PV System Mechanical Design
What the exam tests
Mechanical design covers structural load paths, roof attachments, waterproofing, racking spans, module clamping, thermal movement, and fire-access setbacks. The exam expects you to distinguish dead, live, snow, wind, and seismic loads and to use manufacturer span tables rather than guessing.
Core rules
- Loads: dead load includes modules, racking, and ballast; wind produces uplift and lateral loads; snow and live loads act downward. Use ASCE 7 load combinations and the AHJ-adopted building code.
- Attachments: locate rafters/trusses, use manufacturer-approved lag screws or standoffs, maintain embedment, and flash every penetration. Sealant alone is not flashing.
- Wind zones: corners and edges see higher uplift; reduce attachment spacing or use engineered attachments there. Stay within module clamp zones; mid-clamp and end-clamp ratings differ.
- Rail spans: follow the racking manufacturer's span table for the actual module, snow load, and wind speed; do not exceed cantilever limits.
- Fire access: keep required pathways and ridge setbacks per the adopted IFC/IRC; local amendments often exceed the model code.
- Thermal and galvanic: allow rail and conduit expansion; isolate dissimilar metals to prevent corrosion.
Worked example
An array covers 300 ft². Modules weigh 4 psf and racking 2 psf, so dead load = 6 psf, or 1,800 lb total. The roof's allowable dead load is 10 psf, so it is acceptable for dead load. Design uplift is 25 psf net, or 7,500 lb. With 20 attachments, each carries 375 lb; a lag screw with 500 lb allowable withdrawal capacity is adequate, but edge-zone attachments may need closer spacing.
Traps
Clamping outside the module zone; ignoring corner uplift; relying on sealant instead of flashing; using roof area without checking framing; forgetting fire setbacks; mixing aluminum and galvanized steel without isolation.