Green roof system is the roof of a building that is partially or completely covered with  vegetation, either in soil or in a suitable culture medium. It does not refer to green roof, as that color tile ceilings nor to container gardens. It refers instead to technologies used on roofs to improve habitat or saving energy, ie technologies that serve an ecological function.

Before starting the implementation of any green roof was to meet existing classes to select the most appropriate in each case; as the design, construction, maintenance and evolution of the roof shall be subject to the kind of roof is to be implemented.
Traditionally green roofs were classified into three different types depending on the depth of the substrate and the maintenance requirements: Intensive, semi-intensive and extensive . And these are the main types mentioned commercially. But in addition to these three classes is another, the cover " biodiverse "which shares many of the characteristics of an extensive cover. In addition, within some of the major categories, we are also subclasses.
As already indicated in vegetated covers a green cover (also can be called living cover) is a roof deck or roof (or any other surface structure of a building) on which the vegetation grows artificially or settle, too artificially habitats for wildlife development.

Extensive Green Roof

Extensive green roof system act as a layer with ecological functions, which has benefits, both environmental and economic costs of maintaining the building where it implant (serve as thermal insulation). This system is the lightest of all and cheaper. Still best to install on roofs or in inaccessible steep slopes.
They are characterized by fairly light, with a substrate of reduced thickness and low concentration of nutrients. Suitable for such plants covered with some species of sedum, some perennials and grasses. Overall resistant plants with few requirements for water, fertilizer or the addition of nutrients beyond those obtained naturally from the substrate. Once established vegetation, require minimal maintenance.
These systems are well in renovation projects and for large areas because the plants used are rapidly growing and create areas of dense vegetation. Designers can incorporate water retention systems design to reserve rainwater for irrigation of planted species and encourage their development.
These substrates installed thin allow to collect about 2.5 cm sheet of water after a rainfall event. The soil mixture used for growth has means percentages: 70% of type expanded slate rocks, 20% sand and 10% organic compost. With a thickness between 8 and 10 centimeters and holds a load of 120 to 150 kg / m². Usually no problems weeds or invasive species while the proportion of organic material is not excessive. While the potential for biodiversity is very limited. This system can be installed very well by modules.
Within this type of roof can be distinguished:

Extensive light: characterized by low potential of biodiversity and the minimum holding capacity rainwater.
Extensive super-light: very thin layers, the drainage sheet is only 12 mm and the layer of vegetation only reaches 25 mm. Plant diversity is very limited and tend to dry.

Intensive green roof

Installing intensive green roof is comparable to the construction of a garden in a deck and providing similar to domestic gardens or parks small benefits. So often also they called the green roof. Since the plant variety is broader than in extensive layers require higher structural loads and thicknesses of substrate to retain more water and keep the vegetation. In addition, maintenance must be periodic, with common tasks like gardening and mowing, fertilizing, watering or weeding.
Often they placed on underground car parks or on top of shopping centers (in the US of course) who want to include a play area. These systems require regular watering, and an elaborate drainage system. They are multifunctional, but are mainly designed to give them a recreational use. These covers are generally accessible to the public and more expensive to maintain annually. Many of the designs of these covers include the management of rainwater and even some of re-circulation systems for irrigation runoff.
Intensive green roof is the one that provides greater isolation, which is an advantage in reducing the energy consumption of a building. Also, they filter and purify the air and urban rainwater that sometimes creeps contaminants in precipitation, which has great environmental benefits.

Semi Intensive Green Roof

The features of the semi intensive green roof are a combination of those associated with intensive green roof and extensive green roof, with an average depth of between 120 mm and 250 mm and some maintenance needs. This system is ideal for fine roofs are partially accessible. Often they included in roofs with a social-recreational use so that landscape features are added to make them more attractive. Usually these systems are installed on roofs of metal or concrete / cement which can withstand overweight and waterproof membrane.
A semi intensive green roof is characterized by a higher plant variety, compared to extensive systems. You can support herbaceous plants, grass or small bushes. It requires moderate maintenance and occasional watering. This will depend on the selected plant species. This system can hold more water storm extensive indoor and provides greater ecological wealth.
The mixture comprising this type of light substrate consists of a 55% expanded slate rock type, 25% sand and 20% organic matter and can retain 1-3 cm of rainfall. It is important to ensure that there is good drainage for the plants and their root systems are sufficiently ventilated.
In case there are limitations to water these covers are perennial herbaceous species that can overcome these limitations.