Tom- I've only converted two barns to homes, but in both cases- different municipalities- we were required to show structural engineering calculations for the homes to get permitted. No engineer would sign off on the old timber-despite it being obviously sound and likely of better material than what is available to most of us today.

We were able to circumvent the issue by enclosing the frames with SIP's- actually designing the SIP's to do almost all of the structural work. We used structural connections rather than the usual curtain wall connections most SIP over timber applications use- and we added a ridge beam to carry the roof panels, which ran ridge to eve. Likely overkill structurally, but that's not such a bad thing I guess. Worked well and passified the inspector.