By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.May 2 2024With urbanization expected to reach over 80 % in developed countries by 2030, the impact of new buildings on land, water, and energy needs careful examination. Various sustainable approaches are emerging to mitigate energy consumption and greenhouse gas emissions from the building sector.1
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Efficiently designing roofs with integrated green structures can create resilient, sustainable, and healthy living environments. These engineered green roofs support the growth of various plant types over specialized mediums.1 In this article, we will explore different green roofing materials, their application in sustainable construction, and the related challenges.
Green Roofing Principles
A green roof refers to a rooftop planted with vegetation in a growing medium (called substrate) and consists of multiple layers, each having a unique function.2 At the base lies a waterproof membrane, shielding the building from water infiltration from the layers above, followed by an anti-root barrier to deter aggressive vegetative roots. Then comes a protective layer capable of withstanding construction and operational loads. Above this lies a water storage and drainage layer, ensuring proper ventilation and preventing waterlogging of the overlying substrate. 1
A filter layer acts as a barrier between the drainage layer and the topmost substrate, preventing clogging from small soil particles and vegetation debris. The substrate is crucial for maintaining the biological, physical, and chemical conditions required for optimal vegetation growth. Ultimately, the presence of healthy vegetation atop the substrate marks the completion of a successful green roof structure. The choice of vegetation depends on both climatic conditions and substrate properties.1
Green Roofing Materials
Selecting the right materials for green roofs is crucial to ensure optimal functionality. Among these components, bituminous membranes stand out for their widespread use in waterproofing, offering a blend of water-tightness, elasticity, stability, and UV resistance. When it comes to resisting root penetration and microbial activity, metal sheets excel as anti-root barriers, boasting durability and resilience against environmental pressures.1
For the protective layer, a combination of geogrids, geotextiles, and polystyrene is employed to bear the weight of the layers above while maintaining structural integrity. Drainage and ventilation requirements are met through the use of various granular materials such as pozzolana, pumice, clay, and crushed bricks. Alternatively, innovative modular panels crafted from high-strength synthetic or plastic materials offer efficient water storage and drainage capabilities.1
To ensure effective water permeability, the filter layer is constructed with materials that surpass the substrate's permeability by at least tenfold. Granular materials with high water permeability, coupled with non-woven geotextiles, serve as the ideal choice for this layer.1
The substrate, a pivotal layer for vegetation support, demands a meticulous blend of materials tailored to diverse climatic conditions. Lightweight materials like pumice and perlite are favored for their low bulk density, although a careful balance is necessary to maintain substrate and vegetation stability. Incorporating a mix of inorganic materials such as scoria, vermiculite, and crushed brick ensures optimal substrate performance, enabling robust vegetation growth and maintenance across varying climates.1
Advantages and Applications in Sustainable Construction
Sustainable roof designs not only reduce the energy needed for heating and cooling buildings but also enhance their overall thermal performance. By partially blocking incoming solar radiation and promoting evapotranspiration, green roofs contribute to cooler urban microclimates. The substrate and drainage layers further enhance this thermal insulation, bolstering the roof's heat resistance.2
Beyond energy efficiency, green roofs offer a multitude of social, economic, and environmental benefits to urban areas.1 Economically, they play a crucial role in urban hydrology and stormwater management by absorbing rainfall, reducing runoff, and filtering water, thus alleviating strain on water treatment facilities.2
From a social standpoint, green roofs provide recreational spaces that elevate the quality of urban life and enhance the aesthetics of cities. They contribute to mental well-being, offer areas for relaxation, and mitigate sound pollution through substrate-based sound absorption.2
Environmentally, green roofs serve as natural air purifiers, absorbing vehicular and industrial emissions while increasing oxygen production. They combat the urban heat island effect, mitigate acid rain, and lessen traffic noise pollution.1,2 Moreover, by serving as habitats for small animals, green roofs promote urban biodiversity, enriching the ecological fabric of cities.2
Challenges
Despite their potential, current green roofing techniques face significant barriers to widespread adoption in sustainable construction. A key issue is the absence of robust government policies, leaving the implementation of sustainable practices primarily in the hands of private builders and developers.2 Unfortunately, many developers only consider the aesthetic appeal of green roofs, lacking comprehensive knowledge of their broader benefits.1
Technological challenges also hinder the adoption of green roofs, particularly the disparity between local feasibility and available scientific data. Moreover, there's a pressing need for technological advancements to retrofit green roofs onto older buildings with weaker structures while addressing concerns such as bacterial growth and mosquito breeding.2
Furthermore, the economic benefits of green roofing require sophisticated assessment methods encompassing design, construction, maintenance, and irrigation costs. Complete cost evaluations, including life cycle assessments, are essential to convince both public and private sectors of the long-term benefits and justify the initial investment in sustainable construction methods.2
Latest Developments
Lately, local waste materials have been gaining significant attention in the construction sector. As well as green roof substrates, they can decrease the initial implementation costs and enhance roof functionality. For instance, a recent study in Environmental Research demonstrated that a substrate modified with biochar prepared from agricultural waste enhances the hydrothermal properties of green roofs.3
Another recent article in Water Science & Technology demonstrated the potential of coconut waste as a green roofing material. The researchers used coconut fiber and shell in the filter and drainage layers, respectively, for a green roof and observed that the runoff decreased by 86 % compared to 67 % achieved using commercial materials.4
Future Prospects
Despite the high initial costs, green roofs are economical in the long term due to their energy savings. Further technological advancements and integration with other sustainable rooftop methods can lower the initial costs. For example, combining green roofs with photovoltaics is an emerging trend due to synergistic benefits. While the vegetation keeps the solar panels cool, producing more electricity, the solar panels support vegetation growth and increase abiotic heterogeneity, resulting in higher plant diversity.1,2
Future research on green roof technology should consider the local availability of the required materials. These materials should be sourced from local agricultural and construction waste recycling to decrease costs and enhance long-term performance and sustainability. A life cycle analysis encompassing the recycling process and scientific investigation on the impact of these materials is also necessary for their implementation in green roofing.1
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References and Further Reading
1. Cascone, S. (2019). Green Roof Design: State of the Art on Technology and Materials. Sustainability, 11(11), 3020. https://doi.org/10.3390/su11113020
2. Zhang, G., & He, B.-J. (2021). Towards green roof implementation: Drivers, motivations, barriers and recommendations. Urban Forestry & Urban Greening, 58, 126992. https://doi.org/10.1016/j.ufug.2021.126992
3. Tan, K., & Wang, J. (2023). Substrate modified with biochar improves the hydrothermal properties of green roofs. 216, 114405–114405. https://doi.org/10.1016/j.envres.2022.114405
4. Romali, N. S., Ardzu, F. A. B., & Suzany, M. N. (2023). The potential of coconut waste as green roof materials to improve stormwater runoff. Water Science and Technology. https://doi.org/10.2166/wst.2023.060
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