Green roofs, often perceived as innovative solutions to contemporary urban challenges, play an essential role in transforming the urban landscape. Since the 90s, their integration into urban development policies has sparked growing interest. Indeed, these green spaces on the roofs of buildings contribute not only to the aesthetics of cities but also offer a multitude of ecological benefits. However, the issues surrounding their implementation and evaluation pose a major challenge for urban planners and architects.
Ecological benefits of green roofs: a reality to be measured
Green roofs are often lauded for their multiple advantages, particularly in terms of biodiversity, water retention, and urban cooling. However, the scientific validation of these benefits remains incomplete. The GROOVES (Green ROOfs Verified Ecosystem Services) study, conducted between 2017 and 2019 by the regional agency for biodiversity in Île-de-France, provided a solid foundation on this issue by evaluating both flora and fauna and other ecological functions on 36 roofs of various typologies. It is imperative to understand and objectify these benefits, taking into account different greening systems.
The results of the study indicate that there is a great variety of plants and animals that benefit from these spaces. Approximately 400 plant species were recorded, including common plants like stonecrops (Sedum sp.) and rare species such as compressed restharrow (Ornithopus compressus). The fact that nearly 70% of the plants on some roofs appeared spontaneously testifies to the resilience of urban ecosystems.
Overview of benefits
- Biodiversity: Creation of habitats for many species.
- Rainwater retention: Ability to reduce runoff and filter water.
- Urban cooling: Providing freshness during hot periods.
Green roofs also show significant potential for promoting biodiversity. The study recorded over 600 species of invertebrates, mainly pollinators such as bees and wasps, although diversity is often lower than that observed in other urban environments. This raises questions about the quality of habitats provided by these roofs and their design.
The technical and design challenges of green roofs
To optimize the ecological impact of green roofs, several technical factors must be considered during their design. The thickness and composition of the substrates play a crucial role in the diversity of flora and fauna. Relatively shallow roofs, known as extensive, tend to host less rich biodiversity compared to those with thicker substrates.
Key factors influencing biodiversity
- Substrate thickness: The thicker it is, the more species diversity increases.
- Substrate properties: The type of material used can encourage or limit certain species.
- Architectural design: Design choices influence environmental conditions, such as sun exposure.
The results show that substrates of “agricultural soil” type, with a depth of about 30 cm, provide the best conditions for biodiversity. A design model favoring local natural elements (such as wild vegetation) could harmonize these structures with local wildlife. Thus, rather than relying on standardized products, designers can turn to more ecological solutions that are suited to urban ecosystems.
Comparison of roof types
| Type of roof | Substrate thickness | Biodiversity |
|---|---|---|
| Extensive | 5 to 15 cm | Limited plant diversity |
| Semi-intensive | 15 to 30 cm | Average plant richness |
| Intensive | More than 30 cm | High biodiversity |
Impact of green roofs on the urban climate
Green roofs represent a major asset for improving the urban climate. Thanks to their ability to reduce heat islands, they promote a more pleasant environment for residents. This function, often overlooked, proves to be critically important in the context of growing urbanization and climate change. Evapotranspiration, a fundamental phenomenon with cooling effects, has been measured on several roofs as part of the GROOVES study.
Climatic functions
- Cooling: Temperature reduction in urban environments.
- Humidity regulation: Improvement of air quality.
- Pollution reduction: Contribution to a healthier environment.
However, it should be noted that not all green roof systems are equal in terms of effectiveness. For instance, only six roofs out of fourteen demonstrated significant air cooling capacity. Thus, it is crucial for designers and decision-makers to consider these ecological dimensions to facilitate the fight against the effects of climate change.
Performance evaluation
| Type of roofing | Cooling capacity | Water retention efficiency |
|---|---|---|
| Extensive | Limited | Variable |
| Semi-intensive | Average | Satisfactory |
| Intensive | High | Optimal |
Perspectives and recommendations for effective implementation
To maximize the benefits of green roofs, it is necessary to develop strategies that integrate environmental and sustainability concerns. The creation of solutions like Green Roofs France or Sustainable Rooftop Solutions could serve as a model for countries wishing to promote the use of these roofs. The study identified several recommendations to optimize their impact:
Key recommendations
- Adopt a participatory design: Involve local communities in the design and management.
- Encourage the use of local materials: This reduces the ecological footprint of roofs.
- Educate the public: Raise awareness among citizens about the ecological benefits of green roofs.
Opting for ecological models such as those proposed by EcoToit or NaturRoof is necessary to prevent green roofs from becoming merely decorative elements.
Conclusion of studies
Studies on green roofs reveal undeniable ecological potential while pointing out the limits of current practices. In terms of sustainable urban planning, it becomes essential to integrate these innovative solutions into renovation and construction projects to create a healthier and more welcoming living environment for biodiversity.
