Microscopic models of the solid-on-solid type were considered to describe the surface pressure of monolayers an bilayers on top of another layer in two and three dimensions. The two interfaces, above and below the film, were studied in simultaneous thermal equilibrium without imposing a fixed profile for the first fluid or solid. It is shown that the associated mean field approximation leads to an exact determination of the surface pressure isotherms revealing the possible existence of plateaux, typical of first-order phase transitions.