There are two main types of green roofs, intensive and extensive.
Intensive green roofs have a deep planting bed and can support deeply rooting vegetation as well as pedestrian traffic. This section will solely discuss planting considerations for extensive green roofs.
Extensive green roofs have a porous, light-weight planting medium of shallow depth (40 mm to 200 mm) suitable for a limited number of grass and herbaceous species. Growing medium for extensive roofs varies widely and includes proprietary blends, sand, gravel, crushed brick, compost or organic matter. Extensive green roofs are intended to capture precipitation and roof runoff, require little maintenance, and are designed to succeed with minimal irrigation.
Before construction, a structural engineer must confirm that the load bearing capacity of the building structure and roof deck are sufficient to support the weight of the soil, vegetation and accumulated water or snow, and, if applicable, pedestrians, concrete pavers, etc.
The waterproof membrane must be fully tested before any other layers are applied.
It should be noted that successful green roof applications not only input from a structural engineer is required, but also from landscape architects and landscape contractors experienced with this type of project.
- Vegetation must survive extreme microclimates, including high wind velocities, cold winter temperatures that are not moderated by the ambient heat stored in the ground, and drought.
- Plants must be adapted to shallow well drained soil conditions.
- Conditions most closely resemble the ecology of alpine/subalpine, shoreline, and alvar ecosystems.
- Aesthetics must consider that the practice may be viewed from above or incorporated into a terrace feature.
- Non-native vegetation may be considered due to harsh conditions.
- Drought tolerant through some means other than roots as the planting medium is so shallow, such as succulent species.
- Plants that open their stomates at night to exchange oxygen and carbon dioxide rather than in daytime store water most effectively.
- Plants that are low-growing with spreading roots will be more resistant to wind.
- Vegetation list is comprised of drought-tolerant, perennial and low maintenance species. The list includes a variety of sedums, wildflowers and grasses.
Vegetation must meet Canadian Standards for Nursery Stock, Eighth Edition and Nursery Sod Growers Association of Ontario Specifications.
- Several different types of pre-grown mats are available for green roofs which will provide instant vegetative coverage.
- Plant material should be free of disease, insects, defects or injuries and structurally sound with strong fibrous root systems; roots should be pruned regularly, but not later than one growing season prior to arrival on site.
- If seeding the feature, then seed mixes should be in accordance with Government of Canada Seeds Act and Regulation. To ensure plant coverage within the first year, the City of Toronto green roof construction standard proposes that the green roof should be planted by a seeding rate of 325 seeds/m2, or cuttings distributed not less than 12kg/100m2, or pre-grown plugs installed at not less than 11 plugs/m2.
- The first 2 to 3 weeks is critical to help establish plants and roots.
- A warranty on the vegetation should be included in the construction contract.
- Plants should be replaced as needed to ensure complete coverage and not just at the end of the warranty.