The aim of the study was to evaluate the possibility of using the precipitation index to estimate the outflows and moisture in two green roofs of extensive type with different composition of the vegetation layer (substrate). The research was carried out at the Water Center of Warsaw University of Life Sciences in Warsaw in the period of VIII–XI 2016. The analyzes used daily moisture changes measured by the TDR method. Statistically significant correlations were found between the changes of moisture content of vegetative mats and substrates and the index of precipitation. Such relationships were not observed for outflows from both green roof models and the precipitation indicator. Lower moisture values ??were recorded during the entire observation period of vegetation mat on SPG E-M substrate (mineral composition). The observed moisture differences in both substrates and vegetative mat may be due to the different substrate compositions. The SPG E-E substrate is a mineral-organic compound, while the SPG E-M substrate is a mineral materials.