A method of description of a classification process performed in membrane screens with vibrating sieves is presented in the paper. It is characteristic of these screens that the sieve, stretched on an immobile riddle, is induced pointwise to vibrate. Consequently, the amplitude distribution takes place on the vibrating surface. A result is a separation process that is different than in other screens. Results of studies on the height of material layer on a vibrating sieve surface are discussed in the paper. This is a next project in the research series dedicated to material classification on screens with vibrating sieves performed in the Technical University of Łódź.