A spare wheel is 30-40kg. Two Jerries are 50kg, a hi lift jack (I'm assuming) is 15kg, and swags at approx 10kg each. Let's ignore the shovel and chairs for now and you're still up for over 115kg plus the weight of the rack. To me, that's a heap of weight. Low weight on the roof would be 30kg. I personally wouldn't trust a deefers roof with over about 50kg.

