box at both sides of the armoured plate (6), facing the respective longitudinal edge of the aperture. The seal is formed by a swivelable strip (15, 16) suspended on each longitudinal sides of the aperture.The strip abuts with its free end against the roller shutter armoured plate, due to the position of its point of gravity w.r.t. its suspension. Pref. the two swivel strips are suspended from a profile (5, 10) extending along the aperture in the region of its top longitudinal edge.