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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Incentives and Benefits

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.