Green roofs

Example schematics of extensive green roofs. On the left, a 'built up' system comprising layers: roofing membrane, drainage board, geotextile, planting medium and sedum mat; and on the right a pre-planted plastic module or tray.

Overview

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', '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 in our region.

Types of green roofs: defined by construction depth

Property	Extensive	Intensive
Planting medium depth	10 - 15 cm	> 15 cm
Loading	up to ~250 kg/m2	Limitless where 'roof' is at ground level
Cost	Low	High
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	Will vary highly
Biodiversity benefit	Limited	High
Amenity benefit	Usually visual only i.e. inaccessible	Often accessible

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
• plants

Additional components may include:

• an irrigation system
• pre-formed tray modules

Planning Considerations

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.

Amenity

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[1].

Rooftop farming is also becoming common in some Ontario cities. Again consideration should be given to the proportion of the roof actually planted. Where large open expanses of roof are being cropped, measures may be required to reduce erosion of exposed planting medium. Any digging or furrows will reduce the stormwater benefit.

Biodiversity

Biodiversity opportunities are optimized by planting a variety of species. General advice on this has been prepared by the City of Toronto[2]. 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.

Green roofs should receive as little maintenance as possible. Regular inspection is only required to see that the drains are free from obstruction and that the vegetation coverage is adequate to prevent wind erosion. During detailed design, all areas adjacent to the green roof itself should be kept free from granular material. Sediment accumulates in gravel edging, which then permits the vegetation to take root and spread.

• 

  Example of gravel edging around a roof, which needs weeding.

• 

  Example of gravel edging around a roof drain, which needs weeding.

• 

  The amenity value of this roof is enhanced by the tall and climbing plants screening the high safety fences.

• 
• 

  Very light maintenance has permitted a stand of Mullein to develop on this prairie style green roof, increasing the biodiversity value.

• ==Design==
• Extensive green roofs do not require additional insulation layers. The underlying roof may be of warm, cold or inverted design.
• An alternative drainage layer solution is to use a granular medium to increase the tortuosity of the flow path and slow peak flow rates.
• Maximizing evaporation with spray irrigation is the faster way to empty a stormwater cistern, and provides some cooling benefit for the building and it's surroundings.
• 

  The vegetation on this extensive green roof is designed to withstand the deep shade of being in downtown Toronto.

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  Sloped extensive green roof at Toronto Botanical Gardens, designed with four species of Sedum (Image: Terry McGlade)

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  Same extensive green roof illustrating the increased biodiversity resulting from very light maintenance over 10 years

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Performance

Recently laid Sedum mats on a ground level podium roof.

Detail showing the early root development through the mat to the granular planting medium.

Water quantity

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 [3].

Incentives and Credits

In Ontario

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 [4].

LEED BD + C v.4

LEED offer a relatively large number of points for green roofs compared to other LID technologies.

Sustainable Sites: Open space (1 point) This credit applies to accessible green roofs on tall buildings with little other outdoor space[5].

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[6].

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[7].

Sustainable sites: Rainwater management (up to 3 points)

• 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."
  

OR

• For zero-lot-line projects only, 3 points (or 2 points for Healthcare) will be awarded if the project manages "the runoff from the developed site for the 85th percentile of regional or local rainfall events."

External Links

• Green Roofs for Healthy Cities

Proprietary Systems

In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
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.

• Next Level Stormwater Management
• LiveRoof
• Ginkgo Sustainability
• Zinco
• VitaRoofs

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Abiotic transfer of water vapour to the atmosphere.

Tank used to store rainwater (typically roof runoff) for later use.

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.