Wind loads for membrane roofing systems must be calculated in accordance with Part 4. The tested uplift resistance and factored load should satisfy the requirements of the Commentary entitled “Limit States Design” in the “Structural Commentaries (User’s Guide – NBC 2020: Part 4 of Division B)”.
The test method described in CAN/CSA-A123.21, “Standard Test Method for the Dynamic Wind Uplift Resistance of Membrane-Roofing Systems”, applies only to membrane roofing systems whose components’ resistance to wind uplift is achieved by fasteners or adhesives. It does not apply to roofing systems that use ballasts, such as gravel or pavers, to secure the membrane against wind uplift.
In the case of membrane roofing systems in which the waterproof membrane is attached to the structural deck using mechanical fasteners, the wind-induced forces and the roofing system’s response are time- and space-dependent and, thus, dynamic in nature. Further information on the design and evaluation of such systems can be found in “A Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs”, published by NRC.
The wind uplift resistance obtained from the test method in CAN/CSA-A123.21 is limited to configurations with specific fastener or adhesive patterns. To extrapo l ate the test data to non-tested configurations, refer to ANSI/SPRI WD-1, “W ind Design Standard Practice for Roofing Assemblies”, for a r ational calculation procedure.
Ho wever, in using this ex trapolation procedure, wind load s should be calculated in accordance with the NBC. NRC’s guide for wind design r efe renced above provides further guidance and examples of wind load calculations.