Making progress

A North American group shows how to design durable mechanically attached flexible membrane roof assemblies

For several years, the Canadian National Research Council's Institute for Research in Construction has been managing the Special Interest Group for Dynamic Evaluation of Roofing Systems (SIGDERS)—a consortium of manufacturers, building owners, trade associations and researchers. SIGDERS has helped advance the knowledge for the testing and design of mechanically attached flexible membrane roof assemblies in North America. These types of assemblies, composed of flexible sheets (the waterproofing component) and mechanical attachments to resist wind uplift, are used on more than 30 percent of North American low-slope buildings, according to NRCA's 2004 Project Pinpoint. What follows is an explanation of the SIGDERS test protocol and application recommendations based on the testing. The information is based on the paper "A Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs," co-authored by Tom Smith, president of TLSmith Consulting Inc., Rockton, Ill., and me.

Calculation and evaluation

Determining design wind loads is the first step in designing a roof assembly. Wind loads on buildings are determined in accordance with building codes or standards; in Canada, the National Building Code of Canada 2005 (NBC) is the model code. The reference dynamic pressure for roof covering loads is based on a mean recurrence interval of 50 years. In the U.S., ASCE 7-04 is used. The wind-load calculation procedure for roof coverings can be summarized in six steps:

  1. Calculation of dynamic pressure
  2. Definition of the corner zone
  3. Calculation of external wind pressure component
  4. Calculation of internal wind pressure component
  5. Calculation of net wind pressure
  6. Development of the loading diagram