We are thinking of finishing the attic of an old (100 year)house. I have a concern that the orig roof system (shakes over nailer strips supported by 2x4's 24" o.c.) may not me sufficient to support the weight of gypsum ceiling boards (even the 1/2" sag resistant ones). The framing is not 'true' enough for a nice even surface, so steel hat channels may be used to even things out (more weight + weight of insulation).

Am I being paranoid for worrying about adding an additional load to the roof framing?

Also, if this happens, would venting be an issue? (Or are shakes on nailer strips vented by default? i.e. not very air tight)