Cross bars for Roof top tent

Not all car roof mounting bars are optimized for mounting rooftop tent. 

Whispbar by Yakima Flush Roof Bar - Pair | REI Co-op

 

the one shown above is a flush bar as the bar is flush with the leg, however, this is not optimal for rooftop tents compare to through bar as shown below. Whispbar S15WB Black Through bar Pack | Free KeyAlike

 

Through bar is more optimal for a rooftop tent,  the weight is more evenly distributed across the leg as the bar is through the leg and is wider allow for more balanced support and allow more space for attachment.