Green roof: blanket or habitat?

Does a green roof really serve biodiversity? Not always, to our mind: a sedum mat, however green it photographs, is not an ecosystem. It takes little substrate, the lightweight soil you spread on roofs, to hold it in place. And a single family of succulents to make it flower in June. As for relief, there is none: nothing, then, for life to take hold in. A roof that is truly alive reads like a soil: depths that vary, hollows that hold water, refuges where something can hide. It is not the same project.

The sedum mat has a real merit: it holds back a little rainwater, insulates a little in summer, greens an image. But laid as a thin, continuous layer, it houses no one. A ground beetle looking for the shade of a stone finds none; a solitary bee that digs into warm sand has no sand to dig into; a bird watching for a puddle after the rain finds only flat, dripping foliage, nothing to land on. Green monoculture is a roof that looks like a garden and behaves like a carpet.

What changes everything is relief. A roof designed for the living varies its substrate depth, thin in places, deep in others. That variation alone is enough to grow different species zone by zone: dry ones on the mounds, cooler ones in the hollows. It carries materials that do nothing for waterproofing and everything for shelter: a pile of dead wood, a bed of sand, stones scattered without order. And a puddle a few centimetres deep, drying out in summer and filling again in winter like a small seasonal pond. Nothing spectacular. A diversity of small conditions, side by side, rather than one thin mat everywhere.

The law, here, already asks for more than a blanket. It requires, on a large share of new non-residential buildings, a green or solar roof: 40% of the surface as of this summer, 50% from next summer. And the text does not simply call for green: it demands a system that favours, in so many words, “the preservation and reconquest of biodiversity”. A regulation that writes the word biodiversity is not asking for a decorative mat laid down to tick a box. The exact thresholds and scope must be checked project by project, never taken on faith.

This picks up directly where we left things in “Soil first”: rebuilding a soil on a roof is never the first choice, only the one that remains once natural ground is out of reach. But a rebuilt soil calls for a structure to carry it, a waterproof layer, watering at times, and someone named to look after it. It may as well be worth something, then. The cost gap between a flat mat and a roof with relief lies upstream, at the design stage, not on site. A landscape consultant brought in at sketch stage costs less than a waterproofing lot redone after the fact.

So the next time a roof plan lands on the table, one simple question before drawing: will this roof have relief, or will it stay flat under its green? A mat unrolls. A habitat, by contrast, is drawn.