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.
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 woodbased
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].
Growing Media Trends
Across North America, extensive green roofs, defined by their depth of 15 cm [6 inches] or less, are more commonly used than intensive systems [15cm plus] due to their lower cost and weight. There are two schools of thought in the industry when it comes to growing media composition. The first chooses to follow recommendations from the German landscape association, F.L.L. [Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau], which prescribes a mineral aggregate mix with a low percentage of organic matter, designed to be free draining and to support succulent plants. The other school promotes blends that contain a high proportion of organic matter, designed to support a range of vascular plants [e.g. grasses and flowering herbaceous plants] and have a high water holding capacity.
Interestingly, the North American industry favourites are the FLL blends. Hill et al. [2016] found a number of growing media manufacturers in Canada advocating for mature wood-based compost with up to 50% initial organic content, which points to a growing interest in the benefits organic materials can offer.
“High organic content” refers to a growing medium that contains a minimum of 30% organic content, as measured by loss of ignition [LOI]. LOI is a lab metric commonly used by manufacturers of growing media, that is determined by burning the medium in a furnace. This is quite different from measuring bulk components by volumetric content.
For example, a mix containing 30% organic matter [e.g. compost] and 70% non-organic, matter [.e.g. mineral] content by volume, will have an LOI considerably less than 30%. This is because the compost itself contains only 50-80% combustible material.
While one third of the green roofs examined in this study contained more than 30% combustible organic matter, two thirds of the roofs contained 11% or less, as recommended by FLL guidelines. These FLL-compliant roofs are predominantly planted with succulents. These figures confirm two things; first, that FLL guidelines still dominate the industry and secondly, that there has been little or no experimentation with in-between formulas for growing media in the local market.
The survey by McGlade and Hill [2014] showed a strong trend toward the use of organic material for roofs in southern Ontario, the average among those tested being 53%. This trend may reflect the emphasis that has recently been placed on ecological objectives, such as increased biodiversity, the use of native plants and the attraction of pollinators, as well as the desire to achieve a meadow-like, or naturalized aesthetic.
Myths and Missed Opportunities
What happens to growing media over time?
While some of the findings noted above support current industry practices for the optimal design of green roofs, they also dispel myths that continue to be counter-productive. Contrary to popular belief that mature green roofs experience compaction as particles settle and organic matter decomposes, Hill et al. found no measurable compaction, even in the oldest roof, which was installed 17 years ago.
In fact, they found no correlation between the age of green roof and compaction or settlement of the growing medium. Although natural processes like settling, decomposition and wind erosion will occur, pedogenesis [the constant state of soil reformation through weathering and biochemical processes] and plant decomposition replenishes the growing medium. This dispels the industry myth that growing media with high organic content are more susceptible to compaction. In fact, increased organic matter content was found to have a higher maximum water retention capacity.
Retrofitting cities for flood reduction
Hill et al. [2017] found that the high organic planting medium had up to three times the water retaining capacity of the mineral aggregate when already wet, whether the pre-wetting was due to a previous rain event or irrigation. This makes it a high performing choice for areas subject to extreme weather or persistent and repeated rain events. This high retention capacity supports plant growth, evaporative cooling, drought resilience and greater biodiversity. A network of green roofs in the city that are optimally designed for water retention would be extremely beneficial in curbing stormwater runoff, reducing flooding and improving the quality of downstream natural water systems.
High organic media also tend to be less dense than mineral alternatives, which makes them ideal for reducing the imposed load, particularly on existing structures, without compromising stormwater retention and plant health.
Correlating growing media, plant selection and maintenance practices
As discussed by McGlade and Hill, the original objective for the extensive green roof on the Earth Rangers facility in Woodbridge ON, was to provide stormwater retention and cooling for the building. The FLL growing medium originally specified was planted with succulents only. However, over the first six years the vegetation did not mature, and in fact grew sparse. Clearly the initial design did not meet the performance expectations of either the installer or the client.
Learning from this mistake, the owners replaced an adjacent green roof with a high organic content growing medium to support a mix of native perennial plant species. When analyzing extensive green roofs 10-years of age or older, the composition of the growing medium, rather than its depth, proved to be the most significant design consideration for robust and biodiverse plant growth. Even green roofs with less than 10cm of growing medium, such as on the roof of the Royal Ontario Museum, successfully sustained a broad range of succulents and herbaceous plants, so long as the organic content was at least 50%.
The selection of growing medium has a significant impact on plant succession. The George Vari extensive green roof at Ryerson University in Toronto was planned as a field of daisies, requiring minimal maintenance and no irrigation. Over time, 39 ‘volunteer’ species landed on the roof, transforming the roof into a prairie-like meadow.
While not intended, the high organic growing medium was a significant factor in the naturalization and increased plant biodiversity due to its high-water retention capacity and nutrient availability. Interestingly, while the transformed roof had an ecological value, the building owner was not pleased by the unintended aesthetic outcome, and eventually replaced the roof all together with food crops to meet other pedagogical objectives.
It is important to understand the combined impact of growing media, plant selection and irrigation, or maintenance plans. In this instance, the lack of irrigation, along with inappropriate non-diverse plant selection, led to what was perceived as a complete failure of the green roof. In other words, water-soil-plant relations are interdependent. If one variable is altered, the other two must be adjusted accordingly.
One of the challenges for municipal standards is to clarify the ways in which individual or multiple design factors influence green roof performance, so that designers can make informed choices and tailor their designs to specific site conditions and environmental targets. Hill et al. [2017] demonstrated that daily irrigation decreases stormwater retention capacity, while MacIvor et al [2013] found that having no irrigation decreases plant cover and diversity for non-succulent species.
In both studies however, the organic growing material was proven to be a top performer. So, what is the best green roof configuration? Not surprisingly, the answer is not clear cut, and has much to do with performance objectives. If the primary goal is to maximize water retention, the ideal combination would be the organic growing media, succulent plants and no irrigation. However, if ecological habitat and biodiversity is of greatest importance, then irrigation is a must. In this case, on-demand irrigation, activated via a soil-moisture sensor can contribute to water conservation and water retention capacity.
Conclusion and Recommendations
The composition of growing media is rightly one of the most debated factors in the design of green roofs. The studies cited above demonstrate that growing media composition greatly influences water retention capacity, plant growth and diversity. One of the myths perpetuated around the industry is that organic content contributes to loss of growing medium depth over time, thereby decreasing performance. As a result, most extensive green roof manufacturers still prefer the mineral aggregate growing medium, even though studies show that it is inferior to high organic media with respect to stormwater management, plant growth, biodiversity, and also imposes a higher structural load.
One of the most notable findings is that the organic material significantly exceeds the water retaining capacity of the mineral material even when fully saturated. Given climate change impacts and increased rainfall intensity, aging and inadequate urban water infrastructure, and higher benchmarks for onsite stormwater management, it is critical that we re-examine the default material choices for green roofs holistically.
This means that the selection of growing media must also take into consideration ecological objectives and maintenance requirements. To achieve long-term success, designers must choose the appropriate species mix, growing medium, irrigation and maintenance plan, based on both the local microclimatic conditions and the aesthetic ambitions for the project. Only then can we develop policy to maximize the benefits obtained from green roofs, and achieve them in practice.
