Abstract: A process for manufacturing impermeable strips formed by a plurality of layers in a continuous process wherein at least two single or compound flexible plastic layers e.g. polyethylene or thin webs are made to pass under pressure through the nip between two rollers rotating in opposite directions, the bonding material between the layers being asphalt, or an asphalt mixture, in fluid state distributed uniformly between the plastic surfaces to be joined, the heat of the fluid asphalt being prevented from deteriorating the plastic webs by their being cooled with the application of a water film to the respective surface of each plastic web opposite to the asphalt contacting surfaces.