The invention is directed to a sealing device (1) comprising a waterproofing membrane (2) and a protective film (3) comprising a backing layer (4), which is directly attached to at least part of the bottom surface of the waterproofing membrane (2). The invention is also directed to a method for producing a sealing device, to use of a protective film comprising a backing layer for reducing blocking of a polymeric membrane, and to a method for covering a roof substrate using the sealing devices of the present invention.