Ah, another button that's a bit mislabeled. The bump-button should actually be called "Make bizarre bumpy shape thingy". That's a remnant from way back, should probably not be used, but was left because some mods still use it.
If you want to do roofs similar to how they're done in Northrop, just check how those maps do it, perhaps even copy a roof from there. They're just done by adjusting the z position of an individual point. But a better and nicer way to make a roof would be to use a 3D modeling tool, Blender for example. Any program that can output an .obj format will do.
The obj format that Driftmoon expects is:
-include texture coordinates
Driftmoon doesn't support any kind of bump-mapping (at least yet), but to balance that off it works in most computers.