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, V_asd, as determined in accordance with Section 1609.3.1 and wind exposure.
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.

Determining the Loads Acting on a Rooftop

Simply put, a roof assembly must be able to resist the design wind loads acting on the rooftop. The loads acting on a roof must be calculated in order to select a roof system that has the necessary capacity (i.e., wind uplift resistance). Therefore, step one is to determine the loads acting on the roof of a specific building.

There are a number of factors that determine the design wind uplift loads for the field, perimeter and corners of a roof. In order to determine the wind loads acting on a roof, the architect/designer needs to know the following about a building—location; building code that is in effect at the building’s location; height, length and width; exposure category; use and occupancy; enclosure classification; topographic effects; and ground elevation.

Location. The location of the building within the United States tells us two things which must be determined in specific order. The location directs us to the specific version of the IBC or the applicable building code that is in effect for the project. For example, if the 2006 or 2009 IBC is in effect, then ASCE 7-05 governs. If the 2012 or 2015 IBC is in effect, then ASCE 7-10 governs. If the 2018 IBC is in effect, then ASCE 7-16 governs.

Height, Length, Width. Determining the height, length and width of a building should be straightforward and a vast majority of buildings are predominately square or rectangular in shape, or in general, have square or rectangular roof areas. Note: there are methods to determine the wind loads acting on a roof for non-rectangular or non-square buildings; however, that is outside the scope of this article.

Exposure Category. Exposure category is based on the roughness of a building’s nearby terrain. A terrain’s surface roughness is determined from natural topography, vegetation and the surrounding construction.

ASCE 7 uses three Surface Roughness Category types—called B, C and D—which in turn, defines three Exposure Category types, also called B, C and D.

Exposure Categories B, C and D are generally defined as follows:

• Exposure B is applicable to buildings with a mean roof height of less than or equal to 30 feet and where surface roughness B prevails in the upwind direction for a distance greater than 1,500 feet tor buildings with a mean roof height greater than 30 feet, Exposure B shall apply where surface roughness B prevails in the upwind direction for a distance greater than 2,600 feet or 20 times the height of the building, whichever is greater.
• Exposure C is applicable for all cases where Exposure B and D do not apply.
• Exposure D is applicable where surface roughness D prevails in the upwind direction for a distance greater than 5,000 feet or 20 times the building height, whichever is greater. Exposure D also applies where the ground surface roughness immediately upwind of the site is B or C, and the site is within a distance of 600 feet or 20 times the building height, whichever is greater, from an Exposure D condition.

Use and Occupancy. The use and occupancy of a building is used to determine the “Occupancy Category” in ASCE 7-05 or “Risk Category” in ASCE 7-10 and ASCE 7-16. They are effectively interchangeable terms, however, they are addressed differently. ASCE 7-05 uses Occupancy Category to determine the value to use for the Importance Factor. In ASCE 7-05, Importance Factor is a stand-alone factor in the velocity pressure calculations, and why there is one map in ASCE 7-05. ASCE 7-10 and 7-16 incorporates Risk Category (i.e., importance factor) into the wind speed maps, and that is why there are 3 maps in ASCE 7-10, and 4 maps in ASCE 7-16. In general, the greater the importance of a building, the higher the Importance Factor or Risk Category which results in higher uplift pressures.

Exposure Classification. This factor essentially relates to the possibility that a building will become internally pressurized during a wind event. For ASCE 7-05 and ASCE 7-10, there are three classification types: Open, Partially Enclosed, and Enclosed. ASCE 7 16 amended these classification types by adding another type called, “Partially Open” and also revised some of the definitions. The ASCE 7-16 classification types are Open buildings, Partially Open, Partially Enclosed and Enclosed buildings.

Using “Partially Enclosed” as the building type results in an increase in the design wind pressures in the field of the roof versus an “Enclosed” or “Partially Open” building—all other factors held equal. This is significant. Selecting an “Enclosed” or “Partially Open” building when it could become a “Partially Enclosed” building if doors and windows are blown out during a high wind event could result in a roof system without the adequate capacity to handle the anticipated higher loads.

Topographic Effects. Research and experience has shown that wind speeds can increase significantly due to topographic effects. The wind speed increase is known as a wind speed-up effect. An abrupt change in the topography, such as escarpments, hills or valleys can significantly affect wind speed. ASCE 7 addresses these speed-up effects by applying a multiplier to account for topography in the velocity pressure calculations.

For more in-depth information about determining wind loads, read this blog.

An architect/designer needs to know a building’s location; the building code that is in effect at the building’s location; its height, length, and width; the exposure category; the use and occupancy category; the enclosure classification; any topographic effects; and ground elevation in order to determine the wind loads acting on a roof. Some of these selections may seem straight forward, but some impart a higher resultant design wind load, especially when compounded by similar risk-averse choices. For more information about resilient roof systems, read this blog.

The process of determining the loads acting on a rooftop results in design wind pressures for each roof zone and, importantly, determines the dimensions and overall size of each of the roof zones. Providing this information on the construction documents ensures the contractor and manufacturer (together or separately) can provide an appropriate roof system with the tested capacity that exceeds the design wind pressures in each of the roof zones.

Building Enclosure  |  BuildingEnclosureOnline.com  |  Fall 2021

BACK TO CONTENTS