I want to install solar PV panels on my roof. How do I know if the rafters can handle the extra weight?
If the panels are attached in about the same plane (within 10o) of the roof plane and therefore will not capture snow (drifts) then you may use the prescriptive tables for rafter spans, Tables 5802.5.1 (1) to (8). The dead load (psf) would have to be determined based on the manufacturer’s specifications for the panel weight coupled with the weight of the existing roof material. Use of the on-line rafter span tool http://www.awc.org/calculators/span/calc, which is allowed by the building code, is another method that will produce the same results as the table.