I doubt it - compared to a 100kg person standing on it that is no stress at all.
Maybe if its nose and tail and you leave it in 45deg with the sun on it all the time....
A foot in from each end and normal temps it should be fine

I like storing SUP's on it's rail, instead of flat. Easier for old guys.

N8W said..
Sorry guys I was meaning transporting. Space between rear roof rack and Ute/truck tray rack is 6'. Would that much space between the racks put strain on the middle of the board?

No issue, the forces from the (small) weight of the middle of the board are negligible compared to the forces generated by wind buffeting the board and bumps in the road, especially since they tend to rotate the board (a bump lifts the front then the rear), and the most important issue becomes torque forces on the board.

And for this, the larger the space between the bars, the better. Just picture putting the bars close together, the slightest torque will rip the bars off the roof and may even create pressure dents in the board.