Carrying a canoe on top of your toad?

I have a friend that has a Jeep Grand Cherokee and regularly hauls 2 Kayaks on the roof. Has for years. Must be ok. Don't know the specifics, but just looks like it is tied onto the regular roof rack, front and sides. No tie downs in rear so that the hatch will open most of the way. Can't be rocket science as she has had no prior experience in such endeavors.