In our climate (the Southeast), it's wise to put felt under a metal roof on a barn, otherwise it rains inside whenever the metal hits the dew point.

Depending on where you are (snow loads?), the pitch (since the boards are running up the pitch) and the species of the decking, 3/4 shiplap may be strong enough. Or it may not. Generally speaking, I like to use 1x up until about 36" span, and 2x above that.