I'll throw a curve ball in here, this discussion was brought up at a Workcover meeting I attended a couple of weeks ago, bare with me on this.
Placing objects on a roof rack, simply put, 1/.Do not exceed the manufacturers limits for the roof rack...plus 2/. Do not exceed the vehicle manufacturers limits for tare weight to be carried on the roof...including the weight of the roof rack.
Example
So if you have a roof rack that can carry say 100kg, the Vehicle manufacture only allows 80kg max (gross) on the roof and the roof rack itself weighs 30kg, so technically you can only carry on that vehicles' roof items to the weight 50kg, eventhough the roof rack can take 100kg.
It was brought out in this meeting, that roof design on the latest models of 4x4's has ridgitity of the roof more incorporated into the glass design /placement, than the thickness of the pillars, which by the way is slowly being addressed by the ADR's over the next couple of years.
It was also brought out, that going over rough ground can compound the weight / stress on the roof, plus lifting down a hevy objects from that height to the ground may cause difficulties.
Workcover...what can I say.