Ballasted solar arrays are sized for wind uplift. On a flat commercial roof, the same blocks also decide how the array behaves in an earthquake. A plan checker in a seismic region will ask for that second calculation, and many wind-only ballast sheets do not contain it.
This guide is for the US installer or EPC permit coordinator with a ballasted job on a flat roof. It assumes a seismic comment is in hand or expected. It covers when an unattached array is allowed, what the sliding calculation needs, and when the array has to be attached. For the wind side, read the ASCE 7-22 wind load guide. For which code edition applies in your state, use the ASCE 7-22 adoption matrix.
Quick answer. An unattached ballasted array is allowed only under the ballasted exception in ASCE 7-22 section 13.6.12. The roof must be nearly flat, the array low, the rows interconnected, friction measured, sliding displacement calculated, and clear space kept at roof edges and obstructions. Fail one condition and the array must be attached. Your AHJ may add rules, so confirm edition and adoption first.
What does seismic design mean for a ballasted rooftop array?
ASCE 7 treats a rooftop array as a nonstructural component. It must resist seismic force and tolerate building movement under Chapter 13. The default answer is attachment. Portland’s structural page says panels must be bolted, welded or otherwise positively fastened, without relying on friction. The California Division of the State Architect says the same for non-ballasted support systems and cites ASCE 7 section 13.4.
Ballasted unattached arrays are the exception. Friction holds them in place, so the design question changes. For an attached array, the question is the force on each anchor. For an unattached array, the question is how far the whole array slides, and what it hits when it does.
Two points keep this from being a wind-only job.
- The array weight, ballast included, is seismic mass on the roof.
- Los Angeles County’s plan review list says the most restrictive of the wind and seismic analyses governs the final ballast design.
A note on the SEAOC reports, because mix-ups are common. SEAOC PV2-2017 is the wind design report. The seismic report is SEAOC PV1, titled Structural Seismic Requirements and Commentary for Rooftop Solar Photovoltaic Arrays. The Department of Defense lists PV1-2012 and PV2-2017 side by side in UFC 3-301-01, appendix A-1.4. Citing PV2 for a seismic comment is a wrong answer.
The edition matters too. The 2021 IBC points to ASCE 7-16 and the 2024 IBC to ASCE 7-22, as the adoption matrix explains. The 2022 edition added one provision that changes ballasted designs, covered below. The glossary entry for ASCE 7-22 and the one for ASCE 7-16 define the editions.
Which seismic design category applies to ballasted solar arrays?
Seismic Design Category, or SDC, is a letter from A to F. The structure’s Risk Category and the site’s design spectral accelerations set it. IBC section 1613.2 sends the designer to ASCE 7 for the assignment, or to default maps where the site soil is not poor.
ASCE 7-22 Table 11.6-1 sets the categories from the short-period parameter SDS. The table below gives the values for Risk Category I, II and III. They are printed in the Department of Defense’s nonstructural component seismic design guide (August 2024).
| SDS (g) | SDC, Risk Category I to III |
|---|---|
| Below 0.167 | A |
| 0.167 to below 0.33 | B |
| 0.33 to below 0.50 | C |
| 0.50 or more | D |
Table 11.6-2 repeats the exercise with the one-second parameter SD1. The more severe result governs, and SDC E and F sit above D. The ASCE Hazard Tool returns SDS, SD1 and the category for an address, once you pick the edition, site class and Risk Category.
What changes by category?
- SDC A. The same guide reproduces ASCE 7-22 Table 13.1-1, which exempts all nonstructural components in SDC A.
- SDC C and higher. The remaining exemptions for mechanical and electrical components depend on positive attachment. An unattached array has none.
- IBC text. In the 2021 IBC wording, ballasted non-penetrating systems in SDC C, D, E or F must accommodate seismic displacement. The determination is by nonlinear response-history or other approved analysis, or by shake-table testing.
Do not read a low category as a pass. An AHJ can ask for the analysis on its own checklist. The exemptions apply to the component classes in Table 13.1-1, so the engineer of record decides how the array is classified.
When is an unattached ballasted array allowed under ASCE 7-22 section 13.6.12?
Section 13.6.12 holds the ballasted exception. The Department of Defense cites the section for rooftop PV seismic loading, and DSA cites its numbered exceptions in its 2025 California Building Code interpretation. We could not read the ASCE 7-22 text, which is paywalled. The conditions below come from the Portland structural page, which summarizes the section but states no edition.
- The building is Risk Category I, II or III and six stories or less.
- The roof slope is no more than 1 in 20. Portland allows 1 in 12 with an independent peer review.
- The center of mass of any panel sits below a height limit. The limit is the smaller of 3 feet and half the least plan spacing of the supports.
- The array accommodates seismic displacement relative to the roof.
- Panels are interconnected to resist a horizontal force of 0.2 SDS Wp, where Wp is the component weight. Otherwise each portion of the array needs minimum separations.
- Framing and supports give a force path from each component’s center of mass to the friction location. The design force is the lesser of Fp and 0.6 Wp.
- Panels sit at least 2.0 times the calculated displacement, and not less than 4 feet, from the roof edge. A parapet of 12 inches or more, designed for panel impact, is the alternative. Electrical cables must accommodate the displacement.
- The friction coefficient comes from testing and accounts for weather, including ice and cold.
ASCE 7-22 added a provision on the load path. Solar panels cannot be counted in the path that resists the interconnection force unless they have been evaluated or tested for it. The Sustainable Energy Action Committee’s March 2023 summary of a SEIA presentation warns that this could remove designs where panels span between isolated supports. DSA’s 2026 interpretation requires the force to run through continuous members along each ballast row in both directions. It may run through the panels only if they have been tested.
Slope is where the layers show. The numbers differ by document, and the strictest one governs.
| Source | Maximum roof slope for unattached arrays |
|---|---|
| IBC ballasted PV clause (2021 and 2024 text) | 1 in 12 (4.8 degrees) |
| ASCE 7 exception, per DSA and Portland | 1 in 20 (2.9 degrees); 1 in 12 with testing, analysis and peer review (DSA states this for SDC D) |
| Los Angeles County 2026 review list | Less than 7 degrees, mean roof height under 60 feet |
The 2021 IBC clause is in IBC section 1613.3, and the 2024 text we read numbers it 1613.4. That 2024 text was a public copy of the Illinois adoption, because ICC’s own site blocked us. Confirm the number in your adopted code.
How are seismic displacement and roof-edge separation determined?
An unattached array slides when the roof shakes. The calculation predicts how far, then the layout leaves room for it.
Los Angeles County’s 2026 list and the City of Los Angeles bulletin accept three routes for the displacement of fully unattached arrays.
- the prescriptive method in SEAOC PV1;
- nonlinear response-history analysis;
- shake-table testing.
LA County adds that the input motions must match ASCE 7 forces for nonstructural components on roofs. The City of Los Angeles bulletin also requires a Modification of Building Ordinance form. The form is needed to use the prescriptive PV1 and PV2 methods in place of response-history analysis and wind tunnel tests.
Four inputs move the answer most.
- Friction. The 2012 SEAOC convention paper by Walters and co-authors says friction changes with membrane material, water, snow, frost, ice, dirt and membrane aging. It also says coupon tests of two materials do not predict how a flexible array slides. Full-scale pull tests do. The City of Los Angeles asks for a sliding test by an approved agency under ASTM G115, at critical slopes, wet and dry.
- Roof slope. The same paper reports that movement is almost always down the slope.
- Ground motion. SDS and SD1 from the site.
- Building behavior. The paper notes that large-roof buildings are usually stiff, with low fundamental periods. The effective period shifts if the lateral system yields.
The layout then leaves a gap. The paper names four goals. Arrays must not fall off edges or openings. They must not hit rooftop equipment or other arrays. They must not break electrical continuity. They must not block firefighter access. Gaps at edges, equipment and parapets serve the first, second and fourth goals. Slack in conductors and flexible conduit protect the wiring. DSA requires the slackness details on the approved drawings.
Parapets deserve a warning. The same paper says a realistic check of a parapet’s impact capacity may not be possible, because the array speed at impact is unknown. A parapet is therefore a design element with calculations behind it, not a free setback credit.
Illustrative check of the geometry limits (not for design use)
The numbers below are assumed to show how two limits from the list above combine. Real values come from the adopted code, the racking data and the analysis.
| Item | Case A | Case B |
|---|---|---|
| Least plan spacing of supports (ft) | 4.0 | 7.0 |
| Half of that spacing (ft) | 2.0 | 3.5 |
| Center of mass limit, smaller of 3 ft and half spacing (ft) | 2.0 | 3.0 |
| Assumed center of mass height (ft) | 2.3 | 2.3 |
| Result | Fails, attach or change the frame | Passes this limit |
| Assumed calculated displacement (ft) | 1.5 | 2.8 |
| Twice the displacement (ft) | 3.0 | 5.6 |
| Edge setback, larger of that and 4 ft (ft) | 4.0 | 5.6 |
Case B needs 1.6 feet more edge setback than the 4 foot minimum, and the setback comes out of the usable roof area.
When must a ballasted array be attached?
Attach when any row below fails and no AHJ-approved alternative path exists. The conditions are the Portland and DSA conditions from the section above.
| What the review finds | Unattached path | Action |
|---|---|---|
| Site is SDC A | Table 13.1-1 exempts all components | Record the SDC and confirm the AHJ adds no local rule |
| Roof slope over 1 in 12 | Closed by the IBC clause | Attach, or change the support approach |
| Slope over 1 in 20, up to 1 in 12 | Only with testing, analysis and peer review | Price the peer review, or attach |
| Slope 1 in 20 or flatter | Open, if the other rows pass | Continue |
| Building over six stories or Risk Category IV | Outside the Portland summary of the exception | Read the exception text with the AHJ, expect to attach |
| Center of mass above the limit | Closed | Lower the profile, widen the supports, or attach |
| Rows not interconnected, or interconnection runs through panels | Closed unless panels are tested for the load | Add continuous members, or attach |
| Edge or obstruction clearance cannot be met, no designed parapet | Closed | Change the layout, design the parapet, or attach |
| Friction untested, or ice and frost exposure in a high-seismicity site | Closed | Test the friction, or attach |
| Roof framing fails the existing-building check | Not a separate trigger | Reinforce the framing, or cut ballast |
The last row ties to a different check. The City of Los Angeles bulletin asks the engineer to show two limits. The array may add no more than 5 percent to roof framing demand and 10 percent to the lateral system. The bulletin cites sections 502.5 and 503.3 of the 2023 Los Angeles Existing Building Code. Our structural letter guide walks through the gravity test.
Federal work has a harder line. The Department of Defense’s roofing criteria, UFC 3-110-03, disallow ballasted systems. UFC 3-301-01 deletes the ASCE 7-22 ballasted exception.
Where the answer is attached or hybrid, the wind side of the decision matters too. Attachment at the perimeter and ballast in the field is a common pattern. The ballasted vs penetrating comparison and the flat roof racking guide cover that trade. For attachment into other roof types, see tile roof attachment details. The ballasted racking glossary entry gives the short definition.
What inputs does the seismic calculation need?
Collect these before the structural engineer starts. A missing item is the usual reason a seismic review stalls.
- The adopted code edition, ASCE 7 edition and AHJ amendments, from the adoption matrix and the AHJ itself.
- Risk Category, site class, SDS, SD1 and the resulting SDC.
- Roof slope in every array area, with the direction of fall.
- Roof membrane type and any measured friction data for the exact racking and membrane pair, wet and dry.
- Array weight with ballast, and the height of the center of mass above the roof.
- Support spacing, so the half-spacing limit can be checked.
- A layout with distances from each array to edges, parapets, curbs, skylights, rooftop equipment and other arrays.
- The interconnection members between rows, and test evidence if panels carry any of that load.
- Conduit routing, slack and flexible connection details at every fixed point.
- Existing roof framing data for the gravity and lateral demand checks.
- Roof drainage, so a displaced array cannot block drains or scuppers. DSA states that requirement.
What do AHJs ask for?
The documents below show the range. We checked documents from California and Oregon. We did not find a Washington or Utah document on unattached arrays, so for those states ask the plan reviewer directly.
| AHJ and document | What it asks for on ballasted seismic |
|---|---|
| City of Los Angeles, LADBS Information Bulletin P/GI 2023-027 (effective 1 January 2023) | SEAOC PV1-2012 seismic design, ASTM G115 sliding test, 5 and 10 percent demand checks, recorded owner maintenance affidavit, modification form for prescriptive methods |
| Los Angeles County Public Works, 2026 Solar PV System Plan Review List v1.0 (1 February 2026) | SEAOC PV1-2017 seismic design, slope under 7 degrees, displacement by prescriptive, response-history or shake-table route, wind and seismic both checked |
| California Division of the State Architect, IR 16-8 (revised 7 January 2026, 2025 CBC) | Conditions for use, interconnection through continuous members, friction testing, peer review rules, drainage, wiring slack, for projects under DSA jurisdiction |
| City of Portland, structural design requirements for solar (undated page) | Summary of section 13.6.12 conditions, acceptance of designs that comply with SEAOC PV-1 |
Notice the edition split. The City of Los Angeles cites PV1-2012, and the county cites PV1-2017. The 2017 designation comes from the county’s list, and SEAOC’s own store pages did not load for us to confirm which editions exist. Ask the reviewer which edition the file must cite. Our California AHJ guide covers the wider permit process.
How does this change cost and schedule?
Each seismic condition removes either roof area or schedule certainty. Edge setbacks and array gaps take roof area out of the layout. Friction testing and peer review add time before submission. Moving to attachment adds penetrations, flashing and membrane warranty coordination.
Decide the attachment question at layout stage, not after the plan check comment. A late switch from unattached to attached changes the roof detail, the wind calculation and the schedule for roofing contractor coordination.
Sometimes the roof, the SDC and the AHJ all raise a structural question. The solar civil and structural engineering page describes how roof and racking structural work is scoped. Send the comment letter, the roof drawings and the racking data sheet. Stamping requirements are confirmed at quote stage.
What does this page not verify?
- ASCE 7-22 text. The standard is paywalled. Conditions come from the Portland summary, the DSA interpretation and Department of Defense documents. Check them against the standard before design.
- SEAOC PV1 and PV2. SEAOC sells both. Section numbers and methods come from AHJ documents that cite them.
- IBC 2024 numbering. Read through a public copy of the Illinois adoption.
- Washington and Utah. No AHJ document on unattached arrays was found.
- Review date. These sources were opened on 31 October 2026. Recheck before 30 April 2027.
FAQ
Does SEAOC PV2 cover seismic design?
No. PV2-2017 is the wind design report. The seismic report is PV1. The Department of Defense lists PV1-2012 for seismic and PV2-2017 for wind, and Los Angeles County cites both for ballasted arrays.
Can friction resist seismic force on a rooftop array?
Only through the ballasted exception. Portland requires positive fastening without friction for other arrays. DSA and the City of Los Angeles both cite ASCE 7 section 13.4 for positive anchorage.
Which governs ballast, wind or seismic?
Los Angeles County says the most restrictive analysis governs the final ballast design. Run both. A wind-only sheet cannot answer a seismic comment.
Does a low seismic design category remove the requirement?
In SDC A, the Table 13.1-1 exemption reproduced by the Department of Defense covers all nonstructural components. From SDC C up, exemptions for mechanical and electrical components depend on positive attachment. An AHJ checklist can also ask regardless of category.
Is a ballasted array allowed on every building?
No. The Portland summary limits the exception to Risk Category I to III buildings of six stories or less. The Department of Defense disallows ballasted systems altogether. Check the AHJ and the building type.
Next step
List the roof slope, Risk Category and SDC, the racking and membrane pair, and the nearest edge and obstruction distances. If any item fails the conditions above, plan for attachment or a tested alternative before layout is frozen. For structural scoping, use the solar civil and structural engineering page. For US permit packages, see solar permit design for the USA. To start a conversation, contact the team. Turnaround and scope are agreed at scoping.