Green Roofs: A Strategy for Sustainable Cities

Urban sustainability has emerged as one of the most pressing challenges in recent decades. Cities generate approximately 80% of global GDP but are also responsible for more than 60% of global carbon dioxide emissions and about 80% of total greenhouse gas emissions (UNDP, 2024). As the effects of global climate change intensify, cities around the world are facing air pollution, loss of green spaces, and the growing impact of the Urban Heat Island (UHI) effect. Rapid urbanization has made these issues more critical than ever. Approximately 56% of the world’s population lives in urban areas, and that number is expected to double by 2050 (World Bank, 2025). These conditions place pressure on infrastructure, public health, and environmental resources in urban populations. To address these issues, cities are adopting sustainable and nature-based solutions, such as green roofs.

What are Green Roofs?

Green roofs—also known as vegetative or living roofs are artificial ecosystems designed to bring nature into urban environments. They consist of three main layers: a waterproof membrane, a growing medium (such as soil or substrate), and vegetation. By integrating vegetation into rooftops, cities can improve energy efficiency, enhance air quality, and mitigate the UHI effect, all while optimizing the quality of life in dense urban environments. There are two types of green roofs: extensive green roofs and intensive green roofs.

• Extensive green roofs: Lightweight, low-maintenance systems with shallow soil and drought-tolerant plants.

• Intensive green roofs: Support deeper soil and can sustain shrubs, trees, and gardens, requiring more maintenance and structural support.

   

Energy Savings & Cooling:

Green roofs contribute to energy conservation by reducing both cooling and heating demands. They can lower indoor temperatures by up to 15°C and cut cooling loads by approximately 70% (EPA, 2025). They act as a thermal barrier between the building and the external environment. The soil and vegetation layers absorb solar radiation, reduce heat transfer, and moderate temperature fluctuations throughout the day. In hot weather, green roofs block direct sunlight and absorb less heat than traditional roofing materials. They insulate buildings and provide shade, keeping indoor temperatures cooler, especially in summer, meaning that air conditioning systems don't have to work as hard. This allows the building to stay naturally cooler and conserve a significant amount of energy. This reduces electricity use, lowering energy bills and greenhouse gas emissions. Over time, this contributes significantly to energy conservation at both the building and city-wide levels. In colder weather, the soil and plant layers provide insulation, slowing heat loss from the building and reducing heating demands.

Improving Urban Air:

Plants on green roofs improve air quality by filtering particulate matter (such as dust and air pollutants), contributing to air purification and creating a cleaner environment. The large surface area of the plant’s leaves acts like a natural filter. Fine particles from the air settle onto these surfaces through impaction, interception, and sedimentation. Impaction occurs when airborne particles collide directly with the leaf or stem. Interception happens when particles travelling along the airflow come into close contact with plant surfaces and adhere without direct impact. Meanwhile, sedimentation refers to the gravitational settling of particles onto surfaces such as leaves and soil in low-wind conditions. Some plants used in green roofs have textured or waxy leaves that are effective at capturing airborne dust and soot. Also, moist leaves or surfaces can easily trap particles. Dew or rain can help rinse particles off into the soil, where they may be broken down or held. Moreover, plants take in gases through tiny openings on their leaves called stomata. While they primarily absorb carbon dioxide (CO₂) for photosynthesis, they can also absorb ozone (O₃) and nitrogen dioxide (NO₂), which are harmful gases. This absorption reduces greenhouse gases, providing a healthier atmosphere.

Urban Heat Island Mitigation:

Living roofs mitigate the Urban Heat Island (UHI) effect. UHI refers to the phenomenon where urban areas have significantly higher temperatures than rural areas. Heat-absorbing materials such as concrete and asphalt intensify the UHI effect. Excess heat contributes to heat stress and related health issues, including dehydration, heat exhaustion, and cardiovascular strain. It also negatively impacts air quality by accelerating the formation of ground-level ozone, a harmful pollutant linked to respiratory problems. Plants on green roofs absorb water through their roots and release it as water vapour through their leaves. This process cools the surrounding air, reducing surface and air temperatures around the building. Unlike traditional dark roofs that absorb and retain heat, green roofs reflect more sunlight and have cooler surfaces due to their vegetation and soil. Plants absorb approximately 50% of thermal energy from the sun and reflect 30%. (Jamei et al., 2021). Vegetation shades the roof surface, preventing direct sunlight from heating roofing materials and further lowering surface temperatures. This reduces heat buildup and has a positive impact on the local microclimate around buildings. This lowers the overall urban temperature, leading to an average reduction of up to 3°C across the city (Jamei et al., 2021).

Biodiversity & Well-Being

Green roofs provide habitats for a variety of plants, insects and other small wildlife, supporting biodiversity in cities where green space is often limited. They serve as stepping stones or green corridors that connect fragmented habitats, allowing species to move and thrive. By incorporating native and diverse plant species, green roofs support local ecosystems, promoting pollination and food webs within the city. Flowering plants on green roofs provide nectar and pollen, sustaining vital pollinators crucial for urban agriculture and plant reproduction. Beyond environmental performance, passive visual access to green rooftops from windows or rooftops has been scientifically shown to reduce stress, improve mood, and enhance overall mental health. According to studies in environmental psychology and public health, exposure to greenery and natural landscapes reduces both cortisol levels and symptoms of anxiety and depression, while enhancing cognitive function (Lhoest, 2020). Accessible green roofs can serve as community gardens or recreational spaces. They promote physical activity, social interaction, and outdoor engagement, supporting both physical and social well-being.

A Green Vision for Beirut:

As Beirut continues to face rising temperatures, air pollution, and a lack of green spaces, green roofs offer a promising solution. Turning Beirut’s flat concrete roofs into vibrant green spaces would provide ecological, economic, and social benefits. The strategy is initiated by identifying suitable rooftops, focusing on dense, overheated neighbourhoods and places where greenery is minimal. Public buildings, schools, and hospitals could serve as ideal starting points, demonstrating the function of green roofs and inspiring broader adoption. Local universities and NGOs can play a key role by providing technical guidance and training, while urban planners and architects can incorporate green roofs into building codes and renovation plans. Choosing native plants will ensure that these roofs are sustainable and low-maintenance, an important factor in a city already facing resource constraints. Also, raising awareness in schools, universities, and communities about the environmental and social benefits of green roofs can foster long-term support and engagement. Besides the environmental improvements, this strategy restores a sense of community and resilience in a city that deeply needs it. Green roofs won’t solve all of Beirut’s challenges, but as part of a broader climate adaptation effort, they offer a smart, scalable step toward a more sustainable urban future. It’s time to green Beirut.