Hi Don;

I think you are correct on several accounts. The overall appearance of a building tells a lot about it's age, including the roof pitch. Sometimes there complications like an older barn being Victorianized or updated, or reused materials in a newer barn, so one needs to look at every detail and keep in mind that written documentation about a buildings age is frequently incorrect.
Wars have profound affects on building materials and designs, but whether the Civil War was the period roof pitch changes on barns I cannot attest to.

Jim