When installing tracks on the roof, make sure that both tracks are measured and parallel before drilling multiple holes.  This can be accomplished by drilling and installing the first hole on each track.  Measure the distance beween the tracks, at that first hole, and also at the last hole, makign sure they are both identical.

Be sure to look over the instructions to see which holes you should use for drilling.  As always, when drilling any rack system, measure twice, and drill once!