Wind Design – Low-Slope Membrane Roof Systems (7/30/19)

“NRCA is receiving an increasing number of reports indicating project drawings and specifications incompletely, inadequately or inaccurately address proper wind design for low-slope membrane roof systems”

Mark S. Graham, NRCA's Associate Executive Director of Technical Services, March 2014 Tech Today

Mr. Graham goes on to state in his article, “Responsibility for properly determining and clearly identifying wind design data, including design wind loads for roofing systems, is required by the building code and is clearly that of roof system designers,” in other words, the design professional.  His statement is valid for all types of roofing applications – conventional roofs, protected membrane roofing and vegetated roof assemblies.

Delegated design is often inserted into specifications similar to:

“1.02 PERFORMANCE REQUIREMENTS A. Delegated Design:  Engage a qualified professional engineer, as defined in Division 1 “Quality Requirements,” experienced in the design of ballasted Protected Membrane Roofing systems to meet the specified performance requirements.”

The specified requirements are generally associated with wind pressures identified on the structural drawings.

The basis of Mr. Graham’s statement is critical to understand.  The structure as designed may in fact not allow the roofing assembly to be designed to comply with the specified performance requirements.  Delegation that occurs after design and during the bidding process does not resolve the issue – especially as it relates to vegetated roof assemblies and various overburdens associated with occupied and ballasted roofs.  It simply creates an unanswerable quandary and typically results in bids which exclude the requirement.

How to avoid this dilemma?  Engage a roofing manufacturer technical representative well versed in wind design issues during design development and CD’s and work with them to do the delegated design at that point.  They can help you understand what you need to do with your design to meet the desired wind requirements.

 

American Hydrotech – we can assist you through the process.

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