Introduction
Structural evaluations determine whether a roof can support solar PV systems safely. Oregon’s structural code determines whether a structure is prescriptive or non-prescriptive based on solar system design, roof materials, and roof framing type.
If a structure is classified as prescriptive it means that the structure was built to a prescribed method and can accommodate a solar system without a detailed structural review. If, on the other hand, a structure is classified as non-prescriptive, it means that a third party structural engineer would need to analyze the structure to see if it can accommodate a solar system as is or if the structure requires structural upgrades before proceeding.
Prescriptive Structure Requirements
1) Solar System Design:
- PV modules and racking must weigh less than 4.5 lbs/sq. Ft.
- Modules must be parallel to the roof plane and under 18″ in height.
- All attachments must be connected to structural elements, not decking.
- The racking system can not be a weighed ballasted system.
- Spacing of attachments can not exceed 48” and must be 24” or less when the attachments are located within 3’ of the roof edge, eave, or ridge.
2) Roof Materials:
- The roofing materials must be either metal, single-layer wood shingles, or up to two layers of composition shingles.
3) Roof Framing Type:
- Engineered trusses spaced at 24″ OC automatically qualify as prescriptive.
- Conventional sawn lumber rafters require checks for span and spacing per OSSC Table 2308.7.2(1).
- The roof rafters and ceiling joists must run parallel creating an “A” frame configuration with a ridge board and/or collar tie.
- The rafter slope must be greater than 3/12
- The ceiling joists and roof rafters are at the same spacing and connected where they intersect.
- Support members which reduce the span must be located directly on a bearing wall below
- Hip and valley rafters must be a minimum of a 2” member, be supported at the ridge, and must comply with the span and spacing requirements.
4) Standing Seam Metal Roofs
- Minimum 26-gauge metal panels.
- Panels must be ≤18″ wide and attached to a wood substrate.
- Clamp spacing along seams must not exceed 60″.
If a structure fails to meet the above requirements, it is considered non-prescriptive.
Non-Prescriptive Path
- Structures failing the above prescriptive criteria require engineering review to determine if the existing structure can withstand the additional load of the solar system. The review assesses rafter span, size, spacing, and load distribution for the entire structure.
Conclusion
Compliance with structural codes ensures the safety, durability, and reliability of solar PV installations. If you’re curious whether your structure is prescriptive or non-prescriptive, reach out to one of the specialists at Power Northwest. We can make these determinations in just a few minutes onsite. Contact us today!
