99% chance you will scratch you paint or put a dent on your roof. Use another car if you stuff won't fit in the back.
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If you decide to go forward with it anyway, make sure your roof is washed and put a little foam between the bag and the paint. Brand new urethane foam might work good and give a little extra friction on the paint to keep it from shifting (scratching). Also, if you have something flapping in the wind against your paint, it will scratch it.