The installation of a green roof implies a particular implementation because of the weight of the whole but also of the imperatives of waterproofing and the necessary accesses for maintenance.
The main constraint of adding a growing medium and plants on a roof is the weight. Indeed, a terrace entirely covered with several centimeters of earth must have a sufficiently strong structure of the roof to resist the dry weight but also the soaked substrate. Each installation must be accompanied by a waterproofing membrane as well as a drainage and filtration layer. Only perfectly flat roofs or roofs with a maximum slope of 35° can be vegetated.
A green roof is essentially made up of five components: The supporting structure, which can be wood, concrete or steel; a waterproofing membrane (APP bituminous rather than synthetic); a drainage and filtration layer composed of an embossed polyethylene drainage membrane sloped to meet a drain; a light growth substrate composed of leaf or bark plant compost mixed with light, absorbent stone aggregates; and finally, a plant layer for the actual planting.
Which plants?
If all plants can grow on a roof, some more fragile than others are less used, especially if the roof is inaccessible. In most cases, the recommended plants are native and perennial for a better resistance to the local climate. Ground covers are often used to minimize maintenance and weeding. The most complex installations use drip watering equipment directly into the substrate.
A few specialized companies have recently emerged in France to offer complete systems whose techniques can vary from pre-vegetated mats to automated watering stations.
For which building?
Knowing that the constraints are consequent and the installation cost important, it is preferable to use this technique on buildings to be built, thus foreseen in term of resistance and design to welcome a consequent additional weight. Nevertheless, for extensively planted terraces (without integrated watering), vegetation can be used after construction if the building's supporting structure allows it.
Note: contrary to what one might think, the vegetation of a roof does not alter a building in any way, on the contrary: the stability and waterproofing of green roofs are superior to conventional flat roofs.
