Green roof

A flat roof covered with vegetation, where a build-up of layers - substrate, drainage, filter, root barrier - replaces the mineral ballast with a small ecosystem. It retains and slows rainwater, cools by evapotranspiration, protects the membrane and gives the city back the surface the building occupies.

The green roof turns the roof from an inert surface into a living system. Over the waterproofing a layered build-up is laid - root barrier, drainage and storage, filter, growing substrate, vegetation - calibrated to the type of green: extensive (sedum and grasses, light and almost maintenance-free) or intensive (a true roof garden, with shrubs and trees, heavier and tended).

The first service of the green roof is hydraulic. Substrate and drainage layer absorb and retain a share of the rain, returning it slowly by evaporation; the rest runs off with a strong delay compared with a traditional roof. In the city, where impervious surfaces saturate the sewers during storms, this attenuation of stormwater reduces the discharge peaks and is increasingly required or incentivised by building regulations.

The retained water evaporates through the substrate and the plants (evapotranspiration), removing heat: in summer the surface of a green roof stays much cooler than a dark membrane, reducing the cooling load and the urban heat island. The mass of the build-up adds inertia and decrement delay. For the membrane, finally, the green is a shield: screened from sun, frost and UV, its life lengthens as much as, or more than, in the inverted roof.

Three constraints govern the design. Tightness: a root-resistant membrane (or a dedicated layer) is needed that roots cannot pierce, with a penetration-resistance test. Weights: the water-saturated substrate is heavy (from ~1 kN/m² of the extensive to many kN/m² of the intensive), and must be accounted for in the structure. Maintenance and irrigation: minimal for the extensive, regular for the intensive; in any case perimeter gravel strips and borders around the singular points must be provided, for inspection and fire safety.