The article presents the results of one year of study on a set of pilot green roofs. The roofs were monitored for outflow and water quality of the runoff. The flux data of green roofs were compared to the reference roof consisting of bituminous membrane. The results show similar findings as the literature data, slight release of solids and nutrients. However, the metals can be retained (Zn) or released (Cu, Ni). Atmospheric pollutants like PAH are strongly retained, but on the other side plasticiser from green roof structure can be released. If the obtained data are applied to an urban area of known composition, the substitution of all roofs by green roofs, leads to slight (+19%) increase in the oxydable matter, but significant increase (+60%) in phosphorus loads to the receiving environment. There might be a pseudo effect on heavy metals discharge, but this is rather due to replacement of metallic parts than to roof impact itself. If we consider the real possibilities of roof greening (less than 15% of the total roof surface in Paris) these effects become minimal.