Today, urban runoff is considered as an important source of environment pollution. Roofing materials, in particular the metallic ones are considered as a major source of urban runoff contamination. An accurate evaluation of contaminant flows from roofs is thus required at the city scale. This paper aims to describe the definition of an appropriate methodology for evaluating the zinc emission at the city scale. This methodology is based on combining two different methods. The first one is an automatic classification and the second one is a theoretical urban study site. In order to obtain representative data, the choice of the study site was based on the diversity of land use and the urban and social context. Finally some results and future works will be presented.