25. Structural Loading

Learning objectives

• Identify the loads a roof-mounted array imposes and the governing standards.
• Explain how panels are treated in load combinations.
• Know which structural-documentation path a project needs and when to involve an engineer.

25.1 The governing standards

Rooftop structural requirements come from the International Building Code (IBC)/IRC, which reference ASCE 7 (“Minimum Design Loads and Associated Criteria for Buildings and Other Structures”):

• ASCE 7-16 is referenced by the 2018/2021 IBC. ASCE 7-22 (released Dec 2021) is referenced by the 2024 IBC/IRC. Which applies is AHJ-dependent: confirm the adopted IBC/ASCE edition just as you do the NEC.
• The IBC requires rooftop PV to be treated as dead load in every load combination (§1603.1.8.1 in the 2015/2018 IBC, renumbered §1607.14.4.1 in the 2021/2024 IBC).

25.2 The loads

A roof must withstand the worst factored combination of:

• Dead load (D): permanent weight: roofing, framing, and the array itself (~3–6 psf including racking/ballast).
• Live load (L): temporary loads like maintenance workers (~20 psf typical per IBC for roof access).
• Snow load (S): from ASCE 7 ground-snow maps, adjusted for slope, exposure, thermal, and roof shape. Arrays change drift/accumulation patterns by creating barriers and shed zones.
• Wind load (W): pressure and uplift, driven by basic wind speed, exposure category, building height, and roof zone (field/edge/corner, with corners seeing the highest uplift). ASCE 7-22 added PV-specific wind factors (γE, γA), expanded ground-mount provisions (Section 29.4.5), and a D + 0.7S combination.
• Seismic load (E): in seismic regions, especially for ballasted and ground-mount systems.

25.3 The documentation path

The AHJ accepts one of three structural deliverables:

1. A PE-stamped calculation package (required in high-wind/coastal jurisdictions like Florida and much of California, often on nearly every project).
2. A manufacturer structural letter confirming a pre-engineered racking system fits the site.
3. A simplified permit form referencing published span tables for standard residential construction.

⚠️ When to call an engineer: marginal/old framing, heavy snow or high wind, ground mounts, ballasted commercial arrays, or any time the manufacturer letter’s assumptions don’t match the actual structure. Adding ~3–6 psf to a sound modern roof is usually fine. Assuming so on an unknown roof is how you cause a failure.

25.4 Worked example: a dead-load sanity check

A 19-module array (Case Study A) on a composition-shingle roof. Each module + racking adds ~4 psf distributed; the existing roof carries roofing + framing dead load and was designed for code live/snow loads.

• Array dead load: ~4 psf over the array footprint (≈ 19 × 2 m² ≈ 38 m² ≈ 409 ft² → ~1,640 lb total, spread over many attachment points).
• ⚠️ The question is rarely the average psf (small). What matters is the point loads at each attachment and whether the framing and worst-case wind-uplift at roof corners are satisfied. On a sound modern roof, +4 psf is typically fine; on aged or undersized framing, it is not assume-able.
• Decision: a simplified span-table permit form may suffice for standard residential framing. High-wind/coastal or marginal structure requires a PE-stamped calculation. When unsure, the 20-minute call to the building department (which IBC/ASCE edition? which structural document?) saves a redesign.

25.5 The load types at a glance

   DEAD (D)  - permanent: roofing + framing + ARRAY (~3-6 psf)   [always present]
   LIVE (L)  - temporary: workers/maintenance (~20 psf)
   SNOW (S)  - ASCE 7 ground-snow map x slope/exposure/shape; array alters DRIFT
   WIND (W)  - pressure + UPLIFT; worst at roof CORNERS; ASCE 7-22 adds gE, gA
   SEISMIC(E)- matters for ballasted & ground-mount in seismic zones
        └──► design to the worst FACTORED COMBINATION, not the simple sum

Chapter 25 summary

IBC/IRC + ASCE 7 (7-16 or 7-22 by adoption) govern; panels count as dead load in all combinations. Design for the worst factored mix of dead (~3–6 psf array), live (~20 psf), snow (with array-altered drift), wind (uplift worst at corners; ASCE 7-22 adds PV factors), and seismic. Document via PE stamp, manufacturer letter, or span-table form, and bring in an engineer whenever the structure or loads are marginal.

Practice Problems: Chapter 25

1. In a structural load combination, how is a PV array classified?
2. Roughly what added dead load (psf) does a typical rooftop array with racking impose?
3. On a roof, where does wind uplift tend to be greatest?
4. Which standard do the IBC/IRC reference for load calculations, and which edition pairs with the 2024 IBC?
5. List the three documents an AHJ will typically accept to satisfy structural review.
6. Name three situations that should push you from a simplified span-table form to a PE-stamped calculation.

Solutions: Chapter 25

1. As dead load, included in every load combination (IBC §1603.1.8.1 in 2015/2018, §1607.14.4.1 in 2021/2024).
2. Roughly 3–6 psf including racking (more if ballasted).
3. At the corners (then edges), more than the field of the roof.
4. ASCE 7; the 2024 IBC references ASCE 7-22.
5. A PE-stamped calculation, a manufacturer structural letter, or a simplified span-table permit form.
6. Any three of: high wind/coastal zones, heavy snow, aged/marginal framing, ground mounts, ballasted commercial arrays, or mismatch between the manufacturer letter’s assumptions and the actual roof.