Wind Design for Roof Systems and ASCE 7

Changes to the most recent version of the industry’s wind design standard are having an effect on roofing system design and installation.
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Sponsored by GAF
By James R. Kirby, AIA

Learning Objectives:

  1. Identify the requirements contained in the International Building Code for roof wind design, including edge metal securement requirements.
  2. Discuss the factors used by roof system designers that contribute to the determination of design wind pressures for roof systems.
  3. Recognize the Approval Listing options used to find roof systems that have appropriate wind-resistance capacity.
  4. Demonstrate that the latest version of ASCE 7 increases the design wind pressures for roofs and analyze case studies to show the extent of the increases.

Credits:

HSW
1 AIA LU/HSW
IACET
0.1 IACET CEU*
IIBEC
1 IIBEC CEH

Wind design of roof systems is one of the more complicated things that an architect deals with during the design of a building. And with the latest version of ASCE 7, “Minimum Design Loads For Buildings and Other Structures” (ASCE 7), it has become that much more challenging for roof system designers, roof system manufacturers and roofing contractors. Different editions of building codes exist, and therefore, different versions of ASCE 7 are being used in different parts of the country. The three versions that are currently in use are ASCE 7-05, 7-10 and 7-16, with the last two digits representing the year of publication (e.g., “-05” indicates 2005).

The progression of ASCE 7 during the last two decades had added complexity to what was once a relatively straight-forward calculation. Understanding the similarities and differences between the three versions of ASCE 7 provides for better recognition of the current version’s complexity and allows for more appropriate wind load determination.

Roof systems that have the tested capacity to resist calculated design wind pressures can be found in approval listings (e.g., UL, FM). Recognizing how a safety factor is included in the approval listing is critical to ensuring an appropriate roof system is selected and installed. Conceptually, the goal is to determine the design wind loads, then select the appropriate roof system with a tested resistance greater than the design wind loads. If it were only that simple! Yet while it certainly can be complicated, there are ways to break down the steps of wind design in order to make it much more digestible for architects and specifiers.

This course will discuss and analyze the process for determining wind design pressures and selecting an appropriate roof system from a more-conceptual, what-does-the-book-say approach and, additionally, will provide in-depth analysis for case studies in two cities.

Building Code Requirements

Before we get into a discussion about the wind design process, it’s appropriate to discuss the requirements in the building code. The 2018 IBC (as well as prior versions) has very specific requirements for what is to be included on the construction documents regarding wind design of roof systems.

The 2018 IBC, in Section 1603, Construction Documents, states:

    “1603.1 General. Construction documents shall show the size, section and relative locations of structural members with floor levels, column centers and offsets dimensioned. The design loads and other information pertinent to the structural design required by Sections 1603.1.1 through 1603.1.9 shall be indicated on the construction documents.

      Exception: Construction documents for buildings constructed in accordance with the conventional light-frame construction provisions of Section 2308 shall indicate the following structural design information:

        1. Floor and roof dead and live loads.
        2. Ground snow load, Pg.
        3. Basic design wind speed, V, miles per hour (mph) (km/hr) and allowable stress design wind speed, Vasd, as determined in accordance with Section 1609.3.1 and wind exposure. (Emphasis added.)
        4. Seismic design category and site class.
        5. Flood design data, if located in flood hazard areas established in Section 1612.3.
        6. Design load-bearing values of soils.
        7. Rain load data.”

The 2018 IBC further states, in Section 1603.1.4, Wind design data that the following is to be included on construction documents.

    1603.1.4 Wind design data. The following information related to wind loads shall be shown, regardless of whether wind loads govern the design of the lateral force-resisting system of the structure:

      1. Basic design wind speed, V, miles per hour and allowable stress design wind speed, Vasd, as determined in accordance with Section 1609.3.1.
      2. Risk category.
      3. Wind exposure. Applicable wind direction if more than one wind exposure is utilized.
      4. Applicable internal pressure coefficient.
      5. Design wind pressures to be used for exterior components and cladding materials not specifically designed by the registered design professional responsible for the design of the structure, psf (kN/m2).”

In the end, the design architect’s responsibility is to provide the necessary design wind loads; the manufacturer is responsible for testing roof systems in order to determine wind uplift capacity; and the roofing contractor is responsible for proper installation that follows the construction documents, project specification, and installation instructions.

Wind Loads and Metal Edge Systems. In addition to the requirements for wind design of the roof system itself, the 2018 IBC has specific requirements for edge securement for low-slope roofs. The code language in Section 1504.5, Edge securement for low-slope roofs, states that a roof’s metal edge securement is to be designed and installed in accordance with Chapter 16, Structural Design, and tested to determine its resistance (i.e., wind-resistance capacity). The code language specifically states that metal edge systems are to be tested according to ANSI/SPRI ES-1, Test Standard for Edge Systems Used with Low Slope Roofing Systems.

Why is this a concern? Because it is widely acknowledged within the roof industry that many roof system failures during high wind events initiate at the perimeter edges, the edge securement of the roof is critical to long-term wind-uplift performance. Codification of edge metal securement is the result of years of analysis and inspection of roof system failures after high wind events. Common edge systems for roofs include L-shaped metal, gravel-stop metal and copings used on the top of parapet walls.

ES-1 Specifics. Edge systems for membrane terminations are divided into two separate, large-bucket categories—dependently and independently terminated systems. Dependently terminated systems are used with ballasted roofs, roofs that are adhered with ribbons or spots of adhesives, and mechanically attached roofs where the roof membrane’s attachment locations are greater than 12 inches from the roof edge. Independently terminated systems are used with fully adhered roof membranes and mechanically attached roofs where the attachment locations are 12 inches or less from the roof edge. The latter are considered to have ‘peel’ stops within one foot of the roof edge.

ES-1 includes three test methods (RE-1, RE-2 and RE-3), and each is specific to a type of edge metal and membrane-attachment system. The RE-1 test method determines a metal edge’s ability (i.e., capacity) to restrain the roof membrane (dependently terminated) from the forces created by wind pressures for ballasted roofs and the types of intermittently attached membranes described above. The RE-2 test method determines a metal edge’s capacity to resist outward, horizontal pressures that occur during high winds. RE-2 is used for both dependently and independently terminated roof membranes. The RE-3 test method determines the capacity of a metal coping when both upward and outward loads are applied.

 

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Originally published in Building Enclosure
Originally published in June 2021

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