First Name:
Elena
Last Name:
Cristiano
Type of BP
Technical solution
Typology of Non- Conventional Water Resources (NCWR) What kind of NCWR do you deal with ?:
Rainwater (RW)
What is the collection area (m2)?:
16
What is the material of the collection area?
Water percolated from the soil, collected in the storage layer (PVC roofing membrane)
What is the average annual rainfall (mm)?:
The average volume of water harvested per year varies depending on the rainfall distribution and management of the valve. If the valve is always closed 1.6 m3
What is the material of the storage tank?:
PVC roofing membrane
What is the storage area of the collected rainwater (m3)?:
The storage capacity of harvested rainwater is 16
Number of population concerned by the reuse
40
What are the challenges raised by your Best Practice?
Environmental pollution, Degradation of the Ecosystem (water stress), Vulnerability to climate Change, Other (Please specify below)
Other challenges
Green area degradation
How could you describe your Best Practice?
A polder roof is a type of multilayer blue–green roof designed to temporarily store rainwater on the roof and release it in a controlled manner, mimicking the functioning of land polder systems used in water management. The concept originates from polder systems in low-lying areas (particularly in the Netherlands), where water is retained within a bounded area and drained later through controlled outlets. On a building, the roof acts as the “polder”.
A polder roof prototype (16 m²) has been installed at the University of Cagliari since June 2019. The system consists of CAM cactus vegetation, an 8 cm soil layer (sandy loam), a root-resistant geotextile, and a 10 cm storage layer regulated by a gate that can be remotely controlled.
This nature-based solution helps reduce runoff generation in urban environments and provides high water retention capacity. In addition to the benefits of traditional green roofs, this system allows the collection of rainwater which, after appropriate treatment, can be reused for non-potable domestic purposes. The protype installed at the University of Cagliari is monitored with multiple sensors:
• HOBO thermometers to measure the temperature of the air, soil and structure
• soil moisture ensors (CS650/Drill Drop on datalogger),
• 2 thermal cameras (HIKVISION),
• 1 OPTRIS pyrometer,
• Baro-Diver to measure the outflow.
A polder roof prototype (16 m²) has been installed at the University of Cagliari since June 2019. The system consists of CAM cactus vegetation, an 8 cm soil layer (sandy loam), a root-resistant geotextile, and a 10 cm storage layer regulated by a gate that can be remotely controlled.
This nature-based solution helps reduce runoff generation in urban environments and provides high water retention capacity. In addition to the benefits of traditional green roofs, this system allows the collection of rainwater which, after appropriate treatment, can be reused for non-potable domestic purposes. The protype installed at the University of Cagliari is monitored with multiple sensors:
• HOBO thermometers to measure the temperature of the air, soil and structure
• soil moisture ensors (CS650/Drill Drop on datalogger),
• 2 thermal cameras (HIKVISION),
• 1 OPTRIS pyrometer,
• Baro-Diver to measure the outflow.
Please describe your Best Practice in 5 keywords?
Harvesting; Water reuse;, Sustainable urban development;, Runoff mitigation; , rainwater.
Please provide any links to useful documentations (including website)presenting your Best Practice
In which area has your Best Practice been implemented ?
Urban area, Other (Please specify below)
Other area
Dense Urban
Best Practice location implementation (Country)
Italy
Localisation
POINT (9.11052189302 39.228627219025)
Who are the beneficiaries and/or the target group of your Best Practice ?
Local Population, Municipalities, Other (Please specify below)
Other beneficiaries
University community, including staff, students, attendees, etc.
f the Best Practice has been implemented within a partnership, who were your partners ?
• Metropolder Company (Now Wavin)
• University of Palermo, University of Tuscia, Foreners University of Perugia
• University of Palermo, University of Tuscia, Foreners University of Perugia
Have you involved stakeholders?
No
What are the obstacles to implementation of Best Practice ?
High cost of technological solutions
Did you receive funding for the research and development of the proposed BP?
Yes
Please indicate the source of funding
EU funding, National funding
What difficulties you have faced to access the funding ?
Installed as part of an EU project (no initial cost), but it requires funding for maintenance and upgrade.
Has your Best Practice been validated/upscaled?
Yes, the Best Practice has been validated through field monitoring and modelling, although it has not yet been physically upscaled and currently remains at the prototype stage.
The performance of the PolderRoof installed at the University of Cagliari has been validated through long-term field monitoring between 2019 and 2025. During this period, rainfall and outflow data were continuously collected to assess the hydrological behaviour of the system and its ability to retain and regulate stormwater runoff (Cristiano et al. 2022).
The experimental observations were supported by the calibration of the EHSM ecohydrological model developed by Viola et al. (2017) and Cristiano et al. (2023). The calibrated model confirmed the retention performance of the green roof under different rainfall conditions, including extreme precipitation events.
Additional validation was carried out through field experiments conducted in 2021, which included both artificial and natural rainfall events. These tests enabled detailed analyses of water quantity and water quality, providing further evidence of the system’s effectiveness in stormwater retention and pollutant mitigation.
Although the system has not yet been implemented at large scale, its potential impacts under large-scale deployment have been investigated through scenario analyses. In particular, the costs and benefits of large-scale green roof implementation for stormwater management in cities worldwide have been analysed by Cristiano et al. (2023b). The results highlight the potential contribution of green roofs to urban flood mitigation and sustainable stormwater management.
Overall, the Best Practice has been scientifically validated through monitoring, experiments, and modelling, while large-scale implementation remains a future development opportunity currently supported by research-based assessments of its potential benefits.
Cristiano, E., Annis, A., Apollonio, C., Pumo, D., Urru, S., Viola, F., ... & Nardi, F. (2022). Multilayer blue-green roofs as nature-based solutions for water and thermal insulation management. Hydrology Research, 53(9), 1129-1149.
Cristiano, E., Lai, F., Deidda, R., & Viola, F. (2023a). Management strategies for maximizing the ecohydrological benefits of multilayer blue-green roofs in mediterranean urban areas. Journal of Environmental Management, 343, 118248.
Cristiano, E., Farris, S., Deidda, R., & Viola, F. (2023b). How much green roofs and rainwater harvesting systems can contribute to urban flood mitigation?. Urban Water Journal, 20(2), 140-157.
Viola, F., Hellies, M., & Deidda, R. (2017). Retention performance of green roofs in representative climates worldwide. Journal of Hydrology, 553, 763-772.
The performance of the PolderRoof installed at the University of Cagliari has been validated through long-term field monitoring between 2019 and 2025. During this period, rainfall and outflow data were continuously collected to assess the hydrological behaviour of the system and its ability to retain and regulate stormwater runoff (Cristiano et al. 2022).
The experimental observations were supported by the calibration of the EHSM ecohydrological model developed by Viola et al. (2017) and Cristiano et al. (2023). The calibrated model confirmed the retention performance of the green roof under different rainfall conditions, including extreme precipitation events.
Additional validation was carried out through field experiments conducted in 2021, which included both artificial and natural rainfall events. These tests enabled detailed analyses of water quantity and water quality, providing further evidence of the system’s effectiveness in stormwater retention and pollutant mitigation.
Although the system has not yet been implemented at large scale, its potential impacts under large-scale deployment have been investigated through scenario analyses. In particular, the costs and benefits of large-scale green roof implementation for stormwater management in cities worldwide have been analysed by Cristiano et al. (2023b). The results highlight the potential contribution of green roofs to urban flood mitigation and sustainable stormwater management.
Overall, the Best Practice has been scientifically validated through monitoring, experiments, and modelling, while large-scale implementation remains a future development opportunity currently supported by research-based assessments of its potential benefits.
Cristiano, E., Annis, A., Apollonio, C., Pumo, D., Urru, S., Viola, F., ... & Nardi, F. (2022). Multilayer blue-green roofs as nature-based solutions for water and thermal insulation management. Hydrology Research, 53(9), 1129-1149.
Cristiano, E., Lai, F., Deidda, R., & Viola, F. (2023a). Management strategies for maximizing the ecohydrological benefits of multilayer blue-green roofs in mediterranean urban areas. Journal of Environmental Management, 343, 118248.
Cristiano, E., Farris, S., Deidda, R., & Viola, F. (2023b). How much green roofs and rainwater harvesting systems can contribute to urban flood mitigation?. Urban Water Journal, 20(2), 140-157.
Viola, F., Hellies, M., & Deidda, R. (2017). Retention performance of green roofs in representative climates worldwide. Journal of Hydrology, 553, 763-772.
Is there the potential to exploit/outscale the Best Practice?
Yes, there is strong potential for both replication and upscaling. The PolderRoof system developed by MetroPolder Company and tested at the University of Cagliari is based on modular and adaptable design principles, which allow it to be implemented on a wide range of existing or new buildings.
The technology can be easily replicated at the building scale, as it relies on components that are already widely used in green roof construction, such as vegetated substrate layers and drainage systems, combined with a controllable storage layer. This modular structure makes it feasible to install the system on different roof sizes and building types, particularly in dense urban areas where additional ground-level green infrastructure may be limited.
In addition, the system has significant potential for upscaling at the district or city level. Large-scale adoption across multiple buildings could contribute to urban stormwater management, reducing runoff volumes and peak flows during intense rainfall events. At the same time, widespread implementation could enhance urban climate resilience, biodiversity, and energy performance of buildings.
The prototype installed at the University of Cagliari demonstrates that such systems can be effectively adapted to Mediterranean climatic conditions, suggesting that similar solutions could be replicated in other cities with comparable climates and increasing exposure to extreme rainfall events.
Overall, the combination of modular design, relatively low operational requirements, and multiple environmental benefits makes the PolderRoof a solution with high potential for large-scale replication in urban environments.
The technology can be easily replicated at the building scale, as it relies on components that are already widely used in green roof construction, such as vegetated substrate layers and drainage systems, combined with a controllable storage layer. This modular structure makes it feasible to install the system on different roof sizes and building types, particularly in dense urban areas where additional ground-level green infrastructure may be limited.
In addition, the system has significant potential for upscaling at the district or city level. Large-scale adoption across multiple buildings could contribute to urban stormwater management, reducing runoff volumes and peak flows during intense rainfall events. At the same time, widespread implementation could enhance urban climate resilience, biodiversity, and energy performance of buildings.
The prototype installed at the University of Cagliari demonstrates that such systems can be effectively adapted to Mediterranean climatic conditions, suggesting that similar solutions could be replicated in other cities with comparable climates and increasing exposure to extreme rainfall events.
Overall, the combination of modular design, relatively low operational requirements, and multiple environmental benefits makes the PolderRoof a solution with high potential for large-scale replication in urban environments.
Do you have or know any platform of sharing Best Practice that you would like to link to this inventory platform?
No
Does your Best Practice contribute to an innovation? If so, please provide a short description of the innovative component
The PolderRoof system represents an innovative approach that integrates water management, ecology, and architecture, transforming a conventional roof into a multifunctional blue-green infrastructure.
The innovation lies in the concept of a controlled-storage green roof, where a vegetated layer is combined with a regulated water storage system. The first Mediterranean prototype exemplifies this approach by integrating a green roof layer consisting of 8 cm of sandy soil planted with drought-resistant CAM cacti with a 10 cm controllable storage layer. This storage layer is equipped with a remote-controlled gate, which allows operators to regulate the amount of water retained or released from the roof. This capability is crucial for dynamically managing the available storage capacity, optimizing stormwater retention before rainfall events and enabling runoff capture, reuse, and improved hydrological performance.
Beyond stormwater management, the system simultaneously supports building insulation, urban biodiversity, and climate adaptation benefits, demonstrating how roof surfaces can be transformed into multifunctional urban infrastructure.
Another innovative component of the prototype is the advanced environmental monitoring system installed on the roof. This system includes four HOBO temperature sensors, soil moisture sensors (CS650 connected to a Drill Drop datalogger), thermal cameras, a pyrometer, and a Baro-Diver pressure sensor. These instruments allow continuous monitoring of thermal performance, soil moisture dynamics, and hydrological behaviour, providing high-resolution data to evaluate the roof’s performance under Mediterranean climate conditions and during extreme rainfall events.
Together, the controlled storage technology and integrated monitoring platform make the PolderRoof prototype a cutting-edge example of blue-green infrastructure, advancing research and practical implementation of climate-resilient urban water management systems.
The innovation lies in the concept of a controlled-storage green roof, where a vegetated layer is combined with a regulated water storage system. The first Mediterranean prototype exemplifies this approach by integrating a green roof layer consisting of 8 cm of sandy soil planted with drought-resistant CAM cacti with a 10 cm controllable storage layer. This storage layer is equipped with a remote-controlled gate, which allows operators to regulate the amount of water retained or released from the roof. This capability is crucial for dynamically managing the available storage capacity, optimizing stormwater retention before rainfall events and enabling runoff capture, reuse, and improved hydrological performance.
Beyond stormwater management, the system simultaneously supports building insulation, urban biodiversity, and climate adaptation benefits, demonstrating how roof surfaces can be transformed into multifunctional urban infrastructure.
Another innovative component of the prototype is the advanced environmental monitoring system installed on the roof. This system includes four HOBO temperature sensors, soil moisture sensors (CS650 connected to a Drill Drop datalogger), thermal cameras, a pyrometer, and a Baro-Diver pressure sensor. These instruments allow continuous monitoring of thermal performance, soil moisture dynamics, and hydrological behaviour, providing high-resolution data to evaluate the roof’s performance under Mediterranean climate conditions and during extreme rainfall events.
Together, the controlled storage technology and integrated monitoring platform make the PolderRoof prototype a cutting-edge example of blue-green infrastructure, advancing research and practical implementation of climate-resilient urban water management systems.
What technolog(ies) and/or tool(s) has(ve) been used for your Best Practice ?
MBGR, Multilayer Blue – Green roof, CAM, Acid metabolism, HOBO, Thermometers, CS650, Moisture sensor, HIKVISION, Thermal camera, OPTRIS, Pyrometer
Please indicate the TRL associated with your Best Practice
TRL7 : System prototype demonstration in operational environment
Flow rate (m3/day)
1.50
Flow rate
During controlled experiments: Natural events (1,5 m3/d) Synthetic events - Tap Water (39 m3/d) Inflow based on average annual rainfall: 0,2 m3/d Inflow based on average rainfall event intensity: 0.19-0.76 m3/d
What is the necessary area to implement your Best Practice (m2) ?
16.00
Suspended solids SS (mg/l)
0.00
Comment : Suspended solids SS
During controlled experiments: Natural events (0 mg/L) Synthetic events - Tap Water (0 mg/L)
COD (mg/l), Chemical Oxygen Demand
0.00
Comment : COD
During controlled experiments: Natural events (0 mg/L) Synthetic events - Tap Water (0 mg/L)
Comment : BOD5
NA
Comment : Phosphorus content
NA
Nitrogen content (mg/l)
0.30
During controlled experiments NH₄⁺: Natural events (0.3 mg/L) Synthetic events - Tap Water (0 mg/L)
Pathogens
none
Flow rate (m3/day) of treated NCW
1.50
Flow rate of treated NCW
Not applicable. Outflow depends on rainfall distribution and antecedent soil moisture conditions. An arbitrary flow rate (m³/day) has been set at 1.5, as no suitable value is available for the platform.
Efficiency (BOD5 % Removal)
NA
Comment : Efficiency (COD % Removal):
Increase up to 400 mg/L
Efficiency (SS % Removal):
NA
Efficiency (Salinity % Removal)
Increase up to 10^3 mg/L
Comment : Other (% Removal)
NA
What is the impact on the beneficiaries of your Best Practice ?:
The Polder Roof provides multiple benefits to its beneficiaries by combining urban flood mitigation, climate adaptation, and environmental co-benefits.
By temporarily storing and regulating rainwater during intense precipitation events, the system significantly contributes to urban flood risk reduction, particularly during extreme rainfall (IOR95 = 0.38). This controlled retention and delayed release of stormwater reduces peak runoff entering the drainage network, helping cities become more resilient to climate-induced extreme events.
Beyond flood mitigation, the Polder Roof enhances the quality of life for building users and the surrounding community. The vegetated roof improves urban aesthetics and biodiversity, creating green space in dense urban environments while supporting ecological functions. In addition, vegetation and substrate contribute to air quality improvement and help mitigate the urban heat island effect through evapotranspiration processes.
The system also delivers energy benefits for the building by increasing latent heat flux and evaporative cooling, which can reduce indoor temperatures and potentially lower energy demand for cooling.
Overall, the Polder Roof installed at the University of Cagliari demonstrates how innovative green infrastructure can simultaneously address stormwater management, climate resilience, environmental quality, and energy efficiency, generating tangible benefits for both urban residents and local ecosystems.
By temporarily storing and regulating rainwater during intense precipitation events, the system significantly contributes to urban flood risk reduction, particularly during extreme rainfall (IOR95 = 0.38). This controlled retention and delayed release of stormwater reduces peak runoff entering the drainage network, helping cities become more resilient to climate-induced extreme events.
Beyond flood mitigation, the Polder Roof enhances the quality of life for building users and the surrounding community. The vegetated roof improves urban aesthetics and biodiversity, creating green space in dense urban environments while supporting ecological functions. In addition, vegetation and substrate contribute to air quality improvement and help mitigate the urban heat island effect through evapotranspiration processes.
The system also delivers energy benefits for the building by increasing latent heat flux and evaporative cooling, which can reduce indoor temperatures and potentially lower energy demand for cooling.
Overall, the Polder Roof installed at the University of Cagliari demonstrates how innovative green infrastructure can simultaneously address stormwater management, climate resilience, environmental quality, and energy efficiency, generating tangible benefits for both urban residents and local ecosystems.
Comment : Total Cost:
Provided for free as part of an EU project. Average cost: between 250 € and 450 € per m²
Comment : O&M Cost:
No operational costs. Sensors upgrade from Wavin: 7000 euro
Capital Cost
Provided for free as part of an EU project
0.00
Comment : Average Energy consumption:
To open &close the gate: 0.00025 kWh If the storage layer is full : 0.00020 kWh/m3
Garden Irrigation
NCW not sold
Price of treated NCW
NCW not sold
How your Best Practice is economically feasible ?
Although the PolderRoof system has relatively high initial construction costs, it remains economically feasible thanks to the multiple benefits it provides over its lifetime.
A key factor is the selection of drought-resistant CAM vegetation, which can survive with minimal water input. Combined with the controllable storage layer, this vegetation strategy allows plants to persist without intensive maintenance, effectively eliminating most long-term vegetation management costs. As a result, the system requires very limited operation and maintenance (O&M) compared with many conventional green roof solutions.
Operational costs are further reduced thanks to the automated control gate and minimal irrigation needs, which allow the roof to regulate stormwater storage and release without continuous manual intervention. At the same time, the system provides significant hydrological value by retaining and delaying stormwater runoff, thereby contributing to urban flood mitigation and reducing pressure on drainage infrastructure.
In addition, the presence of integrated filtration and storage capacity enables the reuse of retained rainwater for irrigation and other non-potable uses, generating water savings and further improving the economic performance of the system.
Overall, the combination of low maintenance requirements, water reuse opportunities, and the ecosystem services provided by stormwater retention helps offset the initial capital expenditure, making the PolderRoof a cost-effective solution over its operational lifetime.
A key factor is the selection of drought-resistant CAM vegetation, which can survive with minimal water input. Combined with the controllable storage layer, this vegetation strategy allows plants to persist without intensive maintenance, effectively eliminating most long-term vegetation management costs. As a result, the system requires very limited operation and maintenance (O&M) compared with many conventional green roof solutions.
Operational costs are further reduced thanks to the automated control gate and minimal irrigation needs, which allow the roof to regulate stormwater storage and release without continuous manual intervention. At the same time, the system provides significant hydrological value by retaining and delaying stormwater runoff, thereby contributing to urban flood mitigation and reducing pressure on drainage infrastructure.
In addition, the presence of integrated filtration and storage capacity enables the reuse of retained rainwater for irrigation and other non-potable uses, generating water savings and further improving the economic performance of the system.
Overall, the combination of low maintenance requirements, water reuse opportunities, and the ecosystem services provided by stormwater retention helps offset the initial capital expenditure, making the PolderRoof a cost-effective solution over its operational lifetime.
Is your Best Practice economically viable ?
8
Is your Best Practice environmentally sustainable ?
10
If there was a sustainability assessment carried out, what are the result of this assessment ?
No sustainability assessment has been carried out
Number of jobs created &/or preserved
4.00
Comment : Number of jobs created &/or preserved
3-4 researcher positions during several projects
Please indicate the other various social impact of your Best Practice :
Social Cohesion, Preservation of living environment, Improvment of health conditions
To which Sustainable Development Goals (SDGs) your Best Practice contributes?
SDG6: Clean Water and Sanitation, SDG7: Affordable and Clean Energy, SDG11: Sustainable Cities and Communities, SDG13: Climate Action
The intensive CAM green roof at the University of Cagliari has provided several valuable lessons from its design, implementation, and long-term monitoring. The spontaneous CAM vegetation (Agave americana) has proven highly effective in Mediterranean climates, achieving a retention Index (IOR) of 0.52 without irrigation or regular maintenance. While conventional C3 vegetation can reach slightly higher retention (IOR ≈ 0.71), it requires substantial watering and ongoing care, demonstrating that CAM species offer a low-maintenance, cost-effective alternative for urban green roofs.
Continuous high-resolution monitoring of rainfall and outflow, combined with calibration of the EHSM ecohydrological model, has been essential to validate system performance. The monitoring revealed that antecedent soil moisture strongly influences retention during extreme rainfall events: dry substrate maximizes retention, whereas saturated conditions reduce available storage. This highlights the importance of considering initial soil moisture and operational strategies when designing or evaluating green roofs for stormwater management.
The roof has also demonstrated long-term viability as a self-sustaining system, with its combination of deep sandy substrate and CAM vegetation enabling effective water retention and regulation over decades, without the need for irrigation. Beyond hydrological performance, the roof provides ecological and aesthetic benefits, making it a multifunctional urban infrastructure.
Finally, the University of Cagliari CAM green roof serves as a benchmark for future innovations, such as the CAM PolderRoof, which integrates water reuse tanks while maintaining low-maintenance vegetation and high stormwater retention, showing the potential for scaling and replication in urban Mediterranean environments.
Continuous high-resolution monitoring of rainfall and outflow, combined with calibration of the EHSM ecohydrological model, has been essential to validate system performance. The monitoring revealed that antecedent soil moisture strongly influences retention during extreme rainfall events: dry substrate maximizes retention, whereas saturated conditions reduce available storage. This highlights the importance of considering initial soil moisture and operational strategies when designing or evaluating green roofs for stormwater management.
The roof has also demonstrated long-term viability as a self-sustaining system, with its combination of deep sandy substrate and CAM vegetation enabling effective water retention and regulation over decades, without the need for irrigation. Beyond hydrological performance, the roof provides ecological and aesthetic benefits, making it a multifunctional urban infrastructure.
Finally, the University of Cagliari CAM green roof serves as a benchmark for future innovations, such as the CAM PolderRoof, which integrates water reuse tanks while maintaining low-maintenance vegetation and high stormwater retention, showing the potential for scaling and replication in urban Mediterranean environments.
Have you any recommendation to add?
To promote the wider adoption of green roofs, policy measures should combine financial incentives, public engagement, and regulatory integration. Providing subsidies, grants, or tax incentives, especially for installations on public buildings, can create demonstration sites that encourage broader uptake. At the same time, awareness and communication campaigns are essential to inform citizens, planners, and decision-makers about the multiple benefits of green roofs, including stormwater management, urban heat mitigation, energy savings, and biodiversity support. Finally, integrating green roofs into urban planning regulations and climate adaptation strategies ensures that their installation is systematically considered in both new constructions and retrofits. By aligning financial, social, and regulatory measures, cities can accelerate replication and upscaling, maximizing the environmental, hydrological, and social impacts of green roofs.
Please indicate the acronyms of used &/or developed technologies/Tools:
Nature based process, Soil infiltration, Sustainable Drainage Systems, Physical process, Sedimentation, Evaporation