Green roof systems; what is it and why?

The principle of the green roof system (also called: green roof, green roofing or eco-roof) has existed since prehistoric times. It consists of covering a flat or sloping roof with a vegetated substrate (above 25 °, substrate retention techniques will be used to prevent erosion).

Numerous experiments carried out in Europe since the 1970s have shown that for aesthetic or sustainability objectives, as in the perspective of restoring or protecting biodiversity and the environment in urban areas, the development of an “eco-roof” turned out to be interesting. According to Back to New Roofing – a roof restoration company in Melbourne, eco roofs are very rare, in part due to the solar panel uptake in Australia.

Importantly, a green roof is not waterproof but it is designed to allow any excess water to pass through when the substrate and plants are saturated with it. Therefore, it is essential to have a waterproofing system under the green roof, ideally with rot-proof and root-resistant materials.

Types of green roofs

There are two main categories: extensive green roofs and intensive green roofs. We will also speak of a hybrid or semi-intensive green roof, which represents a combination of more than one category. Ultra-light roofs are considered here extensive. Terrace roofs and garden roofs are optional additions possible for all categories.

Extensive green roof

Made up of a thin layer of rather mineral soil about 3.8 cm to 15 cm (1½ in. To 6 in.), On which we will plant plants very resistant to drought and other climatic conditions of the environment. Several varieties of succulents such as stonecrop (sedums) have proven their worth on roofs but also certain grasses and perennials selected according to the climatic region.

The extensive system is the most accessible due to its reduced cost and moderate weight of 30 to 73 kg / m2 (6 to 20.5 lbs / ft2), when saturated with water. Note that the building structure must still be the subject of a report signed by a structural engineer, who will analyse and calculate the additional load that the roof is able to support.

Intensive green roof

Made up of a thick layer of organic soil over 15 cm (6 in), this system, much more expensive and heavy (147 kg / m2 (30 lbs / ft2) and more), requires a structure of building reinforced accordingly. Mainly located on new buildings designed according to the considerable additional load, this system not only amplifies all the benefits related to the increase in plant mass but also allows market gardening and even the establishment of small trees and shrubs.

Roof terrace and roof garden

To make the most of your green roof and when it is possible to install safe access to the roof, an infinite number of landscaping options are then possible. As it is not recommended to walk directly on most plants, trails can be incorporated with different materials, preferably heavy, which will not be moved by the wind. Ground cover surfaces such as creeping clover, creeping ivy or even low-maintenance turf (lawn grasses are the champions of trampling), create a relaxation area with moderate traffic. Always without forgetting the weight constraint, it is also possible to incorporate various elements such as a vegetable garden, a patio or wooden decking, a gazebo, a fountain ect.

Components / Membranes

A green roof can have very few or several elements, of which the following are the main ones:

The waterproofing membrane

Essential as for all roofs, it protects against infiltration and evacuates excess water when the green roof is saturated with it. The ideal types of membranes for a green roof are thermoplastic membranes and synthetic rubber membranes called EPDM. Elastomer or modified bitumen can also be a valid option, although it is less resistant to stagnant water and must be protected by an anti-root membrane since it is made from bitumen, an organic compound that does not offer not an absolute resistance to the roots

Sacrifice Membrane

Optional, gives additional protection to the waterproofing membrane. A loose TPO or EPDM membrane is ideal, but several other synthetic and/or water-resistant materials can be used for this purpose.

Anti-Root Membrane

Only needed on organic membrane types such as bitumen membranes. Other single-layer membranes like TPO and EPDM are materials with anti-root properties.

Drainage Layer

Allows excess water to drain away when the substrate and plants are saturated. An essential component of almost all green roof systems. Conventional roofing plants rarely appreciate both droughts and floods …


Desirable in terms of rainwater evacuation infrastructure but not necessarily for existing plants, to be selected according to the species chosen andthe climate.


Useful for retaining substrate that would be drained, can also serve as an additional rooting medium.


Water supply should ideally be limited to rainwater, at least for non-market gardening systems. Irrigation by capillary action should also be done naturally by the substrate placed continuously on the surfaces.

Substrate layer (potting soil)

Extensive roofs will prefer a minimum of potting soil mixed with a lot of minerals (sand, gravel, etc.) to improve drainage and thus prevent succulents accustomed to a dry environment from bathing in water. An intensive green roof will require a lot more nutrients and space for its wide variety of plants, so a larger and richer soil. It is of course possible to combine the two types to create a hybrid system with different thicknesses of a substrate.

A layer of vegetation and landscaping

In addition to the plants of your choice, various landscaping can be combined: brown roof with riprap, sand, log … (+ biodiversity), blue roof (+ retention). Although it is possible to walk directly on the ground cover such as ivy, creeping clover or grass, it is preferable to install paths of slabs, stones or other more resistant materials for uses other than maintenance.

Components can be installed in a green roof system in a thousand and one ways, but in a more conventional way by either an all-in-one system in pre-vegetated panels or boxes (always excluding the waterproofing membrane and sacrifice) , or by different thicknesses of material in rolls, substrate added in bulk, plants in rolls, etc. All-in-one systems will have the advantage of leaving much easier and less intrusive access to reach the waterproofing membrane when needed.

Why choose a green roof?

  • Green roofs are prettier than conventional roofs;
  • Green roofs are ecological: there is less water to evacuate and the flow is moderate. In addition, they refresh in spring and summer.
  • Green roofs are economical: by investing in a green roof, you save money, because it doubles the life of your waterproofing system. In addition, you can benefit from grants.