The water supply system of the city of Zurich is planning major construction works at the Moos plant. The green roofs with nature conservation value (> 100 years old, with botanical values of cantonal importance (Landolt 2001)) are to be preserved wherever possible. The Urban Ecology Research Group has developed a concept for the removal, intermediate storage and reinstallation of green roof areas.Grün Stadt Zürich would like to have a comprehensive vegetation analysis carried out prior to the start of construction work as a basis for monitoring the success of the project after completion of the work. On the one hand, the focus will be on the population sizes of target species of nature conservation (all orchid species, approx. 10 other vascular plant species), which have already been exhibited by Elias Landolt as particularly important. At the same time, a vegetation-ecological study by means of vegetation surveys should open up the possibility of recognising changes in the entire species combination, even of species that have not been defined a priori as target species. On this basis, site changes can be detected at an early stage which may also become significant for the target species in the medium term.