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Sustainable building envelope - garden roof system performance
Sustainable building envelope – garden roof system
performance
Liu, K.
NRCC-47354
A version of this document is published in / Une version de ce document se trouve dans: 2004 RCI Building Envelope Symposium, New Orleans, L.A., Nov. 4-5, 2004, pp. 1-14
Sustainable Building Envelope – Garden Roof System Performance
Karen Liu, Ph.D.
National Research Council, Institute for Research in Construction 1500 Montreal Road, Ottawa, Ontario, Canada K1A O6R
What is a Garden Roof System (GRS)?
Garden roof systems (GRS) are specialized roofing systems that support vegetation growth on rooftops. GRS not only add aesthetic appeal to the unused roof space that is available in most urban areas; they also provide multiple benefits in an urban context. From a building’s point of view, the plants and soil protect the roofing membrane from exposure to ultra violet radiation, extreme temperatures and physical damage, thus contributing positively to the roof’s service life. GRS also reduce energy demand on space conditioning, and hence greenhouse gas (GHG) emissions, through direct shading of the roof, evapotranspiration and improved insulation values [1-6]. If widely adopted, GRSs could reduce the urban heat island [7-8] (an elevation of temperature relative to the surrounding rural or natural areas due to the high concentration of heat absorbing dark surfaces such rooftops and pavements) which would further lower energy consumption in the urban area. From a community’s point of view, GRS can be used as a source control tool for the stormwater management strategy in the urban area. Part of the rain is stored in the growing medium temporarily, and to be taken up by the plants and returned to the atmosphere through evapotranspiration [1,6,9-10]. This delays and reduces runoff and takes a load off the city’s storm sewage system. The plants can also remove airborne pollutants and improve the air quality in the urban areas [11].
In addition to the roofing membranes, a GRS consists of several major components, namely, root resistance layer, drainage layer, filter membrane, growing medium and vegetation. Their configuration is show in Figure 1 and their functions are summarized in Table 1. The components act together to provide a suitable environment that supports plant growth while not compromising the waterproofing function of the roofing membrane. GRS can be installed on both conventional and protected membrane systems. In the roofing industry, GRS is generally categorized into extensive and intensive by the weight of the system. Extensive GRS is lightweight, consisting shallow growing medium with small plants (e.g. sedums, herbs and grasses) (Figure 2). These systems require very low maintenance. Intensive GRS is heavyweight and contains much garden soil. The greater soil depth allows growing of bigger plants such as shrubs and trees. These systems require higher maintenance similar to a garden at grade. Table 2 compares the typical load requirement and other features.
What is Sustainability?
There are many definitions of sustainability. Perhaps the most comprehensive and widely used definition was given by the United Nation’s World Commission on Environment and Development in 1987 as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. [12]. The balance of present and future needs is fundamental in sustainability. After speaking to people in all walks of life around the world, the Commission concluded that there was no clear and definite division separating environmental, social and economic issues. Rather, these issues were intertwined, there were links such that problem in one area would affect the other areas. To be more specific for the roofing industry, the Oak Ridge National Laboratory (ORNL) defined “sustainable low-slope roofing” as one “that is designed, constructed, maintained, rehabilitated, or demolished with an emphasis throughout its life-cycle on the efficient use of natural resources and the maintenance of the global environment” for the purpose of the Sustainable Low-Slope Roofing Workshop at ORNL in 1996 [13]. Participants looked at various roofing issues at the time such as energy efficiency, durability, maintenance, reuse and recycle and life cycle analysis. Several papers also advocated that the industry should look beyond low first-cost materials/systems that meet the minimum code
requirements but look more broadly at sustainable issues such as environmental impact, energy efficiency and cradle-to-grave profile [3, 14].
GRS and Sustainability
Buildings account for 30% of energy use and 27% of green house gases (GHG) emission in Canada [15]. Innovation in energy efficient building technologies can significantly reduce the overall energy intensity and GHG emission nationwide, thus contributing positive to Canada’s efforts on climate change mitigation/adaptation. With the rapid growth of many major cities in North American, natural landscape is quickly displaced by development. The City of Toronto, Ontario is the largest municipality in Canada, with a population of about 2.5 million. It is estimated that the tree canopy and natural coverage is approximately 20% while rooftops cover as much as 30-35% of the land surface including existing and proposed development [16]. Similar trends are observed for other major cities in North America. This is clear that the roofing industry can make a significant contribution on sustainability in an urban context. With the rapid adoption of the LEED (Leadership in Energy and Environmental Design) Green Building Rating System in North America, the construction industry is becoming more aware of sustainable building design and construction. As outlined in the Brundtland report Our Common Future, sustainability must be achieved through a balance between 3 domains: environmental, economic and social. Therefore, being “sustainable” is more than being “green” as the three areas are intertwined. This paper will provide highlights of current GRS research in North America. Based on the scientific findings, we will discuss how GRS address sustainability in the three domains.
Environmental
Stormwater Runoff - Quantity
GRS can be used as a source control tool for the stormwater management strategy in the urban areas. The leaves intercept the rain and while the growing medium stores the water temporarily, to be taken up by the plants and returned to the atmosphere through evapotranspiration. The water retention capacity is mainly in the growing medium but the incorporation of “reservoir” type of drainage layer and water retention mat are useful in improving the water retention capacity of GRS with shallow substrates.
Experiment at the National Research Council showed that an extensive GRS with 150 mm (6 in.) of lightweight growing medium could significantly delay stormwater runoff, reduce the peak runoff rate and the runoff volume. Figure 3 shows a rain event of 19 mm (0.75 in.) in 6.5 h recorded at National Research Council’s Field Roofing Facility in Ottawa. The GRS (Runoff-G) delayed the runoff by about 1.5 h and reduced the runoff volume to 2.9 mm (0.11 in.) – a reduction of 85% (Figure 3). It also reduced the runoff flow rate from 2.8 mm/h to 0.5 mm/h as indicated from the slopes of the hydrographs.
Figure 4 shows the total rain and runoff measured at the Field Roofing Facility during spring and summer of 2002. The GRS retained (and diverted through evaporation/evapotranspiration) 245 mm (9.64 in.) out of the 450 mm (17.72 in.) of rain fell during April to September 2002 - a total runoff reduction of 54%. Note that the GRS was not as effective in June as in the other months. This was because it rained steadily during June, and so the growing medium did not have enough time to dry out between rainfall and was saturated with water. This reduced the runoff retention efficiency significantly.
The stormwater management efficiency of GRS depends on many factors such as the roof slope, types and depth of the growing medium, the types of vegetation, the rain pattern (intensity and duration) and the wetting history of the growing medium. An extensive GRS with 140 mm (5.5 in.) of lightweight growing medium installed at a 10% slope at York University in Toronto reduced the total runoff volume by 76% in spring/summer and 37% in the fall [16]. During the spring/summer, the warmer weather increased the evaporation rate in the growing medium, making it drier. This increased the stormwater holding capacity of the growing medium for the next rain event. The stormwater retention capacity of the GRS is mainly in the growing medium but it can be improved by the incorporation of built-in “reservoirs” in the drainage layer and/or a water-retention mat. A study at the Michigan State University showed that using a water-retention mat, extensive GRS with substrate thickness of 25-60 mm (1-2.5 in.) at 2-6.5% slope
could retained 69-74% of rainfall. The study also confirmed that shallower substrate depth and steeper roof slope resulted in greater runoff [17].
A computer simulation study using a Water Balance Model also showed that from a community/regional view point, GRS are potentially very effective in reducing the volume and peak rates of runoff from developed areas in the Greater Vancouver Regions [18]. German experience showed that GRS could be an economical alternative in stormwater management for some cities [19]. In Germany, many towns and cities are required to construct rainwater collection basins because of flood dangers. For Ditzingen, a small city with 25,000 population, it had to build 19 basins over 25 years. The cost per basin amounted to approximately 500,000 Euros but about 17,000 m2 of extensive GRS would retain the same amount of water, at a cost of about 29,000 Euros.
Stormwater Runoff - Quality
Several recent studies have shown that GRS do not improve runoff quality [16, 20-21]. Runoff from an extensive GRS at York University in Toronto was collected and analysed for over 60 water quality parameters [16]. The results indicated that while GRS was effective in reducing loading of suspended solids, nitrogen complexes, aluminium, copper, BOD, manganese and most PAH, however, runoff from the GRS contained higher loading of phosphate, total phosphorous and most metals, cations, anions and COD. A few of the parameters also exceeded the provincial water quality limits in Ontario. Another study in North Carolina found higher loading of total nitrogen and total phosphorus in the GRS runoff than in the rain or runoff from a control roof [20]. It is apparent that the substances were present in the growing medium of the GRS and were leached out during rain events. Since some soil and growing medium might contain high level of pollutants, it is necessary to consult horticultural and environmental experts in selecting growing medium that would minimise the negative impacts on the environment.
Urban Heat Island
The urban heat island refers to an elevation of temperature in the urban core relative to the surrounding rural or natural areas due to the high concentration of heat absorbing dark surfaces such rooftops and pavements [7-8]. The urban heat island poses serious environmental and health concerns to the urban area. It increases the cooling demand in the summer and thus the energy consumption for air conditioning citywide. The high temperature poses health hazards to the young and the elderly and those who have respiratory problems. It also increases the chances of smog formation.
GRS reduce the roof surface temperature by shading and evapotranspiration – a process in which water is evaporated from the leaves and the growing medium, converting the incident solar radiation energy into latent heat (evaporation) instead of sensible heat (high temperature). Environment Canada has shown that GRS was effective in reducing the urban heat island [22]. Using a mesoscale model to simulate the low-level air temperature in Toronto over two hot summer days, the researchers showed that the urban temperature was 2-3°C (4-5°F) warmer in the urban land use region than the surrounding rural areas – the urban heat island effect. With the introduction of a spatially uniform 50% GRS coverage, the model produced a citywide cooling of 0.1-0.8°C (0.2-1.5°F). Furthermore, the model predicted that if enough moisture was available in the growing medium for evaporation, the GRS could reduce the citywide temperature by up to 2°C (4°F ) and expand the region that experienced more than 1°C (2°F) of cooling.
Biodiversity
Natural land has been displaced by development in urban areas and so did the wild life and native plants that lived in it. GRS introduce green space in the urban areas and bring back some of the plants and birds into the cities. Improving biodiversity has been a new driver for GRS in the Great Britain [23]. A Switzerland study showed that GRS provided habitats for insects and birds in the urban area [24]. Based on the observations on spiders and beetles found on GRS and on sites at grade with similar structure and vegetation, it was found that GRS could offer suitable substitute for habitats at grade that were displaced by development. Birds that are commonly found in urban areas as well as those which are naturally found in open landscapes (such as grasslands and riverbanks) were observed on GRS. It was likely that the lack of green space and food in the urban areas that brought the birds to the rooftop green spaces. In addition, some birds were found to breed on the GRS. The researchers suggested that to maximize biodiversity on GRS, attention should be paid to select vegetation, soil types and structures to create habitats that mimic the natural surrounding to optimize the habitat conditions.
Economic
Energy Efficiency
Research at the National Research Council showed that GRS could improve the energy efficiency of the roofing system, thus providing saving in energy consumption for space conditioning. Figure 5 summarizes the average daily energy demand for space conditioning due to heat flow through the roof ONLY as measured from the Field Roofing Facility in Ottawa. The energy efficiency of the GRS (150 mm growing medium planted with grass) was slightly better than that of the Reference Roof in the fall and early winter as the growing medium acted as an insulation layer. However, as the growing medium froze, its insulation value was greatly diminished. Instead, snow coverage provided insulation to the roofing system. The GRS significantly improved the energy efficiency of the roofing system in spring and summer. The growing medium and the plants enhanced the thermal performance by providing shading, insulation and evaporative cooling. The GRS also acted as a thermal mass, which effectively dampened the thermal fluctuations going through the roofing system. The average daily energy demand for space conditioning due to the heat flow through the Reference Roof was 6.0-7.5 kWh/day (20,500-25,600 BTU/day) as shown in Figure 5. However, the growing medium and the plants of the GRS modified the heat flow and reduced the average daily energy demand to less than 1.5 kWh/day (5,100 BTU/day) – a reduction of over 75%.
The Ottawa data indicated that GRS was more effective in reducing heat gain in the spring/summer than heat loss in the fall/winter due to the different thermal mechanisms involved. Observation over 22 months showed that the GRS reduced 95% of the heat gain and 26% of the heat loss as compared to the Reference Roof, with an overall heat flow reduction of 47%. Since an extensive GRS was more effective in reducing heat gain than heat loss, and Ottawa is in a predominantly heating region, it is expected that its effectiveness will be more significant in warmer regions. Note that for warmer region with little snow coverage, dry growing medium can provide insulation values to the roofing system and improve the energy efficiency of the GRS [25]. The actual cost saving depends on the types and efficiency of the heating/cooling equipment and the energy costs.
Durability
An exposed roof membrane absorbs solar radiation during the day and its temperature rises. The GRS blocks the solar radiation from reaching the membrane, thus lowering its temperature. Table 3 compares the number of days out of the observation period (a total of 660 days) when the maximum roof membrane temperature exceeded various levels. During the 22-month observation period, the membrane temperature of the Reference Roof went above 30°C (86°F) over half of the time, above 50°C (122°F) about one third of the time and over 70°C (158°F) in the extreme conditions. In comparison, the membrane under the GRS only went above 30°C (86°F) for 3% of the time and never reached 40°C (104°F). Note that the colour of the membrane was light grey, the temperature of a dark colour membrane would be expected to be even higher.
In addition, the temperature of an exposed membrane tends to fluctuate with the ambient conditions: it absorbs solar radiation during the day and its surface temperature rises; it re-radiates the absorbed heat at night and its surface temperature drops. Daily temperature fluctuations create thermal stresses in the membrane and at the seams. Table 4 compares the median daily membrane temperature fluctuation (daily maximum temperature - daily minimum temperature) of the Reference Roof and the GRS on NRC’s Field Roofing Facility over a 22 month observation period. The exposed membrane in the Reference Roof experienced high daily temperature fluctuation, with a median of 42-47°C (76-85°F). However, the GRS reduced the temperature fluctuation in the roof membrane to a median fluctuation of 5-7°C (9-11°F) throughout the year.
Heat exposure accelerates aging in bituminous material, thus reducing its durability. Daily temperature fluctuations in the membrane create stresses in the membranes and may weaken the seams. Ultra violet radiation changes the chemical composition and degrades the mechanical properties of the bituminous materials. Although long-term durability data is not available, the observations showed that GRS could reduce these environmental stresses on the roof membrane, thus contributing positively to the service life. In Germany, GRS are normally estimated for a 40 years life span, which is significantly higher than the service life of an exposed modified bituminous roof at 15-20 years.
Social
Job Creation
GRS is a major part of the roofing industry in many of the European countries. It employed various trades/professions such as roofing (e.g. engineers, contractors and inspectors), landscaping (e.g. designers, horticulturists and nuseries) and maintenance. In Germany, it is estimated that 14% of all new roofs constructed in 2001 had a GRS. This is equivalent to 13.5 million square meters of roofs. There are more than 1300 businesses in the country providing GRS-related products and services [26]. In Sweden, it is estimated that 70-100 thousands square meters of GRS are built every year [27].
Health and Well Being
Many people believe that tree, shrubs and flowers can foster psychological well being and help reduce stress of urban living. People living in high-density developments are known to be less susceptible to illness if they have a balcony or terrace garden [28]. A study showed that surgical patients who stayed in rooms with a garden view had shorter post-operative hospital stays and took less pain medication than patients with rooms looking out onto brick wall [29]. The International Green Roof Institute in Sweden is heading an initiative to evaluate the horticultural therapy benefits of a health garden installed on an old age home in Malmo, Sweden [27].
Community Participation
A Montreal study showed that GRS could contribute to efficient land use of under-utilized urban spaces, help improve the quality of life of urban dwellers, strengthen social networks and promote equitable access to land and economic growth [30]. GRS could be a participatory, community-oriented and low-cost approach to city greening. Experience from the University Settlement Community rooftop container garden in Montreal showed that the GRS helped to improve social interactions in the community and served as a living classroom for various gardening classes.
GRS and LEED
LEED – Leadership in Energy and Environmental Design is a consensus-based design standard to provide a complete framework for accessing building performance and meeting sustainability goals. It is a rating system with a certification process that encourages wider adoption of green building practices. LEED contains 6 main project design categories:
1. Sustainable sites 2. Water efficiency 3. Energy and atmosphere 4. Materials and resources 5. Indoor environment quality 6. Innovation and design
GRS can contribute directly towards a maximum of 3 points under the sustainable sites category with 1-2 points for stormwater management and 1 point for landscape and exterior design to reduce the urban heat island [31].
Guidelines on GRS
“The Guidelines for the Planning, Executing and Upkeep of Green-Roof Sites” [32] published by the Landscaping and Landscape Development Research Society (FLL) in Germany is probably the most advanced standard on GRS. It contains various information on GRS such as specification, construction, material testing, planting, maintenance and safety. Some of the technical requirements that would be of particular interest to the roofing profession will be highlighted and discussed briefly.
In terms of planning specification, the load bearing capacity, roof slope and thermal insulation of the roof structure are important considerations. The roof slope should be at least 2% for extensive GRS. As the slope increases, the rate of water runoff also increases. For slope greater than 5%, growing medium with fairly high water storage capacity and poor drainage or vegetation that has a low water demand should be used to compensate for the greater water runoff. As the slope increases, special considerations should be taken to protect the GRS against shear and slide. GRS is not recommended for roof angle greater than 45° due to danger of sliding.
Considerations should be given to the plants as well. When planting on non-ventilated roof with no thermal insulation, the underside of the roofing is exposed to sub-zero temperatures so special considerations should be taken to minimize the risk of frost damage to the plants. Also, the materials used must not contain substances that are harmful to the plants or find their way out into the environment. In terms of technical requirements for construction, it is important to protect the membrane against root penetration. This can be achieved by protective sheeting, full surface treatment or the use of non water-permeable concrete. The roots of certain plants can be extremely aggressive such as bamboo and variety of Chinese reeds. Special considerations such as multiple root-penetration barriers should be used. The resistance to root penetration of a GRS would have to pass the “Procedure for investigating resistance to root penetration at green-roof sites. This involves subjecting the root-penetration barrier to aggressive plant roots over a fixed period.
Drainage on GRS also requires special attentions. The roof outlets on GRS should be constructed in a way that is permanently accessible and not be covered with greenery or loose gravel. Plants must not be allowed to grow into the gutters and block the drainage path. Inspection shaft should be installed in roof outlets that are located within the vegetation areas to allow inspection to ensure that no plants have grown over the outlet. Outlets that are located away form the vegetation areas are left lying loose in a strip of gravel, or allowed to lie flush with the upper edge of paving in a paved areas. For roofs with steep slope, the dimensions of the drainage system and the eaves construction should take into the considerations that greater volumes of runoff is expected due to the roof gradient. This is especially true for valley gutter where runoff from two roof surfaces is collected. The use of overhanging plants with vigorous growth should also be avoided in area around the eaves to ensure free drainage.
When the waterproofing and root penetration barriers are not affixed rigidly, the growing medium at the site must prevent them from wind uplift. At edges and corners where the wind uplift is expected to be high, or in situation where the weight of the dry growing medium is, low gravel and slabs could be used. Deeper growing medium is sometimes needed to strategically increase the load in certain areas to prevent wind uplift.
Fire resistance is another important area as dried plant materials are susceptible to fire propagation. According to the FLL Guidelines, extensive GRS may be deemed to have sufficient fire resistance if:
(a) the growing medium meet a composition and depth requirement; (b) the vegetation has a low fire load;
(c) if there is a minimum space of 500 mm (20 in.) between the vegetation areas and any roof penetrations.
In addition, the use of succulents will provide a higher fire resistance than grasses. An irrigation or sprinkler system would further reduce fire risk [25].
Unfortunately, there are currently no formal standards or guidelines on GRS in North America. ASTM International has formed a Green Roof Task Force under its subcommittee E06 on sustainability to develop standards for GRS. Several standards on GRS are being developed with the task group but they are not available for public use yet.
Funding and Incentive Programs Related to GRS
Scientific research showed that GRS could contribute positively to sustainability – environmental, economic and social. GRS has a higher initial cost due to the additional components involved and this is
one of the major barriers in the North American market. In some cases, the energy saving and the improved durability do not necessary justify the high initial cost and long payback period for the building owners. Unfortunately, the current market fails to address some of the non-monetary benefits GRS offer on a community level. For example, the building owner pays for fresh water rather than wastewater discharged into the city’s sewage system. This fee structure unfairly subsidises big box stores where runoff from the roof and parking lots incur a high load on the sewage infrastructure and discourages implementation of innovative technologies to reduce runoff. Also, no monetary provisions were made on the benefits of city greening through GRS such as improved biodiversity and the overall well being of the community. Unless the market would assign appropriate monetary values on these environmental impacts, the true cost-benefit of GRS could not be fairly evaluated. In this case, government should be responsible to take leadership and provide investment to make up for the market failure to acknowledge the significant social and environment benefits that GRS offer.
In Germany, there are many public policies that successfully promote implementation of GRS. These incentive programs often subsidize the cost of GRS and other greening projects on private property by 50-100%. A study by Keeley [26] suggested that some of these policy instruments such as mitigation regulations, stomrwater fees based on impermeable surface area and decoupling plans paired with targeted subsidies held potential for adaptation in North America. In terms of stormwater mitigation, Portland, Oregon has successfully implemented an incentive program (Clean Air Incentive and Discount Program) based on impermeable surface area to encourage the installation of green roofs on commercial, industrial, institutional and residential properties, with the aim of reducing the stormwater runoff problem and relieving the loading on the sewage infrastructure [33].
In Canada, there are currently no government policies that relates directly to GRS, however, there are other policies that concern sustainability, natural conservation, clean air and water and urban renewal [31]. While there are no financial incentives specific for GRS implementation, GRS is generally recognized for its energy efficiency and environmental benefits that it is eligible for programs in these areas. Here is a list of selected Canadian programs that support GRS for various reasons:
• Commercial Building Incentive Program (CBIP), Office of Energy Efficiency, Natural Resources Canada – offers financial incentives for incorporation of energy efficiency features in new commercial/institutional building designs. http://oee.nrcan.gc.ca/newbuildings/cbip.cfm
• Industrial Building Incentive Program, Office of Energy Efficiency, Natural Resources Canada – offers financial incentives for incorporation of energy efficiency features in new industrial building designs. http://oee.nrcan.gc.ca/newbuildings/ibip/ibip.cfm
• Green Municipal Enabling Fund (GMEF), administrated by the Federation of Canadian Municipalities (FCM) – support feasibility studies to access the environmental and economic feasibility of innovative municipal projects. http://www.fcm.ca/english/
• EcoAction, administrated by Environment Canada – help projects that will have measurable positive impacts on the environment (e.g. clean air/water, GHG reduction, climate change). http://www.ec.gc.ca/ecoaction/index_e.htm
• Toronto Atmospheric Fund – support projects that have positive benefits on the local environment. http://www.city.toronto.on.ca/taf/
• Green Roof Financial Incentive Program of the Energy Efficiency Fund by Gaz Metro - offers $1 CDN per square foot incentive for GRS implementation in the province of Quebec.
http://www.fondsee.qc.ca/en/fund.htm
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Acknowledgement
The author would like to acknowledge the members of the Rooftop Garden Consortium – Bakor, Canadian Roofing Contractors’ Association (CRCA), EMCO, Environment Canada, Garland, Hydrotech, IKO, Oak Ridge National Laboratory (ORNL), Public Works Government Services Canada (PWGSC), Roof Consultants Institute (RCI), Soprema and Tremco – and the Climate Change Action Fund for providing financial support and technical expertise for the monitoring experiment at the Field Roofing Facility.
Table 1 Major components and their functions in GRS.
Component Function/Remark
Root Resistant Layer
To control root damage to the membrane. This could be a chemical repellant agent in the formulation of the membrane or a physical root barrier, which can be a layer of PVC, polyester or high-density polypropylene.
Drainage Layer To remove excess water from the growing medium. This can be a layer of gravel, specialized polymer foam panels or highly porous polymeric mat.
Filter Membrane To prevent fine particles in the growing medium from clogging the drainage layer. It is a geotextile material.
Growing Medium To support plant growth. The composition and depth depend on the vegetation selected. Artificial lightweight growing medium can be used to replace regular soil to reduce structural loading.
Vegetation Plants should be selected for their adaptability to the local climate conditions Irrigation system might be needed depending on the plants and climate.
Table 2 Typical weight and features of extensive and intensive GRS.
Loading Classification Typical Weight of System Typical Depth of Growing Medium Features and Functions Level of Maintenance Accessibility Extensive <290 kg/m 2 (< 60 psf) <200 mm (< 8 in.) Ecological setting, viewed from above or surrounding buildings Low maintenance - little or no irrigation needed when plants have been established. Limited - seldom entered except for maintenance Intensive >290 kg/m 2 (> 60 psf) >200 mm (> 8 in.) Complete garden or park-like features High maintenance - irrigation and regular garden maintenance required Accessible -provide green spaces for occupants
Table 3 Statistics on the daily maximum temperature of the roof membranes (grey colour) on the Field Roofing Facility in Ottawa during the observation period (660 days in total).
Reference Roof Garden Roof Ambient
Temperature
Greater Than: No. of
Days % of Days No. of Days % of Days No. of Days % of Days 30°C (86°F) 342 52 18 3 63 10 40°C (104°F) 291 44 0 0 0 0 50°C (122°F) 219 33 0 0 0 0 60°C (140°F) 89 13 0 0 0 0 70°C (158°F) 2 0.3 0 0 0 0
Table 4 Median daily temperature fluctuation of the roof membranes on Field Roofing Facility in Ottawa during the observation period (Nov 22, 2000 – Sep 30, 2002).
Median Daily Temperature Fluctuation
(daily maximum temperature – daily minimum temperature) Observation Period Reference Roof Membrane Garden Roof Membrane Ambient Winter 2001 9°C (16°F) 6°C (11°F) 10°C (18°F) Spring 2001 46°C (83°F) 6°C (11°F) 13°C (23°F) Summer 2001 47°C (84°F) 7°C (13°F) 12°C (22°F) Fall 2001 23°C (41°F) 5°C ( 9°F) 8°C (14°F) Winter 2002 9°C (16°F) 7°C (13°F) 9°C (16°F) Spring 2002 42°C (76°F) 6°C (11°F) 10°C (18°F) Summer 2002 47°C (84°F) 6°C (11°F) 12°C (22°F)
Growing Medium Drainage Layer Filter Membrane Roof Membrane Deck Vegetation Support Panel Thermal Insulation
Vapour Barrier (Retarder) Root Resistance Layer
Figure 1 Principle components of GRS installed on a conventional roofing system.
Figure 2 An extensive GRS installed on the Field Roofing Facility in the NRC campus in Ottawa (2002). Note that the median divider separates the GRS (right) and the Reference Roof (left).
0 5 10 15 20 25 12:00 13:30 15:00 16:30 18:00 19:30 21:00 Time R a in , R u n o ff (m m ) Rain Runoff-G Runoff-R 020927, Rain Event
A summer rain event: 19 mm over 6.5 h
Figure 3 Hydrographs (cumulative rainfall plots) of a summer rain event. Runoff-G and Runoff-R represent runoff recorded from roof sections with and without a GRS (150 mm growing medium with grass), respectively.
Rain and Runoff Measured at FRF (April 2002 - September 2002) 0 50 100 150 200
Apr May Jun Jul Aug Sep
R a in o r R u n o ff ( m m ) Rain Runoff-R Runoff-G
Figure 4 Rainfall and runoff measured at the Field Roofing Facility in Ottawa. Runoff-G and Runoff-R represent runoff recorded from roof sections with and without a GRS (150 mm growing medium with grass), respectively.
Average Daily Heat Flow Through Roof Systems (Nov 22, 2000 - Sep 30, 2002) 0 1 2 3 4 5 6 7 8
Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep
H eat Fl ow ( k W h /d ay) 2000 2001 2002 Reference Green
Figure 5 Heat flow measurement showed that the average daily energy demand due to the heat flow through the GRS was significantly less than that of the Reference Roof in the spring and summer.
