www.rcabc.org  l  27 TECHNICAL UPDATE Taking Wind Seriously M ost of us have experienced strong winds, and if we haven’t, we’ve probably all seen (or experienced) the effects of catastrophic wind events – hurri- canes or tornadoes. And who hasn’t seen The Wizard of Oz? These experiences and stories remind us that wind is a powerful force of nature. Still, most people find wind a mysterious phenomenon, and many don’t realize that even mild winds can do a lot of damage to buildings that are poorly designed or constructed. What is more, wind-generated structural damage has less to do with the actual pushing force of the wind, and more to do with the negative pressure forces wind can generate, if given the right conditions. Those “right conditions” generate powerful destabilizing forces on any surface, similar to lift. Pilots and sailors understand lift, which is what keeps an aircraft in the air and makes it possible for a sailboat to sail into the wind1 . While the concept of lift is interesting and complex, it is important to understand that wind generates a kind of lift effect on any surface, and especially on a horizontal roof surface, because of pressure changes in the air above it – specifically, nega- tive pressure that pulls or sucks the roof surface upward. Wind strength quadruples as wind speed doubles. This means that a wind blowing 20 km/h is four times as strong as a wind blowing 10 km/h. It is also true that wind speed and strength increases with altitude, so that wind felt at roof level will nearly always be more forceful than wind blowing at grade. Consequently, I HAVE A FEELING WE’RE NOT IN KANSAS ANYMORE, TOTO Roof design and wind uplift resistance roof design must naturally evolve in response to building height; not all roofs can be expected to resist the same wind pressure loads. TheNationalBuildingCodeofCanada (NBCC), and the RoofStar Guarantee Standards (located in the online Roofing Practices Manual, or RPM), address the design and construction of low-slope membrane roof assemblies. Good design requires an understanding of the stan- dards, why they matter, and how to successfully apply them. “FM 1-90” or CSA? Understanding the Codes and Standards The 2015 National Building Code intro- duced improved methods for designing wind-resistant roofs. Chief among those changes was the adoption of CAN/CSA A123.21 as a reliable method for testing the effective wind resistance of a roof assembly. Still, some designers, builders and manufacturers have not fully under- stood the changes in the NBCC, why or even if they are important for roof design, and what role the CAN/CSA A123.21 Standard plays in the design process. As a consequence, not everyone is “on board” with the changes, and building specifica- tions written today still invoke the “FM 1-90” standard, sometimes with a nod to the CSA test method, and often with little or no explanation about what to do with either of them. Some of the confusion lies in a failure to appreciate the difference between the whole and its parts. “FM 1-90” (a misnomer we’ll discuss below) is a test standard and a component in a body of broad-coverage building material and construction stan- dards generally referred to as “FM Global Standards,” established by the world-wide commercial property insurer FM Global to manage insurance risk. FM Approvals is the gate-keeping certifying body that approves materials and services for use on FM-insured risks. But, FM’s Standards focus on hazard-specific issues like fire resistance or structural stability, in an effort to mitigate underwriting risk through objective risk managed designs and methods. 1  Sailboats can sail ‘to windward’ (also referred to as ‘to weather’) but not directly into the wind. For more about how sailboats sail to weather, pick up any of a number of introductory books on sailing.