Load-bearing structural support
The load-bearing structural support must be sized to withstand permanent loads and accidental overloading resulting from the weight of the actual garden itself, the weight of irrigation water and the weight of people and equipment required for its maintenance in the event the roof is an amenity space.
Various kinds of load-bearing structural support can be used, such as a traditional or prefabricated reinforced concrete deck, a composite steel deck or timber deck.
Depending on the green roof design, the load-bearing structure can have a slope varying from steep to shallow, or may even have no slope (in the latter case, the slope will be created with concrete screed or with suitably arranged insulating panels).
The purpose of the levelling layer is to level out and compensate for protuberances and unevenness in the surface the roof is being installed on in order to avoid the risk of the waterproofing membrane being punctured.
This layer is produced using geotextiles with a suitable weight and relevant properties.
Root-resistant waterproofing layer
The purpose of the waterproofing layer is to stop rainwater and irrigation water getting inside, as well as protecting the load-bearing structural support from deterioration.
This layer must also be resistant to penetration by roots that it might come into contact with.
The purpose of the protection layer is to protect the waterproofing layer from mechanical damage.
The waterproofing system is exposed to the risk of mechanical damage both during the building of the actual garden and while it is in service. There are green roof systems that, given the nature of the materials and/or systems, do not require this layer to be built or already have protection built in.
Drainage or drainage/water storage layer
The purpose of the drainage layer is to allow excess rainwater or irrigation water to drain away so that the growing medium does not become saturated, which could compromise the correct development of the vegetation's root system.
Many green roof systems feature a water storage layer incorporated into the drainage layer so as to have a reserve of water for "bottom-up" irrigation of the vegetation.
The water storage means sprinkler irrigation can be used at greater intervals and, in certain favourable situations, irrigation can be done away with altogether.
The purpose of the filter layer is to stop the growing medium from being wash away and thus clogging and compromising the operation of the drainage layer or drainage/water storage layer.
This is the growing medium, more specifically the natural substrate comprising a blend of soils in which the roof's vegetation can grow and survive.
Vegetation must be chosen carefully, taking into account the environmental conditions and various factors that can affect its development and survival, such as local climate, exposure, the roof's use and maintenance requirements.