Preface: This is the fourth edition of CSA A123.21, Standard test method for the dynamic wind uplift resistance of membrane-roofing systems. It supersedes the previous editions published in 2014, 2010, and 2004. This Standard is based on the ongoing research and testing that is being carried out at the National Research Council (NRCC). This edition is applicable to the majority of the commercial roof assemblies in North America under three different categories depending on the component attachment to the substrate, namely adhesive applied membrane roofing system, mechanically attached membrane roofing system and partially attached (hybrid) membrane roofing system. CSA Group acknowledges that the development of this Standard was made possible, in part, by the financial support from the consortia members. This Standard has been developed in compliance with Standards Council of Canada requirements for National Standards of Canada. It has been published as a National Standard of Canada by CSA Group. Scope: 1.1 This test method determines the wind uplift resistance of membrane-roofing systems when subjected to dynamic wind load cycles. The test methods are applicable to a) mechanically attached membrane roofing systems; and b) adhered membrane roofing systems. The roofing system consists of a deck and roofing membrane. It includes components such as vapour barriers or retarders, insulation, cover board, etc. It is subjected to a dynamic load sequence that has been developed based on wind pressure records, simulating the effects of wind on membrane-roof assemblies. Note: The roof membrane may be designated as the plane of air tightness. 1.2 The following limitations apply for the test methods in this Standard: a) Method 1: Testing in accordance with this test method is limited to mechanically attached membrane roofing systems having a fastener row separation not greater than 2896 mm (114 in) and fastener in-line spacing not greater than 610 mm (24 in). b) Method 2: Testing in accordance with this test method is limited to adhered membrane roofing systems. 1.3 The values given in SI (metric) units are the standard. The values given in parentheses are for information only. 1.4 In this Standard, "shall" is used to express a requirement, i.e., a provision that the user is obliged to satisfy in order to comply with the standard; "should" is used to express a recommendation or that which is advised but not required; and "may" is used to express an option or that which is permissible within the limits of the Standard. Notes accompanying clauses do not include requirements or alternative requirements; the purpose of a note accompanying a clause is to separate from the text explanatory or informative material. Notes to tables and figures are considered part of the table or figure and may be written as requirements. Annexes are designated normative (mandatory) or informative (non-mandatory) to define their application.