Modern green roofs are a rapidly developing commercial market in Ontario. Their primary benefit as a LID technology is in maximizing evapotranspiration for water balance targets. Green roofs are sometimes referred to as 'ecoroofs', 'vegetated roofs', or 'living roofs'.
Green roofs are ideal for:
- Sites without significant space at ground level for infiltration,
- Zero-lot line projects with outdoor amenity requirements,
- Projects looking for accreditation with LEED v.4
Extensive green roofs are the most commonly used type of green roof used for stormwater management in our region.
|Planting medium depth||10 - 15 cm||> 15 cm|
|Loading||up to ~250 kg/m2||Limitless where 'roof' is at ground level|
|Cost||Typically lower||Higher, including structural accommodations and plant selection|
|Maintenance||Depends highly on the aesthetic expectations of stakeholders||Will be comparable to other landscapes, depending on access requirements.|
|Stormwater benefit||Provides best cost-benefit balance||Varies highly|
|Biodiversity benefit||Lower, depends on planting||Greater potential, depends on planting|
|Amenity benefit||Usually visual only
Intensive green roofs are commonly used for amenity space on condo developments or sometimes as urban farms. They are sometimes referred to as roof gardens and encompass diverse uses, design priorities and technical specifications. In the most extreme examples, many urban parkettes including large shade trees may be included in this class, if they have parking garages beneath. As such the discussion on this page is limited to extensive green roofs.8
The fundamental components of an extensive green roof are:
- a roof able to withstand the load
- a roof membrane resistant to root penetration
- a drainage layer
- a filter layer
- a layer of planting medium
Additional components may include:
- an irrigation system
- pre-formed tray modules
Green roofs offer a variety of co-benefits beyond stormwater management. In urban centers they are often constructed to accommodate a roof terrace or amenity space. In this scenario the direct stormwater capture benefit is restricted to the areas with vegetation planted. Another increasingly popular use for rooftop space is for urban farming. Again, the direct stormwater capture benefit is restricted to the areas with planters. To maximize the utility of a green roof as a low impact development tool, coverage with planting should be maximized. In many cases this means only inaccessible spaces are used.
Roof gardens with a high proportion of impermeable surface are popular in high rise developments. These amenity terraces are often described as green roofs, but the LID benefit applies only to the vegetated areas. The stormwater benefit of all kinds of green roofs is maximized by combined with rainwater harvesting for subsequent irrigation. Sedum and native species have all been shown to thrive with daily irrigation to saturation.
Biodiversity opportunities are optimized by planting a variety of species. General advice on this has been prepared by the City of Toronto. In the long term, the richness of species increases owing to 'volunteer species'. The desirability of this diversity varies with the aesthetic concerns of the green roof owner.
Design for Maintenance
Detailed inspection and maintenance advice can be found in Sustainable Technologies' LID I&M guide . The primary operational concern for operating a green roof is the development of a leak. Green roofs protect the roof membrane from UV damage and should increase the lifespan of the roof. However, in the event that a leak is discovered a section of green roof would have to be removed for access. Some proprietary systems appear to be easier to remove and replace. This may come at a cost, as the rainwater retention of the system is somewhat reliant on continuous coverage of the green roof surface.
The green roof media used in Ontario can be classed according to proportion of composted biological material. Some existing installations use materials which comply with FLL guidelines, whilst others use a much higher proportion of compost.
- Test Method for Saturated Water Permeability of Granular Drainage Media E2396.
|Spray||Maximizes evaporation||Requires higher water quality|
|Drip or capillary||Harvested rainwater is readily used without further treatment
Uses less water
|Planting medium does not 'wick' water sideways readily, so can lead to localized dry areas|
The choice of vegetation on an extensive green roof is insignificant in stormwater management compared to the choice of planting medium or the provision of irrigation. The vegetation should be selected to be resilient to both very wet and very dry periods.Sedum species are the most common choice, demonstrating excellent longevity in systems with or without irrigation. Some projects expect the low growing Sedum to remain in graphic designs according to species and flower color. This is not a realistic expectation without significant maintenance costs. Instead project stakeholders should be prepared early in the design process to embrace the green roof as a living and evolving ecosystem. Designs which incorporate both Sedum and native species can help with this.
Aesthetics of the planting must consider that the practice may be viewed from above.
Green roof plants
|Scientific name||Common name||Soil Moisture
1=Dry 2=Med 3=Wet
|Partial shade tolerance||Colour|
|Aquilegia canadensis||Wild columbine||1-3||Y||1|
|Coreopsis grandiflora 'Early sunrise'||Large-flowered tickseed||1-2||2|
|Coreopsis lanceolata||Lanceleaf coreopsis||1-2||2|
|Equisetum hymale||Rough horsetail||2-3||Y||O|
|Geranium maculatum||Wild geranium||2||Y||1|
|Geranium psilostemon 'Rozanne'||Cranesbill Geranium||2-3||Y||3|
|Leucanthemum superbum||Shasta daisy||1-2||2|
|Liatris spicata||Blazing star||2||3|
|Lobelia siphilitica||Blue cardinal flower||2-3||Y||3|
|Lupinus polyphyllus||Large leaf lupin||1-3||Y||3|
|Monarda fistulosa||Bee balm||1-3||1|
|Papaver rhoeas||Corn poppy||1-3||1|
|Penstemon digitalis||White Beardtongue||1-2||O|
|Pycnanthemum tenuifolium||Narrowleaf mountain mint||1-2||Y||O|
|Rudbeckia hirta||blackeyed Susan||1-3||Y||2|
|Rudbeckia nitida 'Herbstsonne'||Cutleaf coneflower||2||Y||2|
|Rudbeckia laciniata 'Goldquelle'||Golden glow coneflower||2||Y||2|
|Schizachyrium scoparium||Little Bluestem||2||O|
|Symphyotrichum novae-angliae||New England Aster||2||3|
|Trifolium repens||White clover||3||O|
Drains and vegetation free zones
Green roofs Gallery
Green roof performance has not been reported to reduce over time. Controlled studies have instead indicated that maturing green roofs may have improved water retention properties .
- In Southern Ontario rainwater retention of extensive green roofs without irrigation is between 60% and 70%
- Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around 50% .
- Daily irrigation can reduce the annual retention by 20% compared to a roof without irrigation. i.e. 40% for irrigated compared to 60% without irrigation. However, recirculating rainwater from a cistern was estimated to double the annual retention in Florida. The research team modeled 87% retention for a green roof coupled with a cistern, compared to 43% for the green roof alone.
Incentives and credits
City of Toronto updated their 'Eco roof' incentive program in 2017 . It now includes grants for structural assessment and is available to non-profit organisations .
LEED BD + C v.4
Sustainable Sites: Open space (1 point)
This credit applies to accessible green roofs on tall buildings with little other outdoor space.
Sustainable Sites: site development - protect or restore habitat (up to 2 points)
This credit applies to green roofs planted with 'native and adapted vegetation' on tall buildings with little other outdoor space
Sustainable Sites: Heat island reduction (up to 2 points)
Green roofs are weighted as effectively as 'High-Reflectance' roofs in a simple calculation to determine the credit.
- Two points (or 1 point for Healthcare) will be awarded if the project manages "the runoff from the developed site for the 95th percentile of regional or local rainfall events."
- Three points (or 2 points for Healthcare) will be awarded if the project manages "the runoff from the developed site for the 98th percentile of regional or local rainfall events."
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.
- Hill J, Drake J, Sleep B, Margolis L. Influences of Four Extensive Green Roof Design Variables on Stormwater Hydrology. J Hydrol Eng. 2017;22(8):04017019. doi:10.1061/(ASCE)HE.1943-5584.0001534
- T. Van Seters, L. Rocha, D. Smith, G. MacMillan; Evaluation of Green Roofs for Runoff Retention, Runoff Quality, and Leachability, Vol. 44 (1): 33 - 47 (2009). Innovative Approaches to Stormwater Management in Canada