The retention of rainwater is one of the main functions served by green roofs in urban areas. One of the elements influencing the variability of rainwater retention on green roofs is the configuration of the roof, i.e. the combination of drainage and vegetation layers and plants. In the article, laboratory studies regarding the influence of the vegetation layer of the green roof on the retention of rainwater were carried out, and the influence of changes in the initial moisture content in extensive and intensive substrates on retention compared. The analysis of seven randomly selected substrates showed the runoff coefficients to range from 0.59 to 0.71. In the case of retention, statistically significant differences were observed in terms of the rainfall volume as well as initial moisture content.