Could do either. Our Ezi Awn sat on roof bars mounted on to a roof rack rails. Key point is the strength / difference of the mounting points between bars and rack. If they both mount in the same way, then there is probably very little difference although the rack may have more mounting points, even if they are the same that could spread the load. Part of it is not just the weight of the tent but the wind pressure at speed. If you are mounting off bars, one factor will be whether they are directly mounting on to gutter rails, in which case you may need three bars, versus if the bars mount on to roof rails, in which case they are already spreading the mounting point so you probably only need the two bars. We only had two bars on one of the largest Ezi Awns but it went onto the roof rack rails which in turn was bolted to a roll cage.
The only other factor is whether having a rack makes it easier to use. Whilst our Ezi awn would fold out over the bonnet with a ladder down onto the bull bar, we would usually go up the back of the LR on to the roof rack as it was just easier for taking boots off etc. Just what ever works for you.
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