NBS4MED partners visited an Innovative Action project led by the Barcelona Metropolitan Area, exploring how organic municipal waste can be turned into a substrate for naturalised, irrigation-free green roofs.
From Waste to Rooftop: A Circular Approach to Urban Nature
The second day of the NBS4MED study visit in Barcelona brought partners to explore TopSec, an Innovative Action project funded under the EU Urban Initiative and led by the Barcelona Metropolitan Area (AMB). The project addresses two of the most pressing challenges facing Mediterranean cities today. The first is the intensification of heat waves and drought in densely built environments. The second is the growing need to repurpose organic waste beyond conventional composting and agriculture.
The solution showcased is rooted in circularity. The Barcelona metropolitan area produces approximately 1.5 million tonnes of municipal solid waste per year. The organic fraction of this waste is processed into a high-quality biochar with strong agronomic properties. This biochar serves as the substrate for naturalised, seasonal green roofs that retain humidity and require no irrigation, making them viable and affordable even where conventional green roofs have historically been seen as costly and difficult to maintain.
Four Buildings, One Replicable Model
The TopSec project is currently being tested across four diverse building types in the Barcelona metropolitan area, including a public school in Cornellà de Llobregat, an office building in Barcelona’s Zona Franca, a social housing complex in Viladecans and an environmental facility in Sant Adrià de Besòs. Piloting the solution across different structural, social and urban contexts builds evidence for an approach that can be replicated across a wide range of buildings throughout the metropolis and beyond.
Alongside the technical interventions, the project runs an educational programme for the communities of each pilot building, raising awareness of naturalised roofs and the biodiversity they can support.
Relevance for the Mediterranean
For NBS4MED partners, the visit offered a compelling example of how nature-based solutions can reach beyond parks and corridors and into the existing built fabric of the city. The environmental benefits are significant, covering improved thermal insulation, enhanced water management, carbon capture and a measurable contribution to urban biodiversity. What makes the approach particularly relevant for Mediterranean cities is its scalability. The model is replicable and low-maintenance, addressing climate adaptation and waste recovery in a single integrated approach.
Last Update
16/06/2026
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