We propose a novel THz material analysis approach which consists of a combination of a stratified dispersive model, an appropriate measurement configuration and a time-domain based fitting procedure. The measurement configuration is modeled as a stratified system where for each layer the light-matter interaction is realistically described. The method is illustrated for the industrial quality control of (coated) paper sheets and is shown to give highly accurate results in humid air and irrespective of the position of the reference mirror in reflection.