Some European green roof norms and guidelines through the lens of biodiversity: Do ecoregions and plant traits also matter?

Green infrastructure and in particular green roofs are crucial to meet the challenge of sustainable urbanisation fostered by the current European Research and Innovation agenda. Several documents were issued in the last decades in Europe for regulating the design, construction and up-keep of roof greening. In particular, the actual German guidelines have been widely adopted as a reference basis for green roof design and regulation worldwide, because of its exhaustiveness and proven building- and landscaping tradition. With the aim to assess the effectiveness of green roof guidelines and norms in supporting plant and soil biodiversity in different ecoregions, and particularly of the Mediterranean one, the German guidelines, the Swiss and Italian norms are screened and discussed in this paper. The German guidelines were chosen for their traditional referential role, the Swiss norm for its peculiar biodiversity approach, the Italian one for its application on a territory with remarkably heterogeneous environmental conditions, stretching from Alpine to Mediterranean ecosystems. Even if the three documents at comparison addressed to some extent biodiversity-related matters, none of them deepened the relationship between plant species selection (local ecotypes), growing medium composition (materials, granular size and thickness) and system build-ups (multi-layers and/or single-layer systems). This is a crucial point for countries, like Italy, encompassing very different climatic conditions. It was concluded that at the current knowledge a guideline/norm taking into account the peculiarities of green roof design in the Mediterranean ecoregion has to be widely refined.