The urban landscape is known to form a hostile matrix for wild plant communities because of its sealed
infrastructure surfaces. However, green roofs can reduce this hostility by providing new spaces for wildlife
directly on buildings. In this extensive study of 115 green roofs in northern France, we focused on wild
plant communities and the variables that shaped their diversity and their taxonomic and functional
compositions. A total of 176 colonizing vascular plant species were identified; 86% were natives, demonstrating
that green roofs can serve as habitats for wild biodiversity despite their isolation in an urban
landscape in three dimensions. Nonetheless, all types of green roofs were not equal, with the substrate
depth playing a major role in the wild plant diversity. The taxonomic and functional compositions of the
colonizing plant communities were also shaped by the substrate depth, green roof age, surface area, and
height and maintenance intensity at the building scale. We did not detect any effect of the surrounding
potential habitats at the landscape scale. The study of functional traits revealed that the wild plant communities
are adapted to open xero-thermophilous conditions. This study led us to consider an ecological
typology for green roofs referred to as stratum classification, which is based on the vegetal structures living
and colonizing these anthropo-ecosystems. Wild roofs adapted to receive spontaneous species could
play an interesting role in urban biodiversity dynamics if they continue to be developed at large scales
in cities.