Green roofs: 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.

Controlled studies have indicated that maturing green roofs may have improved water retention properties .

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%

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• Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around 50% .
• 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. 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.

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

Water quality

Many green roofs receive only rainwater, which is relatively free of contaminants like nutrients, salt and heavy metals. As such green roofs can contribute contamination, most notably in leaching of nutrients from the growing medium during early establishment. Reported values of total phosphorus in green roof runoff vary from less than 0.1 ppm to over 10 ppm. However 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 for landscape irrigation purposes, so long as it is 'free of solids'. A 'closed loop' system can be created by coupling a rainwater harvesting system with a green roof and harvesting and reusing roof runoff as a source of water for irrigation during dry periods, so the only water leaving the system is through evapotranspiration. This can prevent runoff from leaving the site and thereby prevent contaminant loading to the environment.

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

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.

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.

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

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.

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 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 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.

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.

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.

Relating to the properties, distribution and effects of water on and below the earth’s surface, and in the atmosphere.

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.

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