Green Roof Building Case Study

The ProblemHow to optimize green roof performance

The University of Toronto (U of T) Green Roof Innovation Testing Laboratory (GRIT Lab) was established to investigate and optimize performance of green roofs—roofs covered with plant beds that insulate, absorb precipitation, and provide aesthetic and environmental benefits. The GRIT Lab conducts ongoing experiments consisting of a weather station and 33 raised planting beds that are 1.22 m by 2.44 m (4 ft by 8 ft), with different soil media, amounts of soil, vegetation types, and irrigation regimes. The goal is to evaluate four main variables for green roofs in Southern Ontario:

  1. Stormwater management
  2. Evaporative cooling
  3. Biodiversity
  4. Lifecycle costs

The Solution—Data collection from 33 test beds

This application uses one CR3000 datalogger and two CR1000 dataloggers, along with several AM16/32B multiplexers to measure the almost 300 sensors used to compare the following four parameters:

  1. Growing media type (FLL standard versus high organic content)
  2. Growing media depth (4 in. versus 6 in.)
  3. Vegetation community (sedum versus native and biodiverse prairie-meadow mix)
  4. Irrigation regimes (none, timer activated, soil moisture sensor activated) 

Each bed is instrumented with several temperature sensors, installed at different depths in and under the soil media and heights above the soil. These instruments measure heat flux through the different soil media. An infrared radiometer also measures the average temperature of the vegetation.

Stormwater management and irrigation regimes are also being investigated using high-capacity tipping buckets installed beneath each raised bed to determine runoff from the different soil media. Soil-moisture sensors track how much moisture the soil retains. A climate station measures general weather conditions and rainfall, and a flow meter installed on a pressurized irrigation system approximates amounts of water entering each bed to determine runoff. Different irrigation regimes, soil media, and plant types should have a profound effect on runoff and water retention, especially between storms or watering.

Automated weather stations monitor 33 raised planting beds with different soil media, amounts of soil, vegetation types, and irrigation regimes.

Aerial view of Chicago City Hall green roof on the west end of the City Hall-County Building. Courtesy of the City of Chicago.

Chicago City Hall

Chicago, IL

Building Constructed: 1911

Roof type: Semi-intensive, test/research

Size: 20,300 sqft

Year Installed: 2001

The green roof on the Chicago City Hall building was installed as part of a Environmental Protection Agency study and initiative to reduce the urban heat island effect in the city and improve air quality. The one–block wide, twelve–story building had a substantial roof area that was ideal for planting and data collection. The green roof only covers the City Hall half of the City Hall-County Building, allowing for comparative testing of the green roof and traditional roof.

Plantings are set back from the roof to limit visibility from the street below and allow access for maintenance of the building facades. Courtesy of the City of Chicago.

As a semi–intensive green roof, a combination of 20,000 herbaceous plants, 112 shrubs, and 2 trees were planted. The growing medium varied from a depth of 3 inches for the extensive plantings to 24 inches for the intensive plantings, and the semi–intensive areas averaged 8 inches in depth. Sedums and grasses were planted in the extensive areas, shrubs and deeper rooted plants in the semi–intensive areas, and the two trees in the deepest, intensive areas. The intensive areas were installed on cantilevered platforms over structural columns to support the additional weight. Portions of the roof that were not planted are used to collect rainwater for periods of drought which is stored in two 150 gallon cisterns. A supplemental irrigation system was used to establish the plants and can provide further supplemental water during periods of extreme drought.

Initially the city wanted to make the green roof accessible to building users. Despite the height of the building, the very low parapets meant any railing, if not set back from the parapet, would be highly visible as well as pose safety concerns. A railing set further back from the edge of the roof would have also reduced the area available for the green roof. Currently the green roof is only accessible for maintenance.

A variety of plantings were used on the green roof of Chicago City Hall. Courtesy of the City of Chicago.

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