Three green roof techniques
A green roof system consists of an assembly of materials and plants installed on a roof (or a flat roof) with the aim of ensuring the sustainability of the vegetation and the building. One of the most important elements in a roof is the bearing structure (concrete slab, steel decking, timber panels, etc.)
which, depending on its nature and technical design, is able to bear different loads and is suited to different uses. A waterproofing membrane, which is also root-proof, is indispensable to guarantee a lasting system. Thermal insulation, generally placed underneath the waterproofing membrane, completes the roof build-up.
Three different techniques of green roof concepts co-exist in France, and the table below summarises their characteristics:
|substrate thickness||from 4 to 15 cm||from 12 to 30 cm||> 30cm|
|weight||from 60 to 180 kg/m²||150 to 350 kg/m²||> 600 kg/m²|
|type of support admissible||concrete, steel, timber||concrete, steel, timber||concrete|
|choice of vegetation||restricted||wide||very wide|
|overall cost of roof||economical||average||high|
Extensive green roof
This is a recent solution (1980s), radically different from the others: the concept of the garden is abandoned in favour of a notion of a carpet of vegetation (mainly sedum-based), with a natural look and acting as an ecosystem. The notion of low maintenance is also a priority. The growing media are reduced to a minimum (very thin layer of material selected), but suited to a restricted list of compatible plants. This is what is known as an extensive green roof. The notion of a very limited (and precisely defined) load makes its use possible on all types of bearing structure, even lightweight ones, and in many cases on existing flat roofs (substitution of a complete green roof system for the gravel protecting the waterproofing membrane). Maintenance is usually limited to 1 to 3 visits a year, which do not involve complex work.
Extensive green roofs are covered by the "Professional rules for green roofs" published by the CSFE. They can be installed on concrete, steel and timber structures and on slopes of up to 20%.
Semi-intensive green roof
This is an "improvement" on the roof garden, insofar as the growing media are carefully selected (specific substrates replace soil, and the drainage layer usually also serves to retain water). The choice of vegetation (ground-covering plants for example) and the overall design are intended to allow easier maintenance than the "traditional" solution.
Semi-intensive green roofs are not covered by current regulations. Manufacturers' installation specifications may allow for their installation on lightweight support structures (steel, timber), as long as precise control of the loads induced allows it
Intensive green roof or "traditional" roof garden
This consists of transposing onto a roof the elements of a garden which could be created on the ground (thick layer of soil, large plants, laws, trees), whilst taking extra measures such as providing a root-proof waterproofing membrane, a drainage layer and filter layer, a peripheral sterile area, and of course taking into account the very high permanent loads due to the soil. Maintenance is "intensive", that is to say at least equivalent to what it would be for the same garden at ground level, to which can be added the particular constraints of taking materials up to the roof and removing waste.Intensive green roofs are covered by standard NF P 84-204- (ref DTU 43.1). They may only be installed on a concrete support structure with a slope of less than 5%.