I'm evaluating my 1941 Colonial home's roof for a potential solar installation. The diagram below outlines my home's architecture; note the raised ceiling joists that run parallel with the common rafters.
When measuring the horizontal span from the purlin to the ceiling joist/rafter mating (6'), the Minnesota Standardized Load Tables for Rooftop Solar suggests that my roof (16" OC, Douglas Fir Historical, 50psf ground snow load, wind exposure B, 7:12 pitch) has 6.0 psf of additional load capacity, which seems adequate for solar.
However, if the span is to be interpreted as the horizontal distance from the purlin to the exterior wall (8'6"), then the span is beyond the scope of the table and is unlikely to support the additional weight without sistered members.
Which interpretation of span is correct for my application? If the latter, would sistering the 8'6" spans with 2x4s be sufficient to support the additional weight?
