The Impact of Green Roofs on the Quality of Rainwater and Operational Problems – Case Study

Shopping centres usually occupy large areas of land development. However, their roofs are rarely used for purposes related to rainwater management and introduction of active biological surfaces. In Bielsko-Biała, thanks to the initiative of the originator and architect, an intense green roof was created on the rooftop of a shopping centre. In addition to the primary role of the roof as a garden that makes the housing estate more attractive, it also collects some of the rainwater and delays its outflow to the sewage system. Plants, shrubs and trees planted on the roof are watered almost exclusively with water collected in a tank located under the building, intended for retaining rainwater. These waters are pretreated before they reach the tank, i.e. they filter through the root layer of plants growing on the roof, and through the biologically active layer (substrate). This paper highlights some of the operating problems of the selected green roof. The authors focused on assessing the toxicity of rainwater flowing down from hardened surfaces on the roof of the shopping centre and the area occupied by the green roof. Relevant toxicity studies on these waters have been carried out in this regard. The research hypothesis that rainwater percolating through the selected soil material is not toxic and stimulates test plant growth was put forward. Tests of semichronic toxicity with the seeds of white mustard (Sinapis alba L.) prove that all the rainwater samples showed a stimulating effect on the test plant. The average root growth stimulation value during the period considered was –125%. It was confirmed that rainwater did not show toxic properties for white mustard seeds (Sinapis alba L.) and can be used for watering the garden constituting the green roof of a selected research facility. In the case of rainwater pH, it was found that the pH is adequate and has no toxic effect on the selected white mustard test plant (Sinapis alba L.). It confirms that the rainwater retained in this way can be used to water the green roof. The selected research facility meets the requirements set for green roofs.