More thoughts;

Some board roofs probably did develop leaks (but only when it rained) and were soon shingled over.

Please keep an eye out for vertically boarded roofs which appear weathered, especially if there is evidence of a batten applied to the edges of the boards, that is if the edges are less weathered than the middle part of the boards.

Also, a barn near me had closly spaced common purlins and could have been shingled directly onto the purlins with no sheathing, but was vertically boarded whin it was disassembled a few years ago.

Jim