The concept of the green roof has been around since prehistoric times. It consists in covering a flat or low sloped roof with a vegetated substrate (beyond 25 °, substrate retention techniques will be used to avoid erosion).
Many experiments conducted in Europe since the 1970s have shown that for aesthetic purposes or sustainability, as well as with the outlook of restoration or protection of the biodiversity and the urban environment, the development of a green roof is an interesting alternative.
An important point to note, a green roof is not a waterproofing solution in itself and is even designed to remove all excess water when the substrate and the plants are saturated. It is therefore essential to have a sealing system under the green roof, ideally with rot-proof (synthetic) materials.
With our division specialized in green roofs, we are able to offer turnkey solutions for waterproofing and vegetate any surface. From the low maintenance vegetation system to the wide commercial area, we have the equipment and expertise to give life to your projects of any size.
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.
Components / membranes
A green roof system can have very few or several elements, separately or all-in-one, which may include following major:
- Waterproofing membrane – Essential for any roof, it protects against leaks and flushes out the overage water when the green roof is saturated. Ideal membranes for a green roof are synthetic membranes like thermoplastic TPO and in some cases, rubber membranes known as EPDM work very well. Elastomeric membrane or modified bitumen could also be an 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 offering low resistance to roots. A conventional asphalt roof is obviously not recommended for the same reason, and not sustainable enough in addition to being harmful to the environment.
- Protection membrane – Optional, gives added protection for the waterproofing membrane. TPO membrane or loose EPDM again are ideal for their non-rotting properties but several other synthetic and / or water resistant materials can be used for this purpose.
- Anti-root membrane – Only needed on organic roofing types such as bitumen based systems. Other monolayer membranes such as TPO and EPDM materials have anti-root properties.
- Drainage layer – Provides a free path for excess water when the substrate and plants are saturated. Essential component of both all-in-one and layered systems. Very few roof plants strive in dry and soggy soil conditions.
- Water retention – Desirable for the stormwater disposal infrastructure but not necessarily for the plants, to be selected based on chosen species and climate.
- Filtration – Useful for holding a substrate that would otherwise be drained, can also serve as additional rooting medium.
- Irrigation – Water intake should ideally be limited to rainwater, at least for garden systems without vegetables. Irrigation by capillarity should also occur naturally by the substrate added in continuity on the surfaces.
- Substrate (soil) – Extensive roofs prefer minimal soil mixed with a lot of minerals (sand, gravel, etc.) to improve drainage and avoid that succulent plants accustomed to a dry environment soak in water. However, intensive roofs require a lot more nutrients and space for their wide variety of plants, thus a thicker, richer soil. It is of course possible to combine the two to make a hybrid roof, with sections of different substrate thickness and composition.
- Plants and landscaping – Besides the infinite variety of plants to choose from, various landscaping designs can be amalgamated: brown roof with riprap, sand, log … (to add biodiversity), blue roof (to add retention). Although it is possible to walk directly on the soil cover like ivy, rampant clover or grass, it is best to install paths made of slabs, stones or other heavy and durable materials for uses other than maintenance.
Components 3 to 9 can be implemented into a green roof system in a thousand ways, but more conventionally by either all-in-one panels or prevegetalized boxes (always excluding the waterproofing and protection membranes) or by different thicknesses of roll materials, addition of bulk substrate, plants in rolls, etc. All-in-one systems have the advantage of allowing a much easier and less intrusive access to the waterproofing if needed in case of waterleakage.
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.