Solar (C-46) · Lesson 6 of 6

Roof Loads Under a PV Array: The Two Load Cases

"Roof Loads Under a PV Array: The Two Load Cases" is lesson 6 of 6 in the Solar (C-46) Learning Hub. It opens with "Why the Roof Must Be Checked Before You Mount" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Why the Roof Must Be Checked Before You Mount
  2. Two Load Cases Where the Array Covers the Roof
  3. Get the Roof Live Load from Table R301.6
  4. Wind, Fire Class and Penetrations
  5. Buildings Under the CBC: Section 1607.22

The worked example in the course applies this lesson to a realistic job in 7 steps, from "Site A governing load" through "Site B governing load".

What you'll be able to do

  • Explain the main idea of "Roof Loads Under a PV Array: The Two Load Cases" in plain language you can use on the exam.
  • Apply this step with confidence: "Get the Roof Live Load from Table R301.6".
  • Apply this step with confidence: "Wind, Fire Class and Penetrations".
  • Apply this step with confidence: "Buildings Under the CBC: Section 1607.22".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Roof Loads Under a PV ArrayPV + racking: 3 psfRoof dead load: 10 psf5 in 12, 150 sq ft per rafterRafters designed for 10 + 30 = 40 psfTWO LOAD CASES (R329.4.1.1), psfLive load = 16 psf (Table R301.6: 4 to 12 rise,0 to 200 sq ft tributary area)Site A: snow 30Site B: 0Case 110+3+30 = 4310+3+0 = 13Case 210+30 = 4010+16 = 26Governs43 (Case 1)26 (Case 2)Case 1 = dead load + PV + snowCase 2 = dead load (no PV) + greater of live or snowSite A: 43 psf is over the 40 psf design load: get the framing reviewedSite B: 26 psf is under 40 psf: OKSource: 2025 California Residential Code R329.4.1.1, R301.6, Table R301.6CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Why the Roof Must Be Checked Before You Mount

2025 California Residential Code R329.4.1 says rooftop PV systems shall be designed to structurally support the system and withstand the applicable gravity loads, and the roof they sit on must be designed and constructed to support the loads imposed by the PV system. Panels, racking and ballast add dead load, and snow can drift against them. Checking the load before you attach anything is the contractor's way of avoiding an overloaded rafter.

Step 2: Two Load Cases Where the Array Covers the Roof

R329.4.1.1 says roof areas NOT covered by PV are designed for dead loads and roof loads under R301.4 and R301.6. Roof areas that ARE covered by PV are designed for two cases. Case 1: dead load INCLUDING the PV weight, plus snow load from Table R301.2. Case 2: dead load EXCLUDING the PV weight, plus the roof live load or the snow load, whichever is greater, per R301.6. R301.4 says to use the actual weights of materials and equipment for dead load.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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