28 l ROOFINGBC l SUMMER 2018 TECHNICAL UPDATE Like the so-called “FM 1-90,” the CAN/CSA A123.21 Standard is a method of testing roof assemblies, and it too is a component of a body of broad-ranging standards – the National Building Code of Canada. CAN/CSA A123.21 forms part of the requirements under Part 5 Environmental Separation. However, the National Building Code (Canada) is quite different in its intent and scope from FM Approvals; in short, it is the national template for building construction, focused primarily on occupant and public safety and liveability. The NBCC also interfaces with other national standards, including the National Energy Code, the Canadian Electrical Code (Part I), the National Fire Code, and the National Plumbing Code. While “FM 1-90” and the CAN/ CSA A123.21 Standard are often both referenced in design specifications, they are in fact quite dissimilar, and while each is appropriate for the parent code it supports, Canadian construction design should be focused exclusively on the Canadian standard and the NBCC. To fully appreciate why, one needs to understand the fundamental differences between the two test standards, and how they fit into their respective parent codes. FM Standard 4470 is a broad-based set of requirements for Class 1 (non-com- bustible) roof assemblies. Negative wind pressure resistance just happens to be one characteristic the assembly must be tested for. The procedures for the wind resistance test are detailed in the Standard, and cross-referenced to another FM Standard, 4474 (American National Standard for Evaluating the Simulated Wind Uplift Resistance of Roof Assemblies Using Static Positive and/or Negative Differential Pressures). In both Standards, roof assemblies are tested in a 12’x24’ mock-up and must be able to withstand a maximum “pressure differential” of 90 lb/ sf. The test, as the title indicates, utilizes a static state of pressure. The word “static” is the key to under- standing the wind uplift requirements in FM Standards 4470 and 4474. The test procedure outlined in Appendix C of FM 4474 requires a test period of only 60 seconds, at a static rate of pressure. The same requirement is stated in Standard 4470, 4.3 Wind Uplift Resistance. While a static test may be useful for underwriting purposes, it doesn’t repre- sent real-world wind dynamics. Wind never blows at a constant rate; it gusts, pushes hard for a while, and then backs off, only to repeat this in endless varia- tions. As wind speed modulates, so does the negative pressure it exerts on a roof surface. The variability in wind speed and pressure occurs because winds are gener- ated both by moving atmospheric pressure systems and by solar radiation that causes local or regional thermals (ascending warm air columns). Pressure variability is also a product of wind deflection caused by structures, geography and colliding air masses. SIDGERS understood this, and developed a dynamic test method – CAN/CSA A123.21 – that simulates the natural cycles of wind variability. Whereas FM Global standards are risk-management focused and propri- etary, the CAN/CSA A123.21 standard is part of a building code focused on public and building occupancy safety. Furthermore, the CAN/CSA standard is focused solely on the effects of wind, and ignores other factors such as fire resis- tance or live loading; those are concerns addressed in other parts of the National Building or Fire Code. And because the CSA test method is solely about wind, its aim is to determine how much negative wind pressure a roof assembly is capable of resisting, even when that pressure is modulated in multiple cycles. To do that, the CSA-based test replicates real-world wind dynamics for up to 5,000 cycles over the course of the test. A test can last up to five hours, depending upon whether or when a roof assembly fails. The CAN/CSA A123.21 test method, as a component part of the NBCC and its approach to wind-resistant roof design, instills confidence in the building design community, because it simulates real wind dynamics. This, in part, is why the RoofStar Guarantee Standards have adopted the NBCC requirements and strongly recommend the use of Tested Assemblies.