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The Saxon research project MaNuGrün has examined ten roof systems in 222 trials – the findings help to better assess the impact of green roofs on the sewerage system and the water cycle.
Whether green roofs help to retain rainwater and relieve pressure on the sewerage system during heavy rainfall depends largely on the specific roof structure. This is demonstrated by the recently completed ‘MaNuGrün’ research project. A test facility was set up for this purpose, featuring ten different roof structures, on which a total of 222 heavy rainfall tests were carried out. The project involved Leipzig Waterworks, the Education and Demonstration Centre for Decentralised Infrastructure (BDZ), the Leipzig Research and Transfer Centre (FTZ Leipzig) at the Leipzig University of Applied Sciences (HTWK Leipzig), the Helmholtz Centre for Environmental Research (UFZ) and Dresden City Drainage (SeDD). The four-year project has been funded by the German Federal Environmental Foundation (DBU) since December 2021. Together, the industry partners have analysed the measurement results and laid the groundwork for better taking into account the impact of green roofs on rainwater management in future.
“MaNuGrün demonstrates how important it is to bring together the various areas of expertise from water management, research and knowledge transfer. Green roofs can thus be a key component of climate-adapted urban development. However, the extent of their contribution to rainwater management depends on the specific system and local requirements,” says Prof. Dr Ulrich Meyer, Managing Director of Wasserwerke.
Ten roof structures put through heavy rainfall testing
How much water do different green roofs retain? To what extent do they delay runoff? And what impact does this have on the hydraulic load on the sewerage system? To answer these questions, eight commercially available green roof systems, as well as a gravel roof and a conventional roof, were compared.
Leipzig Waterworks set up the test facility, installed the various roof systems and carried out the rainfall tests. From June to September 2025, a specially developed system simulated heavy rainfall events of varying intensity – ranging from a one-in-a-year to a one-in-a-century downpour. In addition, different initial conditions of the roofs were taken into account: The tests took place following pre-saturation at intervals of one, four or 72 hours. Sensors recorded, amongst other things, inflows and outflows.
Scientific supervision was provided by HTWK Leipzig and the UFZ, each with different areas of focus. HTWK Leipzig investigated, in particular, how the retention capacity of green roofs affects sewer hydraulics. The UFZ drew up water balances for the individual roof systems in order to record their water balance more accurately. The BDZ coordinated the project and was responsible for drawing up a practical guide for local authorities, municipalities and planning offices. Dresden’s urban drainage authority contributed to the funding of the project.
In this way, MaNuGrün combined the practical construction and operation of the test facility with the scientific analysis of the measurement data and the application of the results to municipal practice.
Not all green roofs are the same
The results show that all the green roofs investigated were able to significantly reduce peak runoff compared with conventional roofs and gravel roofs. An intensive, restricted green roof proved particularly effective. In one of the scenarios studied, a reduction in runoff of around 90 per cent and a delay in the peak runoff of around 20 minutes were measured. The green roofs also showed a significant reduction in the total volume of water that ran off. Retention was particularly high on roofs with a vegetation layer of at least ten centimetres.
At the same time, the studies make it clear that vegetation alone is not the decisive factor. Rather, substrate depth, roof structure, drainage and any restriction significantly influence how much rainwater a roof can retain and to what extent it can delay runoff. Which system is best suited therefore depends on the specific requirements for rainwater management.
Measurement data improves sewer network planning
A key benefit of the project’s findings is the ability to better reflect the impact of different green roofs in the planning of drainage systems. In standard sewer network models, green roofs have so far been taken into account in a somewhat oversimplified manner. However, the studies show that this can lead to actual runoff being significantly overestimated – in some of the cases studied, by up to eight times. “The aim of water-sensitive urban development is to achieve a balance between evaporation, runoff and groundwater recharge – similar to what occurs in nature. Rainwater should be retained, allowed to evaporate or seep away. Green roofs are an effective way of achieving this,” says Prof. Dr.-Ing. Hubertus Milke, Professor of Water Resources Management and Urban Water Management at HTWK Leipzig, who supervised the project on behalf of the university.
Based on the measurement data, HTWK Leipzig developed a modelling approach that depicts the storage, runoff and evaporation processes of a green roof more realistically. For most of the extensive green roofs studied, the calibration showed good to very good agreement between the simulation and the measurements. The results of the UFZ’s water balances complement the analysis of the water balance of the individual roof systems.
This represents an important step forward in practice: The more accurately the effects of green roofs can be modelled in sewer network models, the more reliably local authorities and drainage operators can assess the extent to which these measures can help relieve the burden on the sewerage system.
From research findings to decision-making support
To ensure that the findings can be utilised beyond the scope of the research project, the BDZ has produced a practical guide for local authorities, municipalities and planning offices. It compares the runoff behaviour of different roof types, quantifies runoff attenuation and time delay, and classifies the roof structures studied into different categories of effectiveness. “The aim is to support planners and private stakeholders in selecting suitable green roof systems and planning rainwater management measures,” says BDZ Managing Director Dr Gabriele Stich.
Prof. Dr.-Ing. Hubertus Milke
HTWK Leipzig, Head of the Institute for Hydraulic Engineering and Urban Water Management (IWS)
Phone: +49 341 3076-6230
E-Mail: hubertus.milke@htwk-leipzig.de
https://doi.org/10.5281/zenodo.21454393
https://www.htwk-leipzig.de/en/research-1/research-projects/manugreen Research Project MaNuGrün at HTWK Leipzig
The green roof test facility on the grounds of the BDZ e.V. Education and Demonstration Centre for D ...
Copyright: Leipziger Gruppe
A wide variety of irrigation trials were carried out on the ten green roof areas.
Copyright: Leipziger Gruppe
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