Extend the room for roof generation a little outside the outer walls before generating. It gives you a roof where you can attach raingutters.
It doesn't work for all houses but the generated roof is an object like all other furniture: you can resize it to get the same result.

Using the interior surface like you describe is great if you just want a ceiling. That comes in handy if you have a roof like I described that extends outside the exterior walls but you want a different ceiling fort the part inside the house and the part that extends outside the house. Just add this second roof at a little lower elevation for an interior ceiling. That hides the ceiling that extends outside the house so you can give different textures to the parts inside and outside the house.