Plan Review: Structural Letter, Ridge Setback Amendment, and a Module Substitution
A plan set is submitted for an 11.2 kW flush-mounted array on a 1995 two-story dwelling with a 4:12 gable roof, asphalt shingles, and rafters at 24 in. on center. The structural letter, signed by a PE, states: "Array dead load is 2.5 psf including racking and attachments; attachments at 48 in. on center; design snow load 25 psf; no structural reinforcement is required." The bill of materials lists 28 modules at 49 lb each, 20 rails at 16 ft and 2.2 lb/ft, and 60 attachment feet at 1.0 lb each. The roof plan shows an array footprint of 620 sq ft and places the upper array edge 18 in. from the ridge. The jurisdiction has adopted a local amendment requiring a 36 in. clear setback from ridges on one- and two-family dwellings. Sheet E-1 schedules 405 W modules; the datasheet included in the submittal is for a 410 W module with a different length and width.
1. Check the dead load against the structural letter. Can you approve the structural documentation as submitted?
Read the worked answer
No. The submitted equipment does not match the load the engineer analyzed.
Compute from the bill of materials:
- Modules: 28 x 49 lb = 1,372 lb
- Rails: 20 x 16 ft x 2.2 lb/ft = 704 lb
- Attachment feet: 60 x 1.0 lb = 60 lb
- Total: 2,136 lb
- Distributed over the stated 620 sq ft footprint: 2,136 / 620 = 3.45 psf
The letter analyzed 2.5 psf. The submitted design is about 38% heavier, roughly 0.95 psf over the analyzed value. A letter that does not describe the design being permitted does not cover it.
Two more questions to put to the engineer:
- Which area did the 2.5 psf use? If the engineer divided by a larger gross roof area instead of the array footprint, the letter is even further off.
- Attachment spacing of 48 in. on center matches the letter, but the load per attachment rises with the heavier assembly, so uplift and withdrawal values must be rechecked, not just the uniform dead load.
Approve only after a revised letter covering the actual weights, or a bill of materials revised to meet 2.5 psf.
2. How do you handle the 18 in. dimension at the ridge and the local amendment?
Read the worked answer
The dimension fails the adopted local requirement, but the amendment has to be confirmed before the plan is returned.
Steps:
- Verify the amendment text, its effective date, and the edition of the model code it amends. If the permit was submitted before the effective date, ask the AHJ in writing whether it applies.
- Confirm it applies to one- and two-family dwellings, which this is, and that it is a ridge setback rather than an eave or access-pathway requirement. They are separate provisions and are often confused.
- Compare the drawn dimension with the requirement: 18 in. < 36 in., so the layout as drawn does not comply.
The fix is a redesign, not a field adjustment: shift or shorten the upper row, which changes module count, string configuration, and possibly the interconnection calculation. Do not accept a note that the crew will "keep it back in the field," because the plan set is what gets inspected and the racking layout is set from it.
Record the measurement basis (from the ridge cap or the ridge line), the amendment citation, and the specific sheet that must be revised.
3. Sheet E-1 schedules 405 W modules while the datasheet is for a 410 W module. What are the consequences and what do you require?
Read the worked answer
A different module is not a clerical substitution. It propagates through the design.
Consequences:
- Physical: different length and width change the row layout, the footprint, and every edge and ridge dimension, including the setback problem in the previous task.
- Electrical: 28 x 410 W = 11.48 kW versus the scheduled 11.2 kW, which changes the interconnection review and the continuous output used for the breaker and busbar calculation. Different cell counts change Voc and Vmp, which changes the string voltage at the record low design temperature.
- Structural: different module weight changes the dead load calculation.
- Listing and labeling: the panel label and the placard must match what is installed.
Require:
- The datasheet for the module actually being installed, with the model number matching Sheet E-1.
- A revised roof layout drawn to the correct module dimensions, with the ridge and eave setbacks dimensioned.
- Recomputed string voltage at the design low temperature against the inverter maximum input voltage.
- A statement that the interconnection review and the breaker sizing were based on the final DC rating.
If the installer wants to use the 410 W module, the plan set is revised; if the 405 W module is intended, the datasheet is replaced. Either way, the documents must describe one system.