While it may seem counterintuitive, the wind load on a flat roof has a stronger impact than on pitched roofs. Wind creates negative pressure on flat roofing, exerting an upward pressure that puts stress on the entire structure. Whether you’re adding a roof to a new building or receiving a partial or full replacement on your existing commercial building, it is essential that your roof (and the system that adheres it to your building) can withstand the strong wind forces it will face. In short, your roofing products need to meet the requirements of the CSA A123.21 standard.
This standard is commonly misunderstood. It is erroneously considered to be comparable to a standard widely called FM 1-90. However, FM 1-90 is a misnomer, not a specific test. The CSA A123.21 is the only standard acceptable under the National Building Code of Canada (NBCC) to meet wind load requirements for flat roofing as of2015.
You may be surprised to hear that this CSA Group standard results from about twenty years of research on how wind impacts roofs in the field, or in the real world, as opposed to laboratory conditions. Unlike other similar roof standards, this standard requires dynamic wind uplift testing. Dynamic testing most closely simulates real-world or in-field wind conditions that roofing systems and buildings face.
If you’re not very familiar with this standard, you may wonder what it entails and how you can ensure your roofing system meets its requirements. We’ll explore the value of the standard and how your designers and roofers should implement it in this article.
The Need for the CSA A123.21 Standard
Wind pressures affect flat roofs differently than pitched roofs. Flat roofs may experience negative pressure or lift. Wind creates this force by blowing across and away from the roof, instead of onto the roof (blowing onto the roof would cause positive pressure, but blowing across and away from the roof creates negative pressure). You may be able to witness these positive and negative pressure forces as “membrane flutter” where the roofing membrane appears to wiggle as the wind moves across it.
As flat roofing systems have sealed components that are adhered or fastened to the other structural elements of the building, the uplift force that causes this phenomenon stresses the entire structure.
Further, wind strength increases with height. The taller the building, the more stress wind places on its roof and the more potential for damage. This is especially true for roofs that are taller than the surrounding structures such that they are in open terrain. In this case wind will affect the building more strongly.
Because of the challenges with wind load on flat roofing systems, it was necessary to come up with a unique wind load test for flat roofing systems, and ideally, one that mimicked real-world wind conditions and failures as much as possible. Before the standard was created, the Oak Ridge National Laboratory did a survey in the United States that revealed the flat roofing industry, insurance industry, and other stakeholders felt that there was a need for a dynamic wind test.
Development of the CSA A123.21 Standard
To develop this test, the Institute for Research in Construction (IRC) created an organization to perform the necessary research. In 1994, they formed the Special Interest Group for Dynamic Evaluation of Roofing System (SIGDERS), which included manufacturers, building owners, insurance companies and more. When SIGDERS began, members sought to design a test that would meet several criteria, including:
- Closely mimic real-world wind effects
- Create failure modes that were seen in the field
- Apply to a wide variety of roofing materials and components
- Allow researchers to process results in no more than a day
A chief concern of SIGDERS was that other wind tests used only static pressure. The group would create a test that was dynamic, simulating the gusts and fluctuations of wind pressure that occur in the field. With a more realistic testing method, manufacturers and designers would be able to create roofing systems and buildings which better resisted real wind effects. Building owners and roofers would also be more informed when recommending and selecting roof systems that can withstand specific wind conditions.
It took ten years, but in 2004 the CSA Group created the CSA A123.21 standard based on the testing and research completed by SIGDERS.
Between 2004 and 2012, Canadian roofers, architects, and building owners may have used the FM 4470 test to assess a roof system’s performance under wind load. In 2012, the SPRI, Single-Ply Roofing Industry organization, released a bulletin that the FM 4470 test was static and that the dynamic CSA A123.21 test should be used instead. Indeed, the National Building Code of Canada (NBCC) did require a dynamic test at the time.
In 2014, the CSA Group adjusted the standard. There were updated requirements for calculating the negative pressure loads placed on buildings by wind and how you should choose a roof system based on this information. This version of the test could also test new kinds of roofs, including those with steel, wood and concrete decks.
Following this, in 2015, the CSA A123.21 test integrated with the National Building Code of Canada (NBCC).
Specifics of the CSA A123.21 Standard
The test itself is a measure of the dynamic wind uplift resistance for membrane roofing systems. This includes mechanically attached, adhered and partially attached (hybrid) membrane roofing systems. A sample roof assembly of 12 by 24 feet is tested to a maximum pressure differential of 100 lbs/sqft.
The test is meant to discover the maximum wind conditions a roof can withstand before it fails. Roofing systems undergoing the test must remain intact through an entire wind pressure level to pass the level. The personnel who conduct the test generate reports on wind performance for the roofing system manufacturer to analyze.
How to Implement the Standard
As a commercial building owner or roofer, your architects or designers and roofing manufacturers perform most of the work of implementing the standard. On new installations, your designers should measure the wind load on your roof. How do you calculate wind load on a flat roof? They can use the IKO’s Wind Uplift Tool to do so.
Once they understand your building’s specific wind loads, the designers should choose roofing systems to meet those loads. Manufacturers release information about a specific roof assembly’s performance in the dynamic wind uplift resistance test in the relevant products’ information or summary sheet. IKO also includes a reduction for the safety margin of 1.5 on these product sheets, making it simple for your designer to see whether the roof assembly which the product is part of meets your building’s requirements immediately.
Thirdly, if the roofing system your designers selected does not already have a specified method to secure the roof assembly to the building (using fasteners or adhesives), then your designers need to choose one and then get approval from the manufacturer.
How the CSA A123.21 Compares to Other Test Methods
The most common confusion about the CSA A123.21 standard is its comparison to the FM 1-90 standard. While the FM 1-90 is widely cited, it is a misnomer and not a specific test method.
In this context, FM stands for FM Global Standards. It is a system of standards meant to assess and mitigate the risk of specific hazards, such as fire and structural collapse, on buildings.
FM 1-90 is derived from the FM Standard 4470, which includes a wide breadth of standards for all noncombustible roof assemblies. Negative wind pressure resistance is just one aspect of the FM 4470 standards. The “1” in FM 1-90 is derived from “Class 1” materials (noncombustible roof systems), and “90” is derived from a requirement to have a negative wind pressure resistance of 90 lbs/sqft. Thus, this is not a specific test at all. Further, FM Global Standards reference various minimums for negative wind pressure resistance, sometimes as low as 60 lbs/sqft.
You may have also heard of FM Standard 4474 “American National Standard for Evaluating the Simulated Wind Uplift Resistance of Roof Assemblies Using Static Positive and/or Negative Differential Pressures.” This test is static and therefore does not meet the National Building Code of Canada’s requirement for a dynamic test.
Ultimately, CSA A123.21 is the only standard that meets the requirements for wind load testing of flat roofing systems under the National Building Code of Canada.
Wind Uplift Resistance is Critical
It is important for commercial roofers and building owners to understand that FM 1-90 is not just a misnomer but also does not guarantee that your roofing system and building will meet Canadian national building code requirements for wind uplift resistance. As wind can be such a powerful force on flat roofing, it is essential that your roof meet the CSA A123.21 standards.
IKO offers other information about the other perils your roof faces beyond wind, and how you can better protect it. That includes protecting buildings from lightning strikes and assessing and removing snow loads from roofs.
