Categories of green roofs
There are two main categories: Extensive green roofs and intensive green roofs. There is also the hybrid or semi-intensive green roof, which is an amalgam of several systems. Ultralight roofs are considered extensive. A roof terrace and a roof garden are optional additions for both categories.
- Extensive green roof: Consists of a thin layer of a rather mineral compost about 3.8 cm to 15 cm (1½ in to 6 in) thick, in which very tough plants are being planted.
Those plants are ideally native but other rustic species resistant to drought and/or other climatic conditions of the environment can be used. Several varieties of succulents such as stonecrop (sedum) are proven on roofs but also some perennial grasses and plants selected according to climatic region.
The extensive system is the most accessible due to its low cost and its moderate weight of 30-73 kg / m2 (6 to 20.5 lbs / ft2) when saturated with water. Note that the building’s structure must still be subject to a report signed by a qualified structural engineer, who will analyze and estimate the additional load that the roof is able to support.
Intensive green roof: Consists of a thick layer of organic soil 15 cm (6 in), this system much more expensive and heavier (147 kg / m2 (30 lbs / ft2) and more) requires a reinforced building structure. Mainly installed on new buildings designed according with this considerable extra load in mind, this system not only maximizes all the benefits related to the plant mass increase but also allows vegetable gardening and even planting small trees and shrubs.
Roof terrace and roof garden: To make the most of a green roof and when it is possible to install a safe roof access, endless landscaping configurations are achievable. As it is not advisable to walk directly on most plants, pathways can be incorporated with different heavy materials, which will not be displaced by the wind. Groundcover plants such as white clover, creeping ivy or low maintenance grass (grass turf is the trampling champions), can create a relaxation area for moderate traffic. Without forgetting weight stress, it is also possible to incorporate various elements such as a vegetable garden, patio or wood deck, a gazebo, a fountain, or even a water basin or pond.
When access to the roof is planned for purposes other than maintenance (rest area, vegetable garden, etc.), it is mandatory in Canada to install a guardrail or parapet with a minimum height of 1.07 m (3.5 ft). It is also essential to inquire about municipal regulations, knowing that some municipalities require for instance that the guardrail is not visible from the street and is set back from the roof’s edge.
Some important facts to note:
- A green roof system is not strictly waterproof per se and is even designed to evacuate any excess water when the substrate and the plants are saturated. A waterproofing system under the green roof is therefore required and will ideally be made of a rot-proof material for the touching part receiving the excess water from the green roof.
- Any greening project should be subject to a structural analysis by a structural engineer to ensure that the entire building (and not only the roof) can support the extra weight, even for an extensive green roof.
- During periods of drought, minimal monitoring should be provided for a system without mechanical irrigation, especially during the first two years. A few inspections per year or as needed are necessary for all types of green roofs to prevent unwanted people from taking root there.