Digging Deep

Unearthing the Truth About Green Roof Growing Media

The Green Roof Innovation Testing Laboratory [GRIT Lab] at the University of Toronto, John H. Daniels Faculty of Architecture, Landscape, and Design, is focused on investigating the environmental performance associated with ‘green’ and ‘clean’ technologies such as green roofs, green walls, and green roof integrated photovoltaic arrays.

By Hadi El-Shayeb, Liat Margolis, and Jennifer Drake

Green roof technology is widely recognized as a key component of sustainable building design across North America. The wide range of benefits associated with green roofs is captured to varying degrees in the U.S. Green Building Council’s LEED rating system. USGBC acknowledges that not all green roofs are made equal and the extent to which a green roof can help earn credits varies accordingly. The LEED guide offers general design direction, while municipal standards tend to establish specifications that address regional conditions and priorities.

Although the City of Toronto is a North American pioneer in green roof design policy, the City’s 2009 Green Roof Bylaw set few locally nuanced performance targets. The Bylaw stipulates that all developments above 2,000 sq. m. [21,528 sq. ft.] must have 20-60% of their roof area installed with a green roof. Beyond coverage, the Bylaw simply requires that the selected plant species survive for at least 3 years and maintain a minimum of 80% vegetated cover.

This benchmark is problematic, as it may result in green roofs that fail to deliver the other important benefits widely associated with them, namely: stormwater management, reduction of roof surface temperature, and provision of nesting and foraging habitat for a diverse range of native pollinating species. Municipal standards, and hence design practice, could benefit greatly from practical information that clearly defines the influence of individual or multiple design factors on a range of environmental performance targets.
Arguably, the most important design factor is the composition of the growing medium. Green roof growing media are generally defined as lightweight engineered materials designed to withstand wind erosion and support plant growth in very shallow depths.

However, one can find a range of proprietary recipes that contain a broad array of local and imported mineral and biological ingredients, including shale, sand, brick, peat, wood bark, coconut fibre, and others. Each recipe results in different biochemical and structural attributes including nutrient availability and water holding capacity. As research shows, the planting media greatly impacts the hydrological, thermal, and ecological quality of a green roof.

This article presents research undertaken by the University of Toronto Green Roof Innovation Testing Laboratory [GRIT Lab]. The GRIT Lab was established with the goal of investigating the environmental performance of green roofs specifically for the Toronto context. However, with Toronto ranking second among North American cities in green roof area, and with many Ontario manufacturers working across the continent, the results of this research are transferable to similar climate regions.

GRIT Lab brings together researchers and students from multiple fields including landscape architecture, engineering, biology, forestry, and planning and partners with the industry and policy makers. Its first study investigated four green roof design parameters distributed in 33 test beds, each instrumented with sensors to measure temperature, soil moisture and water discharge:

  • growing medium type [mineral-based and wood-based compost];
  • planting type [sedum plants, as well as grass and herbaceous flowering plants];
  • depth [100 and 150 mm [4 and 6 in.]; and
  • irrigation practice [none, daily, and on-demand].

In parallel, multiple studies were undertaken on constructed green roofs across the City of Toronto – ranging in ages up to 17 years old – to survey common industry practices, compare initial design with the long-term character of mature roofs, and dig deeper into industry debates about growing media.

Liat Margolis is Director, Master of Landscape Architecture Program and Associate Dean, Research at the University of Toronto. Hadi El-Shayeb, GRIT Lab is Assistant Researcher and Dr. Jennifer Drake, Assistant Professor of Civil Engineering at the University of waterloo.


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