January 23, 2020
by Abhishek Gaur, PhD, Flonja Shyti, EIT, and Bas A. Baskaran, PhD, P.Eng.
The National Building Code of Canada (NBC) and the National Energy Code of Canada for Buildings (NECB) provide minimum building requirements at Canadian locations based on their historical climate characteristics. Observational records suggest Canada has, over the last 70 years or so, on average warmed at a rate twice the global average, meaning Canada is at higher risk than many other countries to the climate change effects. Future projections suggest a significant shift in key climate variables and their extremes in the years to come—the degree of which will depend on the amount of greenhouse gases (GHGs) emitted by the global community (Read the article “The representative concentration pathways: An overview” by Detlef P. van Vuuren, Jae Edmonds, and Mikiko Kainuma.).
The National Research Council Canada (NRC) is currently undertaking work to integrate climate resiliency into building and infrastructure design, guides, and codes as part of the Climate-resilient Buildings and Core Public Infrastructure initiative. The authors present a framework for climate-resilient design of commercial roofs. This framework will become part of the Canadian Standards Association (CSA) A123.26, Performance Requirements for Climate Resilience of Low Slope Membrane Roofing Systems.
The framework has three distinct parts:
Determination of climate zone severity classes of NBC locations
As shown in Figure 1, the Table C2 of NBC 2015 provides values of design parameters useful for the determination of building and infrastructure elements at 678 locations across Canada. Two design parameters included in the table—1/50 hourly wind pressures and 1/50 one-day rain—were selected for climate resilient performance requirements. These climate elements were chosen because they are of critical importance for resilient roofing design. In other words, the roof has to stay where it belongs without getting blown off by wind. If it remains part of the building envelope, it has to be watertight to provide a livable indoor environment from rain events.
Current values in Table C2 are based on historical observations of climate at those locations. As part of a memorandum of understanding (MoU) between the Environment and Climate Change Canada (ECCC) and NRC to meet the deliverables of Climate-resilient Buildings and Core Public Infrastructure (CRB-CPI) project, ECCC has provided NRC with, among other information and guidelines, future projected changes in design parameters included in Table C2 of NBC. The projected changes have been calculated between a baseline (1986-2016) and multiple future time-periods coincident with potential 0.5, 1, 1.5, 2, 2.5, 3, and 3.5 C (0.9, 1.8, 2.7, 3.6, 4.5, 5.4, and 6.3 F) future increases in global average temperatures (Figure 2).
The climate zone severity classes were derived using this future projected design parameters data on the basis of the projected changes in extreme wind and rainfall variables. The following steps were undertaken to obtain climate zone severity classes for NBC locations.
1. Obtain the range of projected future changes in climate extremes
The range of projected changes in extreme wind and rain were obtained by analyzing the projected changes in 1/50 hourly wind pressures and 1/50 one-day rainfall, respectively. Projected changes at all NBC locations corresponding to global temperature increases of up to 3.5 C were taken into consideration when finding the range of projected changes.
2. Formulate climate zone severity classes
Three climate zone severity classes—normal, severe, and extreme—were formulated separately for wind and rain extremes based on projected changes. The definitions of the extreme wind and rain severity classes were finalized by expert judgment and active discussions with key government and public stakeholders.
3. Assign NBC locations with climate zone severity classes
NBC locations associated with different sets of projected changes in extreme wind and rainfall under different levels of temperature increases were categorized into one of the three categories of climate zone severity classes. This led to the identification of the total number of stations falling into each climate zone severity class for different levels of future temperature increases.
A summary of the projected changes in 1/50 hourly wind pressures at NBC locations is shown in Figure 3 for two extreme levels of projected global temperature increases. The first map shows the projected changes under 0.5 C increase in global temperatures whereas the second one illustrates the projected changes under 3.5 C increase in globally averaged temperatures. From the figure, it is evident extreme wind pressures will change across the NBC locations in the future. More intense changes are projected to occur with larger temperature increases (3.5 C in this case) than smaller rises (0.5 C in this scenario). Overall, the projected changes in 1/50 hourly wind pressures across all NBC locations and under global temperature increases of up to 3.5 C can increase as high as 33 per cent.
Similar to wind, a summary of the projected changes in 1/50 one-day rain at NBC locations is shown in Figure 4 for two extreme levels of future projected global temperature increases. The top panel shows the projected changes under 0.5 C increase in global temperatures whereas the bottom shows the projected changes under 3.5 C increase in globally averaged temperatures. From the figure, it can be seen extreme one-day rainfall is projected to increase across Canada and the magnitude of projected change is higher for larger temperature increases (3.5 C in this case) than smaller ones (0.5 C in this situation). Overall, the projected changes in 1/50 one-day rain across all NBC locations and under global temperature increases of up to 3.5 C were found to range from four to 62 per cent.
Based on the above analysis and in consultation with the roofing community, three climate zone severity classes were defined as normal, severe, and extreme . In the case of wind severity the, ranges are:
The three climate zones for rain severity ranges are:
Finally, the total numbers of NBC locations falling into each of the three climate zone severity classes were established for different degrees of future global temperature increases. These are summarized in Figure 5 where the per cent of total NBC locations (678) falling under each of the three climate zone severity classes are provided. It can be noted more and more NBC locations are projected to switch from normal to severe and extreme classes as the global temperature increases from 0.5 to 3.5 C. It is also determined the switch in climate zone severity classes in the case of extreme rainfall, especially from normal to severe is more drastic and consistent in the case of extreme rainfall than extreme wind. The climate zone severity classes identified for each NBC location and under different levels of future temperature increases were combined with a resiliency index and guidelines for several performance levels. Since the roof systems are designed to last 15 to 20 years, it is recommended climate zone severity classes corresponding to 2 C rise in global temperatures are used as this level of increase is projected to occur around 2050. However, practitioners have the option of choosing classes associated with larger increases in global temperatures in design if they were interested in constructing roof systems for longer time-periods.
Determination of roofing resilience index
Once a climate load is determined, the building designer must then select a roofing resilience index. Figure 6 outlines the definitions of resilience index for the low-slope membrane roofs (LSMR). A roof designed to the minimum requirements specified in building codes (NBC and/or NECB) falls under the resilience index of one. Roofs designed to withstand emergencies and the harshest climates have a resilience index of three, and systems with a resilience index of two fall between minimum and maximum required design specifications. While the climate load is dictated by the building location, the level of the resilience index is selected by the roof designer.
After the climate load is determined and the resilience index is selected, a performance level requirement is generated, as shown in Figure 7. Performance levels are gold (highest), silver (intermediate), and bronze (minimal). For any LSMR design, performance levels can be different for wind, rain, and thermal load, as demonstrated in the example box of Figure 7.
The example output box shown in Figure 7 requires a performance level of gold for wind, silver for rain, and bronze for thermal protection. Only NBC and NECB requirements need to be addressed to meet the bronze requirements for thermal performance. To meet the gold-wind and silver-rain performance requirements, additional specifications from CSA A123.26 must be fulfilled. Figure 8 shows a summary of which clauses pertain to each performance level and requirement.
For the above LSMR design example, clauses 6.1.1 to 6.3.3 from Figure 8 must be met to satisfy the wind performance requirements; and clauses 7.1.7 to 7.1.3 and 7.2.1 to 7.2.6 must be fulfilled to satisfy the rain performance requirements. To show an example of what these clauses entail, clause 6.1.8 for gold level wind-roof performance specifies:
A “peel-stop” bar should be placed over the roof membrane near the edge flashing/coping to provide secondary protection against membrane lifting and peeling, as shown in Figure 9. The bar has to be anchored to the parapet or deck. The spacing is recommended between 102 to 305 mm (4 to 12 in.) on centre. Between each bar a space of a few inches should be left and the bar should be stripped over with a stripping ply.
Similarly, for the design to meet silver (and gold) rainwater resistance performance requirements, clause 7.2.1 specifies:
The designed drainage roof slope shall be a minimum of two per cent.
To summarize, the climate load is determined based on a building’s location, and the resilience index is selected by the designer. These are used together to determine the performance level of a roof. If a performance level of bronze is required, only the relevant NBC and NECB specifications need to be considered. If a silver or gold performance level is required, extra specifications from CSA A123.26 are required.
Updating the wind roof calculator on internet tool
NRC currently hosts a freely available Wind Roof Calculator on Internet (RCI) tool that calculates wind loads on roofs based on NBC 2015 as well as various details about the building’s location and applications. WIND-RCI is widely used by the roofing community for wind load design of LSMR. This existing tool is now being updated such that it will facilitate the designer to streamline the process of selecting a performance level. In addition to the tool’s existing capabilities it will also identify the appropriate severity classes (normal/severe/extreme) based on the city and province, and it will allow designers to select the resiliency index they deem appropriate. The tool will then determine the necessary gold, silver, or bronze performance levels for wind, rain, and thermal conditions.
Abhishek Gaur, PhD, is a research officer with the Construction Research Centre of the National Research Council Canada (NRC). At NRC, he is responsible for providing climate data needed to more effectively test and design climate resilient building systems and for facilitating climate data access to building practitioners by developing effective user-interactive tools. He obtained his masters and PhD degrees from Western University. Gaur can be reached at email@example.com.
Flonja Shyti, EIT, is a civil engineer in training who received her degree from the University of Ottawa. She works in the roofing and insulation group in the Construction Research Centre at NRC. She helps characterize building material properties and determine wind loads based on the National Building Code (NBC) and the American Society of Civil Engineers (ASCE) 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Shyti can be reached at firstname.lastname@example.org.
Bas A. Baskaran, PhD, P.Eng., is a group leader at NRC, where he researches the performance of roofing systems and insulation. He is an adjunct professor at the University of Ottawa, and a member of Roofing Committee on Weather Issues (RICOWI), RCI Inc., Single Ply Roofing Industry (SPRI), and several other technical committees. Baskaran is a research advisor to various task groups of the National Building Code of Canada (NBC). He was recognized by Her Majesty Queen Elizabeth II with a Diamond Jubilee medal for his contribution to fellow Canadians. Baskaran can be reached at email@example.com.
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