I would only put my bikes on roof for short trips, because
a) constant high speeds dry out all the bearings/lube
b) bikes get covered in dust, dead bugs and stone chips, and
c) sooner or later you will drive under a low tree, carport, bridge or whatever.......
Basically offroading with bikes outside your vehicle will stuff them wherever you hang them, and I wouldn't do it for any great distance. I stick them in the back of the 4by on long trips but then I have no kids so plenty of room.
Cheers,
Cam