Green roofs without soil, sustainable innovation

I green roofs represent an increasingly relevant technological solution to address urban environmental challenges, offering significant benefits in terms of energy efficiency, mitigation of the island effect heat and management of the waters meteoric.

A recent research conducted by the University of Perugia (Muscas et al., 2022) has analyzed the environmental impact of an innovative system of green roof without soil (Pratotetto®) which uses the species Zoysia tenuifolia, through the methodology Life Cycle Assessment (LCA).

Research Methodology and LCA Approach

The study by Muscas et al. (2022) took a more general approach Cradle to Gate, following the ISO 14040:2006 and ISO 14044:2006 standards for thelife cycle analysisThe research was conducted using the SimaPro 8.4.0.0 software, applying three different evaluation methodologies: Ecological Footprint V1.01, IPCC 2013 GWP 100y V1.0 e ReCiPe H V1.1.

The methodology ReCiPe, from the term Recipe (recipe), is a method that translates the complex LCI inventories (Life Cycle Inventory) in a limited number of environmental impact indicators, for a two-level analysis:

• midpoint (midpoint), which focuses on single environmental problems such as climate change or acidification;
• endpoint-level (final point), which assesses three main areas of protection: human health, ecosystem quality and resource scarcity. The suffix 'H' indicates the perspective Hierarchist (hierarchical), which expresses the most common scientific consensus and adopts moderate precautionary approaches for methodological assumptions.

The functional unit established for the analysis is 1 m² of Pratotetto®, a modular system that stands out for its extremely low weight compared to other green roof solutions. The LCA analysis includes all production processes, input of raw materials and the output up to the factory gate (excluding the transport, installation and maintenance phases due to their case-specific variability).

Weight characteristics of the Pratotetto® system

One of the distinguishing features of the Pratotetto® system is its considerably reduced weight compared to traditional green roof systems:

• conventional extensive green roofs have weights ranging from 73 to 270 kg/m² for shallow systems;
• intensive green roofs can reach weights between 390 and over 730 kg/m² with substrates of at least 15 cm depth;
• the Pratotetto® system is estimated to weigh less than 40 kg/m².

The drastic reduction in weight of the Pratotetto® system was achieved through the replacement of the traditional growth substrate (Plot)  with  a double layer of recycled felt often just 4 cm. This feature makes it ideal for:

• building retrofit interventions, namely the redevelopment of existing buildings through the integration of innovative solutions such as green roofs, without the need for demolition;
• widespread applicability, even on structures not designed to support the loads of conventional green roofs, such as the historic buildings that characterise the Italian building heritage.

Botanical characteristics of the Zoysia tenuifolia

La Zoysia tenuifolia Thiel, commonly known as maskwort or Korean velvet grass, is a perennial grass belonging to the family of poaceaeFrom a morphological point of view, the Zoysia tenuifolia stands out for:

• its extremely thin leaf blades, typically less than 2 mm wide, which give the lawn a very delicate texture and a characteristic velvety appearance;
• a stoloniferous bearing – that is to say with horizontal expansion, at ground level or just below – and very slow growth, so as to form thick carpets which naturally inhibit the growth of weeds.

The choice of Zoysia tenuifolia for lawns and green roofs it is mainly due to:

• high tolerance to the water stress and high temperatures. Thanks to its tropical origin, Zpysia has in fact developed a very efficient C4 type photosynthetic metabolism;
• an exceptional mechanical strength,  trampling and wear from physical contact. And it is therefore used in football fields and other sports facilities in subtropical and tropical areas.

The remarkable ability to form dense carpets and the adaptability to physical and climatic stress conditions significantly reduce the needs for agronomic management and maintenance.

Innovative features of the soilless system

The system Pratotetto® presents innovative technological features that differentiate it from traditional green roofs. The growth substrate is made up of a felted material made from recycled and heat-treated textile fibres, which provides thermal and acoustic insulating properties.

The system drip irrigation is integrated between two layers of felt, while a non-woven geotextile layer acts as an anti-root barrier. Under the root barrier, a cuspated mat in high density polyethylene (HDPE) ensures drainage of excess water.

Life cycle inventory and analyzed components

The LCI inventory identified four main components of the system:

• Zoysia Cultivation and Maintenance. The agronomic model considered included the use of nitrogen (0,039 kg), phosphate (0,004 kg) and potassium (0,006 kg) fertilizers, as well as specific herbicides such as pendimethalin and pyridine compounds. Mechanical operations include mowing, cultivation, fertilization and irrigation (132 liters/year per functional unit);
• Drip irrigation system. Polyethylene pipes with integrated drippers, to meet a requirement of 0,08 metres of equivalent HDPE DN 200 pipe per m²;
• Felt production. Sterilization process of recycled fibers at 180°C without the use of water, chemicals or adhesives, with an energy consumption of 9,814 MJ per kg of product.
• Draining layerDrainage geocomposite with a cuspidate structure coupled with a filtering geotextile, made of 565 g/m² of HDPE and 150 g/m² of polypropylene (PP) fibres.

Impact assessment results

The analysis ReCiPe midpoint highlighted that felt production represents the component with the highest impact, with 4,44 kg CO₂ eq./m², ozone depletion, terrestrial acidification and particulate matter formation, mainly due to the high energy requirement for the treatment of recycled fibres.

Cultivation of the zoysia impacts mainly on natural soil transformation and terrestrial acidification, in addition to marine and freshwater ecotoxicity. Normalized data show that environmental persistence of nitrogen nutrients contributes to marine eutrophication, while herbicide treatments influence ecotoxicity categories.

The analysis ReCiPe endpoint-level confirmed that the drainage layer has the highest value in terms of additional costs for future resource production, while the felt production has the greatest impact on human health and the loss of ecosystem species. single score total is 735,92 mPt, with felt contributing 48% (356,64 mPt).

Climate change impact and carbon footprint

According to the methodology IPCC 2013 GWP 100y, the production of 1 m² of Pratotetto® involves a global warming potential of 7,66 kg CO₂ eq. The distribution of impacts sees the felt as the main contributor (4,47 kg CO₂ eq., 58%), followed by the drainage layer (1,77 kg CO₂ eq., 23%), the drip irrigation system (0,73 kg CO₂ eq., 10%) and the cultivation of Zoysia (0,70 kg CO₂ eq., 9%).

The analysis Ecological Footprint confirmed the high impact of felt production on the carbon dioxide category, while the other components have a greater influence on the less relevant categories of nuclear energy and land occupation.

Sensitivity analysis and alternative scenarios

The Sensitivity analysis evaluated three hypothetical scenarios to verify the influence of variations in the components with the greatest impact:

• 1 scenario. Using drainage mat with 60% recycled HDPE – 9,02% reduction in total impact.
• 2 scenario. Use of 100% photovoltaic electricity for felt production – significant reduction, – 48,87%.
• 3 scenario. 30% increase in plant protection active ingredients and irrigation – 0,20% increase.

The results demonstrate that the impacts of Pratotetto® are more sensitive to variations in the energy mix used rather than the origin of the materials, highlighting the importance of renewable energy sources.

Comparison with scientific literature

The results obtained are in line with the few data available in the literature, taking into account that the LCA studies applied to green roofs are still limited and often include multiple life cycle stages:

• Manso et al. (2018) analyzed the Geogreen commercial system, identifying the support layer as the main cause of impact;
• Rincón et al. (2014) compared different types of green roofs with various substrates, highlighting how recycled rubber, more similar to the substrate analyzed, presents the greatest environmental load in the production phase.

Implications for building sustainability

The analysis highlights the high performance of the Pratotetto® soil-free system in terms of sustainability, particularly for its applicability in scenarios of building retrofit thanks to its low weight. The system minimizes the layers required for the assembly of a green roof, making it suitable for many existing buildings that lack sufficient load-bearing capacity for traditional systems.

The research highlights the importance of reuse of waste materials of the textile industry, contributing to the circular economy. However, the impact of felt is caused exclusively by the electricity needed to process the recycled fibres, suggesting that further reduction of the impact would be possible by using renewable energy sources.

Recommendations and future developments

The study (Muscas et al., 2022) recommends several strategies to further improve the environmental sustainability, of the Pratotetto® soil-free green roof system:

1. Energy transition. Implementation of renewable sources for the fiber treatment phase, thus significantly reducing the overall impact of production;
2. Integrated agronomic management. Improving fertilization and treatment practices for Zoysia through integrated or organic management approaches;
3. Materials Optimization. Use of plastic materials with a higher share of recycled sources for the drainage layer;
4. MonitoringFinally, further studies including operational and end-of-life phases are recommended for a complete LCA assessment.

Conclusions

The research represents a significant contribution to the understanding of environmental impacts of innovative green roof systems, providing essential quantitative data for informed decisions in the building sector. The Pratotetto® system demonstrates interesting potential for the urban climate mitigation, with a production impact of 7,66 kg CO₂ eq./m² which can be further reduced through energy mix optimization.

The soilless (without land) with Zoysia tenuifolia It appears to be a promising technological solution for the widespread implementation of green infrastructure in urban contexts, particularly in Mediterranean climates, with ultimate benefit for the health and well-being of the inhabitants. The LCA methodology provides a robust framework for evaluating similar technologies, contributing to the development of more sustainable and climate-resilient building solutions.

The research also highlights the importance oftechnological innovation in the green roof sector, demonstrating how the integration of recycled materials and advanced cultivation techniques can significantly contribute to the environmental sustainability of the built environment, supporting the objectives of decarbonization of the construction sector.

Dario Dongo

Bibliography

• Bianchini, F., & Hewage, K. (2012). How 'green' are the green roofs? Lifecycle analysis of green roof materials. Building and Environment, 48 (1), 57-65. https://doi.org/10.1016/j.buildenv.2011.08.019
• Catalano, C., Laudicina, V. A., Badalucco, L., & Guarino, R. (2018). Some European green roof norms and guidelines through the lens of biodiversity: Do ecoregions and plant traits also matter? Ecological Engineering, 115, 15-26. https://doi.org/10.1016/j.ecoleng.2018.01.006
• Cook-Patton, S. C., & Bauerle, T. L. (2012). Potential benefits of plant diversity on vegetated roofs: A literature review. Journal of Environmental Management, 106, 85-92. https://doi.org/10.1016/j.jenvman.2012.04.003
• Dunnett, N., & Kingsbury, N. (2008). Planting green roofs and living walls. Timber Press. ISBN: 978-0-88192-882-9
• Fantozzi, F., Rocchi, L., Brunetti, M., Peron, F., & Saro, O. (2021). Do green roofs really provide significant energy saving in a Mediterranean climate? Critical evaluation based on different case studies. Frontiers of Architectural Research, 10 (2), 447-465. https://doi.org/10.1016/j.foar.2021.01.006
• Fernandez-Cañero, R., Emilsson, T., Fernandez-Barba, C., & Herrera Machuca, M. Á. (2013). Green roof systems: A study of public attitudes and preferences in southern Spain. Journal of Environmental Management, 128, 106-115. https://doi.org/10.1016/j.jenvman.2013.04.052
• Kolokotsa, D., Santamouris, M., & Zerefos, S. C. (2013). Green and cool roofs' urban heat island mitigation potential in European climates for office buildings under free floating conditions. Solar Energy, 95, 118-130. https://doi.org/10.1016/j.solener.2013.06.001
• Korol, E., & Shushunova, N. (2016). Benefits of a modular green roof technology. Procedia Engineering, 161, 1820-1826. https://doi.org/10.1016/j.proeng.2016.08.673
• Li, W. C., & Yeung, K. K. A. (2014). A comprehensive study of green roof performance from environmental perspective. International Journal of Sustainable Built Environment, 3 (1), 127-134. https://doi.org/10.1016/j.ijsbe.2014.05.001
• Manso, M., Teotónio, I., Silva, C. M., & Cruz, C. O. (2018). Life cycle analysis of a new modular greening system. Science of the Total Environment, 627, 1146-1153. https://doi.org/10.1016/j.scitotenv.2018.01.198
• Mentens, J., Raes, D., & Hermy, M. (2006). Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landscape and Urban Planning, 77 (3), 217-226. https://doi.org/10.1016/j.landurbplan.2005.02.010
• Muscas Desirée, Landi, F.F.A., Fabiani, C., Grohmann, D., & Rocchi, L. (2022). Zoysia soil-less green roof's performance: A life cycle assessment. Environmental and Climate Technologies, 26 (1), 955-967. https://doi.org/10.2478/rtuect-2022-0072
• Niachou, A., Papakonstantinou, K., Santamouris, M., Tsangrassoulis, A., & Mihalakakou, G. (2001). Analysis of the green roof thermal properties and investigation of its energy performance. Energy and Buildings, 33 (7), 719-729. https://doi.org/10.1016/S0378-7788(01)00062-7
• Patton, A. J., Schwartz, B. M., & Kenworthy, K. E. (2017). Zoysiagrass (Zoysia spp.) history, utilization, and improvement in the United States: A review. Crop Science, 57, 37-72. https://doi.org/10.2135/cropsci2017.02.0074
• Rincón, L., Coma, J., Pérez, G., Castell, A., Boer, D., & Cabeza, L. F. (2014). Environmental performance of recycled rubber as drainage layer in extensive green roofs. A comparative life cycle assessment. Building and Environment, 74, 22-30. https://doi.org/10.1016/j.buildenv.2014.01.001
• Shafique, M., Kim, R., & Rafiq, M. (2018). An overview of life cycle assessment of green roofs. Journal of Cleaning Production, 250, 119471. https://doi.org/10.1016/j.jclepro.2019.119471
• Vacek, P., Struhala, K., & Matějka, L. (2017). Life-cycle study on semi-intensive green roofs. Journal of Cleaning Production, 154, 203-213. https://doi.org/10.1016/j.jclepro.2017.03.188
• Vijayaraghavan, K. (2016). Green roofs: A critical review on the role of components, benefits, limitations and trends. Renewable and Sustainable Energy Reviews, 57, 740-752. https://doi.org/10.1016/j.rser.2015.12.119
• Wherley, B. G., Gardner, D. S., Metz, S. P., & Walz, A. L. (2011). Low-input performance of zoysiagrass (Zoysia spp.) cultivars maintained under dense tree shade. HortScience, 46 (7), 1033-1037. https://doi.org/10.21273/HORTSCI.46.7.1033
• Yang, H. S., Kang, J., & Choi, M. S. (2012). Acoustic effects of green roof systems on a low-profiled structure at street level. Building and Environment, 50, 44-55. https://doi.org/10.1016/j.buildenv.2011.10.004

Dario Dongo

More postsBio

Dario Dongo, lawyer and journalist, PhD in international food law, founder of WIISE (FARE - GIFT - Food Times) and Égalité.

• Equality: Inclusion and the fight against cultural and architectural barriers
• Disability and Independent Living, EU Report
• Disability reform in Italy: the state of the art
• EU Budget 2026: Welfare, Not Weapons