The main objective of the project is to study the drainage-retar- ding capacity of the vegetated roofs and their effect on the water quality of the drainage water for Flemish climatic condi- tions. Besides that the development of the vegetation will be studied. The project is milited to extensive vegetated roofs but does take different slopes and orientations into account. To meet the ovjective the research consists of: - development of a water balance- and heavy metals balance model for vegetated roofs; - simulation of the effect of vegetated roofs on the quality and quantity of the drainage water. Measurements on an experimental set-up will be used to develop, test and validate the various submodels describing the processes of water drainage, retention, evaporation and uptake and the retention of heavy metals.