Susan Stark, Senior Manager of Training, IronRidge
What is ASCE 7-16 and how does it affect residential solar projects in Florida? Every six years, the American Society of Civil Engineers / Structural Engineering Institute publishes ASCE/SEI 7 - Minimum Design Loads and Associated Criteria for Buildings and Other Structures. The 2016 revision has now been adopted into Building Codes throughout the nation, replacing the 2010 publication.
While adoption throughout the US, especially in the Residential Code has been gradual, ASCE 7-16 has already been incorporated into the 7th Edition (2020) Florida Building Code, Building (FBCB) and Residential (FBCR). In short, the switch to ASCE 7-16 is here and it brings new complexities when calculating roof attachment spans for different roof sections (see Figure 1 below).
Steep-slope roofs are now delineated into three categories by ASCE 7-16: pitches of 8-20 degrees (1.7 to 4.5 in 12 rise in run), 21-27 degrees (4.6 to 6.3 in 12 rise in run), and 28-45 degrees (6.4 to 12 in 12 rise in run).
ASCE 7-16 defines numerous roof configurations, however the two most common types of steep slope roofs are defined as:
■ Hip roof: characterized by all sides sloping downward to the walls, usually with a gentle slope.
■ Gable roof: consists of two pitched or sloping sides, which meet along the roofline ridge and are open at the end.
In addition to delineating between different roof types, ASCE 7-16 has also increased the number of roof zones based on wind
tunnel testing results (which we’ll discuss in a later section). What was previously known as Zone 1 (field) remains in the same location for both hip and gable roofs, Zones 2 and 3 have now been further segmented.
While these changes don’t sound very impactful, the new design criteria changes how the zone dimensions are calculated (see Figure 2 on page 38). As you can see in the diagram, there are now additional roof zones for both hip and gable roofs:
■ For hip roofs, Zones 1 and 3 locations remain unchanged from 7-10 to 7-16. Zone 2 has been further segmented into 2e and 2r.
■ For gable roofs, Zone 1 location remains unchanged from 7-10 to 7-16. Zones 2 and 3 have been further segmented into 2e, 2r, 2n, 3e and 3r.
But here’s where real complications arise. Finding roof zone dimensions are now based on the following calculation:
a = 10 percent of least horizontal dimension or 0.4h, whichever is smaller, but not less than either 4 percent of least horizontal dimension or 3 feet. If an overhang exists, the edge distance shall be measured from the outside
edge of the overhang. The horizontal dimensions used to compute the edge distance shall not include the overhang distance.
For hip roofs with a roof slope between 7 and 20 degrees, tables are broken up between buildings with a height to width ratio (h/B) greater than 0.8 or less than 0.5. If the building the array is being mounted on falls between these values, compare both tables and choose the higher value.
Is your head spinning yet? Why all the changes?
The reasoning behind the updated roof zones is due to the availability of more comprehensive data and better analysis techniques, which revealed localized variations in pressure based on the specific roof location. ASCE 7-16 therefore has changed wind loads across the US, with impacts varying greatly by region.
Although wind speeds have been reduced for many parts of the US under ASCE 7-16, hurricane-prone regions (including Florida) saw no reduction in wind speed. Complexity was increased by additional roof zones with their positions affected by roof pitch and
with each roof profile having its own set. Add to that that zones must be calculated based on building dimensions and it becomes evident that properly designing solar arrays has become much more challenging.
The updated ASCE 7-16 solar-specific findings determined:
1. A decrease of pressures between upper and lower surfaces of a solar array for non-exposed/edge modules
2. An increase of wind pressures caused by roof edges and large gaps between modules on the roof.
In the first situation, testing found that a solar array on a roof acts as a barrier, creating a pressure equalization between the lower surfaces and top surfaces of an array. This counteracts uplift forces so that net wind pressures are decreased. But in the second
scenario, testing revealed different results at array module gaps and edges, so new definitions were necessary, defining treatments for forces on modules that are exposed or at an edge:
■ Exposed: Horizontal distance from its free edge (the edge with no connectivity to other modules) to the facing roof edge (such as eave, ridge, side or hip) is greater than 0.5h (h is ASCE defined building height) AND if the distance from this free edge to any other adjacent array or panel is greater than 4 feet.
■ Edge: Distance to a roof edge (such as eave, ridge, side or hip) less than two times the distance from the module top surface to the roof surface: e.g., 5-inch high array = 10 inches minimum distance from the roof edge.
Using the definitions in the table on page 38 (see Figure 3), modules at the roof edge or large gaps in the array will see an increase in the net wind pressures compared to ASCE 7-10. This is due to modules on the edge being exposed to higher wind pressures. Gaps in the array allow wind to reattach to the roof and disrupt the pressure equalization that would otherwise be lowering the pressures. The designer of the PV system must carefully consider these and other factors when designing the array:
■ PV modules that can withstand the design pressures
■ Roof attachment spans for the project may vary by roof zone and exposed module conditions
■ Actual span lengths used must be within the capacity of the roof attachment (sheathing attached vs rafter-attached PV mounts can result in up to four times the number of attachments).
Residential PV installers often encounter previously unknown obstructions or issues on the day of installation and are typically forced to re-design on the fly. Having a plan set that depicts the roof zones, as well as understanding the definitions of edge and
exposed modules will be important tools for lead installers.
IronRidge’s free Design Assistant tool can quickly and easily provide site-specific attachment reaction forces and maximum allowable spans for your project. With just a few inputs, attachment spacing in all roof zones can be determined and used to design your project. The project-relevant certification letters and a bill of materials is generated automatically for submittal. To find our latest certification letters as well as additional resources on ASCE 7-16, please visit www.ironridge.com/asce716.
Susan Stark started her career in solar in 2010 and has been NABCEP Certified since 2013. Currently Senior Manager, Training, at IronRidge, she is responsible for training installers and distributors on the IronRidge and QuickMount product brands. She has earned four Professional Certificates from Solar Energy International (SEI), including the Solar Professionals Trainer Certificate. Susan is also an NRCA RISE Certified Solar Roofing Professional. Prior to entering the solar industry, Susan was the co-owner of a construction and safety equipment distribution company. IronRidge, an Esdec company, designs and manufactures mounting and racking equipment for residential and commercial PV systems. For over 20 years, they have worked closely with solar professionals to build strong, simple and cost-effective products. IronRidge is NSF Certified to ISO 9001, maintaining the highest of quality management standards.
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