Abstract: An arrangement for sealing an opening in a roof deck by mounting a baffle plate in the roof deck so as to provide a throughgoing ducting channel, the roof deck having at least one raised seam joint intersecting the opening, the baffle plate being dimensioned such that it extends beyond the periphery of the opening in substantially all directions thereby defining a zone of overlap which extends around substantially the entire periphery of the opening, the arrangement including a capping member for capping and sealing the area where the baffle plate, the raised seam joint and the opening converge, a backing and stiffening member for being positioned on the opposite surface of the roof deck from the baffle plate and at least partially within the zone of overlap, and a fastening device for extending through and securing together the capping member, the baffle plate, the roof deck and the backing and stiffening member within the zone of overlap.