Green roofs

Overview

Green roofs were traditionally employed for insulation and increased resiliency in cold climate regions of Europe. Modern green roofs are a rapidly developing commercial market in Ontario. Their primary benefit 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

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Planning Considerations

Green roofs offer a variety of co-benefits beyond stormwater management. In urban centres 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 in 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.

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Design

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.

Planting

The choice of vegetation on an extensive green roof is insigificant 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 without irrigation.

However, 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 huge maintenance costs. Instead project stakeholders should be prepared early in the design process to embrace the green roof as a living and evolving ecosystem.

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Performance

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. 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'.

Where rainwater is harvested and reused to irrigate a green roof, there is no significant discharge of stormwater from the system, and no loading of contaminants downstream.

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Incentives and Credits

In Ontario

City of Toronto updated their 'Eco roof' incentive program in 2017 to include grants for structural assessment and increased their scope to non-profit organisations [1].

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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.

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 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.

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.

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.

(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).

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

A biological community, including humans and their natural environment.