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 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 "-xx" representing the year of publication.
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 wind loads 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.
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:
The 2018 IBC further states, in Section 1603.1.4, Wind design data that the following is to be included on construction documents.
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 (See Determining Resistance, below); and the roofing contractor is responsible for proper installation that follows the construction documents and installation instructions.
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.
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 blog.
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 ft. and where Surface Roughness B prevails in the upwind direction for a distance greater than 1,500 ft. For buildings with a mean roof height greater than 30 ft., Exposure B shall apply where Surface Roughness B prevails in the upwind direction for a distance greater than 2,600 ft. 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 ft. 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 ft. 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.
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 of about one third 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 appropriate capacity to handle the anticipated higher loads.
Figure 1: Graphic showing external and internal air flow possibilities
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-adverse choices. For more information about Resilient Roof Systems, read this blog.
Revisions to ASCE 7-16
Eventually, we will all use ASCE 7-16 as the basis for determining design wind loads for our roofs. To that end, we will need to understand what has remained the same, what is changed, and what has been added to the latest version of ASCE 7.
Basic differences between versions of ASCE
There are some noteworthy differences between the three ASCE 7 editions and they include: the wind speed maps, roof zones, enclosure classifications, and external pressure coefficients.
Wind speed maps
Simply put, for the contiguous U.S., ASCE 7-05 has one wind speed map and it is based on Allowable Stress Design. ASCE 7-10 has three wind maps, based on Risk Category I, Risk Category II, and Risk Categories III and IV, and they are based on Strength Design. ASCE 7-16 has four wind speed maps, one for each Risk Category and they are also based on Strength Design.
Note: This blog is not going to try to explain the difference between ASD and Strength Design loads. It's a hardy structural engineering discussion! However, the appropriateness of using ASD values with roofing systems and the adjustment of the Strength Design to ASD values are addressed in the 2018 IBC and in ASCE 7-16.
ASCE 7-05 and ASCE 7-10 have three roof zones: field, perimeter and corner, see Figure 2. The dimensions of the zones are mostly determined by a building's length and width. ASCE 7-16 added another zone and it presents the potential to have four roof zones: interior, field, perimeter and corner, see Figure 3. ASCE 7-16 also revised how the dimensions of the zones are sized; it is based on a building's height. Figure 4 shows possible roof-zone configurations based on ASCE 7-16.
Figure 2: Roof zone layout for ASCE 7-05 and ASCE 7-10
Figure 3: 4-Roof-Zone layout for ASCE 7-16
Figure 4: Possible roof-zone configurations based on ASCE 7-16
As covered previously, ASCE 7-05 and ASCE 7-10 have three classification types: Open, Partially Enclosed, and Enclosed, while ASCE 7-16 added Partially Open and slightly modified the definitions. These classifications determine the values to use for the Internal Pressure Coefficient, GCpi. These are shown in Figure 5.
Figure 5: Interior Pressure Coefficients, GCpi, for ASCE 7-16
External Pressure Coefficients (GCp)
The values for External Pressure Coefficients have been significantly increased in ASCE 7-16. This is where much of the concern with ASCE 7-16 lies—the increase in the External Pressure Coefficients—and how the increases will affect design wind pressures. As seen in Figure 6, the GCp values for Field of the roof increased by 70%, for the Perimeter by 28%, and for the Corners by 14%. Because of the different configurations of the roof zones and other factors that are intended to allow for a correction (i.e., a reduction) in velocity pressure, it is hard to state—broadly—a percentage that loads may increase. However, the factors selected by a conservative owner (e.g., choosing Partially Enclosed) also have an effect on the design wind loads. Each project is different, so results will vary. The good news is that the roofing industry has numerous roof assemblies that have tested capacity to meet the design wind loads established under the direction of ASCE 7-16.
Figure 6: External Pressure Coefficients, GCp, for ASCE 7-16
Determining Velocity Pressure
All of the previously discussed assumptions and selections and characterizations of the building are used to determine the Velocity Pressure for a roof project. The Velocity Pressure is the 'foundational' load that is used to determine the design wind pressures for each zone of a rooftop. It's important to recognize there are two basic steps used to determine design wind pressures acting on a roof. The first step is to determine velocity pressure; the second step is to use velocity pressure to determine design wind loads for roof zones (e.g., field, perimeters, and corners).
The equation to determine velocity pressure varies slightly in ASCE 7-05, ASCE 7-10 and ASCE 7-16. ASCE 7 uses the following base equation to determine velocity pressure (qh):
qh = 0.00256 (Kz)(Kzt) (Kd) (V2)(I)
qh = velocity pressure at mean roof height
Kz = exposure coefficient based on exposure and height
Kzt = topography factor (likely 1.0)
Kd = wind directionality factor (Components and Cladding uses 0.85)
V = basic wind speed for the location
I = Importance Factor (based on Occupancy Category)
Each version of ASCE 7 uses a variation of the above equation. In ASCE 7-05 for example, I—the Importance Factor—is in ASCE 7-05 only. The Importance Factor was absorbed into the wind maps, which means for ASCE 7-10 and ASCE 7-16, the Velocity, V, is adjusted within the wind speed maps. Also new in ASCE 7-16, a ground elevation factor (Ke) can be used to reduce pressures at higher elevations, or it can more conservatively be set to 1.0.
Determining Design Uplift Pressures
This is the second step in determining design wind pressures. After determining the velocity pressures, the next step is to calculate the design uplift pressures specific to the interior (if applicable), field, perimeter, and corner zones of a roof. Design uplift pressures are adjusted by multiplying the velocity pressure (qh) by the appropriate external pressure coefficients (e.g., GCp), as shown in Figure 6. The external pressure coefficient values are based on roof zones and the appropriate "effective wind area" (which we won't go into in this blog). Effective wind area is the tributary area for the element being considered, and 10 sq. ft. is typically used for roof systems.
The internal pressure coefficient values are based on the building design (i.e., the enclosure classification). See Figure 5.
This process results in Design Wind Pressures for each roof zone and determines the dimensions for each of the zones (although not discussed in here). Providing this information on the construction documents ensures the contractor and manufacturer (together or separately) can provide an appropriate roof system with tested capacity.
- Don't mix and match methods; for instance, don't use the wrong wind map with the online application that you are using. (See Determining Resistance below for more information.)
- Select roof systems that have capacity greater than the loads acting on the building.
- Select roof systems that have been tested in accordance with code-approved test methods by accredited testing laboratories.
- Ensure that an appropriate safety factor is included on either the load side or the resistance side. Select a roof system with a tested capacity that meets or exceeds the design wind loads. Use approval listings to select the appropriate roof system.
- In situations where a specific version of ASCE 7 is not mandatory, using the most recent version of ASCE 7 is recommended.
For additional information regarding the changes to the 2005, 2010 and 2016 editions of ASCE 7, refer to the following articles by Thomas L. Smith published in Professional Roofing Magazine: "ASCE 7 update" (June 2008); "Mapping the 2010 wind changes" (August 2010); and "How do I load thee?" (October 2017), respectively.
The primary method for determining a roof system's wind uplift resistance (aka, capacity) is through physical testing. The test methods to determine wind resistance are listed in the IBC Section 1504, Performance Requirements.
In the 2003 and 2006 IBC, for wind resistance of nonballasted roofs, the codes state that built-up, modified bitumen, adhered or mechanically attached single-ply, through fastened metal panel roof systems, and other types of membrane roof coverings shall be tested in accordance with FM 44504, FM 44705, UL 5807 or UL 18978.
In the 2009, 2012, 2015, and 2018 versions of the IBC, for wind resistance of nonballasted roofs, the codes state that built-up, modified bitumen, adhered or mechanically attached single-ply roof systems, metal panel roof systems applied to a solid or closely fitted deck and other types of membrane roof coverings shall be tested in accordance with FM 44746, UL 5807, or UL 18978.
These tests are run by approved testing agencies. FM Approvals, Underwriters Laboratory, Intertek, NEMO, PRI, and others can perform testing—according to the code-approved test methods—that can be used to determine a roof system's capacity.
It is important that the testing method used to determine the capacity of a roof system is listed in the applicable building code.
The tested roof systems are found in approval listings. Approval listings are maintained by various entities, such as government agencies, testing laboratories, and even a trade association. Example of government agencies with approval listings include: Florida Department of Business and Professional Regulation, Miami Dade County, and Texas Department of Insurance. Testing laboratories that have listings of rated roofing assemblies include Underwriters Laboratories and FM Approvals. And lastly, SPRI sponsors the Directory of Roofing Assemblies (DORA) which is an online database of tested assemblies.
For more information on wind design and the new ASCE 7, register for the Continuing Education Center webinar, Wind Design of Roof Systems, sponsored by GAF and presented by Jennifer Keegan, AIA, and James R. Kirby, AIA