Green roofs

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 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

The fundamental components of a 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.

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  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.

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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 stormwater capture benefit is restricted to the areas with vegetation planted. Another increasingly popular use for rooftop space is for urban farming. Again, the 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. Within the scope of green roof coverage, there is a further distinction made between 'intensive' systems over 15 cm deep, and the more commonly employed 'extensive' systems, which are 10-15 cm deep. Specialized, shallow systems exist but their application is limited.

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.

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

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

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

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

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Design

Roof

Flat roofs should be graded without depressions, with positive drainage ≥2% (1:50) towards roof drains. For roofs with pitch greater than 10% (1:10) additional check dams or cellular components should be included in the design. These structures reduce the flow rate of the draining water, and help to stabilize green roof components. Green roofs can be installed on slopes greater than 20% (1:5), but specialized design advice should be sought for the addition of components required to secure the green roof in place.

Extensive green roofs do not require additional insulation layers. The underlying roof may be of warm, cold or inverted design.

Extensive green roofs add load of around 70 - 300 kg/m². A structural engineer should be consulted during design to account for the distributed loads including snow accumulation and live loads including maintenance staff.

Roof membranes should be waterproof, root resistant, resilient to temperature change, and comply with appropriate CGSB standards as specified in the Ontario Building Code. In most cases a new roof with a modern membrane will not require a separate root penetration barrier. In retrofit scenarios an additional root barrier may be recommended to protect an older roof membrane.

Drainage Layer

The underlying drainage layer is most often a preformed plastic sheet, formed to include depressions for water storage and perforations to drain excess water. This design has the advantage of being most lightweight, but has minimal impact on flow rates once the water has percolated onto the roof membrane below.

An alternative drainage layer solution is to use a granular medium to increase the tortuosity of the flow path and slow peak flow rates.

Planting Medium

In many proprietary systems the default option for planting medium will be a granular material with very low organic matter content. However, many companies can arrange for a high organic content alternative to be substituted if requested. The benefits of 'high organic' media are:

1. Increased water holding capacity, which benefits both vegetation health and stormwater retention,
2. Often lighter weight, even when saturated,

The disadvantages of a 'high organic' planting medium are:

1. Increased phosphorous concentration in runoff water, however this may be mitigated by the reduced runoff volume resulting in a lower loading overall,
2. The lightweight material is more prone to wind erosion, and this should be accounted for in the initial design, alternatives include erosion control blankets, soil tackifiers, or Sedum mats.

Increasing the depth of planting medium from 10 cm to 15 cm has been shown to benefit the vegetation[3]. Stormwater retention was not improved with this increase in depth[4].

Irrigation

Regular irrigation has been shown to substantially reduce the stormwater capture benefit of an extensive green roof[5]. One way to reduce the irrigation used on green roofs is through the use of electronic technologies. Responsive sensors that suppress irrigation after a rainstorm are routinely attached to green roofs to conserve water. Improvements can be made by instead using a 'soil' moisture sensor to trigger irrigation. State-of-the-art management systems now use predicted weather data to prevent irrigation ahead of storm events[6].

Due to their limited water retention capacity, many green roofs are coupled with a cistern to capture the excess water. It then becomes desirable to use as much harvested water to regain the cistern capacity, Green roofs can be irrigated to saturation daily throughout the growing season without damaging the vegetation.

Irrigation technology	Benefits	Disadvantages
Spray	Maximizes evaporation	Requires higher water quality standard.
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

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.

Planting

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.

See: Planting for Green Roofs

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  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

Water Quantity

Water Balance

The key hydrologic benefit which green roofs have over other forms of LID is the proportion of the water returned to the atmosphere through evapotranspiration.

• In Southern Ontario rainwater retention of extensive green roofs without irrigation is between 60% and 70%[7][8][9].
• Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around 50% [Jen][10].
• Using a compost based planting medium improves retention by around 10% i.e. 60 % for compost compared to 50% for granular.
• 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[11]. However, recirculating rainwater from a cistern was estimated to double the annual retention in Florida[12]. The research team modeled 87% retention for a green roof coupled with a cistern, compared to 43% for the green roof alone.

Peak Flow Control

An appropriate NRCS curve numbers for green roofs without irrigation in Southern Ontario is 90 [13][14].

Water Quality

Many green roofs receive only rainwater, which is relatively clean when it lands. As such green roofs can contribute contamination, most notably in nutrient leaching during early establishment. Reported values of total phosphorous in green roof runoff vary from less than 0.1 ppm to over 10 ppm. But, in dense urban centres, green roofs are increasingly being used to receive irrigation from harvested rainwater. Current Ontario Building Code permits the use of rooftop runoff to be reused in this manner, so long as it is 'free of solids'.

A 'closed loop' system can be created by coupling a rainwater harvesting system to a green roof. by catching and reusing runoff, the only water leaving the system is through evapotranspiration. This prevents any runoff from leaving the site and so prevents any nutrient loading to the environment.

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  Total phosphorous concentrations reported in green roof runoff (abstracted from Hill 2017)

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  Recently laid Sedum mats on a ground level podium roof.

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

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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 [15].

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[16].

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[17].

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[18].

Water Efficiency: 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."

SITES v.2

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See Also

• Rainwater Harvesting

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External Links

• Green Roofs for Healthy Cities

Proprietary Systems

Links are provided for information only. Inclusion of a company or product does not constitute endorsement.
You may inform us of any omissions using the link below:

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

SEND US YOUR FEEDBACK ABOUT THIS PAGE

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.

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

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.

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.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

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.

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.

Includes the protection of soil from dislocation by water, wind or other agents.

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

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

Abiotic transfer of water vapour to the atmosphere.

A biological community, including humans and their natural environment.

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

The practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.