Rock mineral wool-based green roofs to improve the quality of urban water runoff

Green roofs are nature-based solutions that allow greenery to be integrated into the building envelope, making it possible to re-nature cities while providing multiple benefits. However, whether green roofs are a source or sink of pollution in the urban environment is still a controversy. One of the causes of the possible deterioration of the quality of runoff water from green roofs is the substrate. Green roofs based on rock mineral wool (RMW) growing media require thinner substrate layers or can even be substrate-less. In the present study, four green roof systems based on RMW have been studied over the course of 2 years. Their performance, in terms of leachate quality, has been compared with two traditional roofs, a green roof with pozzolana as a draining material and a gravel-ballasted conventional flat roof. Limit values for wastewater quality from international regulations were considered benchmark. The main conclusions were that after the first flush, which was observed for all solutions, generally exceeding the limit values, RMW-based solutions performed better than traditional solutions. Furthermore, the average values of leachates from all tested green roofs and especially those from RMW solutions fall within the limits set by international regulations.