Solar permit packages routinely include structural calculations for gravity loads, wind uplift, and snow. Seismic is the fourth load case, and in many jurisdictions, it is the one most frequently missing from first-submission plan sets.
AHJs in seismic design categories C through F require documentation that the solar array and its attachment system can resist seismic forces in addition to the standard load cases. Plan sets that omit seismic calculations in these zones generate correction requests that add weeks to the permit timeline.
Seismic Design Categories: What They Are
Seismic Design Categories (SDCs) classify buildings and structures based on the seismic hazard at the site and the consequence of failure. SDCs range from A (lowest hazard) to F (highest hazard combined with high occupancy). The classification is determined using ASCE 7 maps of spectral response acceleration values and a site class assignment based on soil conditions.
SDC A and B: Seismic is unlikely to govern the design. Many AHJs in these categories do not require seismic calculations for rooftop solar.
SDC C: Seismic calculations are required for nonstructural components including rooftop solar arrays. This category covers a large portion of the interior United States, including parts of the Midwest and Southeast.
SDC D, E, and F: Seismic calculations are required, and the demands are significantly higher. These categories cover most of California, the Pacific Northwest, Alaska, and seismically active zones throughout the West.
How Seismic Affects Solar Array Design
Rooftop solar panels are classified as nonstructural components under ASCE 7 Chapter 13. The seismic design force on a nonstructural component is calculated as a function of the component’s weight, the component amplification factor, the building’s seismic response coefficient, and the height of the component within the structure.
For a rooftop solar array, the weight of the panels, racking, and associated hardware is multiplied by the applicable seismic coefficient to produce the design force. That force is applied horizontally (and in some cases vertically) and must be resisted by the attachment system.
This has direct implications for the mounting hardware specification. Lag bolts, standoffs, and rail connections that are sized only for gravity and wind loads may not be adequate to resist the seismic demand. In high-SDC zones, the seismic load often governs the attachment design, meaning more or larger fasteners than a wind-only analysis would require.
What the Seismic Calculation Document Needs to Show
AHJs in seismic zones expect the following in a plan set that includes seismic analysis:
Site-specific seismic parameters. The design short-period spectral acceleration (Ss) and the one-second spectral acceleration (S1) for the project location, the site class, and the resulting SDC assignment. These values are available from USGS seismic hazard tools for any US location.
Component amplification factor and response modification coefficient. ASCE 7 Table 13.5-1 provides component amplification factors and response modification coefficients for mechanical and electrical components on structures. The correct table values for roof-mounted solar must be identified and applied.
Design seismic force. The calculated horizontal seismic force (Fp) applied to the array, derived from the parameters above and the component weight.
Attachment verification. A calculation demonstrating that the specified mounting hardware can resist the seismic force without exceeding allowable load values. For penetrating anchor systems, this means verifying the lag bolt or through-bolt capacity in both shear and combined shear and withdrawal.
Ballasted system stability. For ballasted flat-roof systems, the seismic analysis must verify that the ballast weight is sufficient to prevent sliding of the array under the design seismic force. This is a different calculation than the wind-driven sliding check, and both must be performed.
Residential vs. Commercial Seismic Requirements
For residential solar projects in SDC C and above, seismic calculations are required but are typically incorporated into the structural roof evaluation package. Many engineering firms that provide structural roof evaluations for solar do not routinely include seismic analysis unless the AHJ specifically requests it. This is a gap that generates correction requests in high-SDC jurisdictions.
For commercial rooftop solar projects, seismic analysis is expected as a standard part of the engineering package in SDC C through F. Commercial AHJs in seismic zones typically have reviewers familiar with ASCE 7 Chapter 13 requirements and will flag missing seismic documentation quickly.
AHJ-Specific Seismic Documentation Requirements
Beyond the standard ASCE 7 framework, some AHJs in high-seismic zones have additional requirements. California jurisdictions following the CBC (California Building Code) apply site-specific seismic criteria and may require additional documentation of equipment listing or seismic certification for the racking system. Some AHJs require a separate seismic certification letter or a declaration by the engineer that the mounting system has been evaluated for the site-specific seismic demand. Our permit design team tracks AHJ-specific seismic documentation requirements across all 50 states.
Getting Seismic Right the First Time
The most effective way to avoid seismic-related correction requests is to include a complete seismic analysis in the plan set before submission, regardless of whether the AHJ has explicitly requested it. In SDC C through F jurisdictions, seismic documentation is a standard expectation. A complete solar plan set built for a high-seismic jurisdiction includes seismic analysis as part of the structural section, not as an afterthought.
Right Angle Engineering includes seismic analysis in all structural engineering packages for projects in SDC C and above. Our licensed structural PEs calculate site-specific seismic parameters and verify attachment adequacy for the design seismic demand. Start a project with our team to get a complete package that addresses every applicable load case.
Frequently Asked Questions
Does every solar installation require seismic calculations?
No. Seismic calculations are required for solar installations in Seismic Design Categories C through F. Projects in SDC A and B are generally exempt from seismic requirements for rooftop nonstructural components. The SDC for a specific site is determined using ASCE 7 seismic hazard data and the site soil classification.
How is the seismic force on a solar array calculated?
The seismic design force on a rooftop solar array is calculated using ASCE 7 Chapter 13 for nonstructural components. The force (Fp) is a function of the component weight, component amplification factor, seismic response coefficient, and height of the component within the structure. The result is a horizontal force that the mounting system must resist.
Do ballasted solar systems need seismic calculations?
Yes. Ballasted systems on flat roofs require seismic analysis to verify that the array will not slide under the design horizontal seismic force. This check is separate from the wind-driven sliding check and must account for the seismic demand specific to the site.
What is the difference between SDC and seismic zone?
Seismic zones were used in older building codes (pre-IBC). The current IBC and ASCE 7 framework uses Seismic Design Categories (SDC A through F) based on spectral response acceleration values at the site. SDC is the correct term for current code-compliant seismic analysis.
My AHJ did not mention seismic requirements. Do I still need to include seismic calculations?
Yes, if the project is in SDC C or above. AHJs do not always explicitly list seismic as a requirement on their submittal checklists, but if the jurisdiction has adopted the current IBC or CBC, seismic requirements for nonstructural components apply. Omitting them does not waive the requirement; it generates a correction request when the reviewer catches the omission.
References
International Code Council. International Building Code (IBC), Chapter 16.
Right Angle Engineering. Structural Roof Evaluations for Solar.