Green roofs are increasingly popular in both new and modernised buildings. They significantly reduce the outflow of stormwater from buildings and change its composition. Wherever an urbanised area is equipped with a separate sewage system, usually stormwater goes directly to the receiver without treatment, which may affect the quality of water in the receiver. The article presents results of research carried out on the green roof of a building in Lodz, Poland. During rainfall, the flow rate from the roof was measured. With the use of the US EPA software Stormwater Management Model (SWMM) a model of the green roof was created and calibrated using rainfall data from the city’s pluviometric network. Based on the measurements of the roof runoff, as well as SWMM modelling, the degree of outfall reduction was determined. Samples of roof runoff were collected to study the characteristics of rainwater, including pH, electrical conductivity, organic compounds, nitrogen, phosphorus, and suspended solids. The results were compared with the quality of runoff from a traditional roof. Except ammonium nitrogen, values of the examined quality indicators was higher in the case of the green roof but the pollution load of almost all contaminants, except phosphorus, were lower due to a significant reduction in the volume of stormwater outflow (62–91%). The quality of stormwater discharged from the green roof improved with its age.

Flood risk management are considerably influenced by several factors, such as all sources of flooding, social circum-stances, policy and even the potential for local economic growth. To encourage government, business, community and oth-er parties to continue investing in flood risk management projects, it is necessary to give understanding that the projects can also provide economic benefits through systematic predictions and assessments of costs, benefits and social values, espe-cially on flood-affected communities. This study aims: (1) to develop knowledge and understanding on small-scale flood risk management project in Malang City, Indonesia, and; (2) to assess the economic efficiency of the project investment considering all benefits, both monetary and non-monetary. The research method is a mixed method combining quantitative questionnaires (N = 53 from 162 families) with qualitative in-depth interviews (N = 10) and field observations. The runoff discharge and the inundation depth were calculated using hydrology and hydraulic analysis, while the economic efficiency was analysed using cost benefit analysis (CBA). The results show that the community-based flood risk management system can reduce the flood risk up to 30% compared to before the implementation of that system. This system also provides direct financial benefits through the use of drainage channels for fish and vegetables farming. It causes the increase of the net so-cial benefit about 70–90% and the net present value (NPV) greater than zero (NPV > 0). Therefore, the project investment is recommended to be proceeded.