Plans called for trusses, the framer built 2x6 rafters: is that legal and is it safe?

In r/Carpentry, the owner wrote that the approved plans showed engineered trusses and a ridge in the middle of the building. The framer says the stick roof meets code and that he has the paperwork. With a centered ridge, each rafter covers roughly 12 feet. The International Residential Code sizes rafters from span tables in Section R802.4.1, and the span is measured level, between the top of the wall and the ridge.

Snow load settles most of the argument. In the IRC tables, as compiled by EstimationPro, a 2x6 rafter of No. 2 Douglas fir-larch, spaced 16 inches apart, spans 14 feet 7 inches with a 20 psf roof live load, 12 feet 1 inch at 30 psf of ground snow and 9 feet 10 inches at 50 psf. A load that falls between two tables rounds up. Michigan's residential code gives every county its own ground snow load, from 20 psf in some southeast Michigan areas up to 90 and 100 psf in the Keweenaw, according to Stellar Roofing; most of Kent County is 35 psf. Above 70 psf the roof must be engineered.

The tables take for granted that ceiling joists or rafter ties sit at plate height, where they keep the rafters from spreading the walls. Move the ties a third of the way up to the ridge and the allowable span drops by a third. The IRC also calls for a ridge board at least as deep as the cut end of the rafter, and for heel-joint nailing from Table R802.5.2(1) unless a beam or wall holds up the ridge.

Then there is the permit. Section R106.4 of the IRC requires changes that depart from the approved construction documents to be resubmitted for approval. Ask the framer which table row he used, check the grade stamp on the lumber and look up your county's snow load, and get the building department to sign off on the revision before the framing inspection.