My roof racks are only 25'' apart.. You shouldn't have a problem with just one board (even being 14x30) IMO.. I've had no problems in the past 8 yrs and 150,000 Kim's that I've been doing it.
I do have a tow-bar support that I mostly only use when stacking boards to reduce the wobble driving over speed-bumps etc.. I often have two or three boards and even four on the odd occasion.
I drive daily on the freeway to the beach with my 14' board on the roof and don't use the rear support without a problem.. Just make sure you use pads between the board and the racks so any wobble will be absorbed by the pads rather than go straight through the roof racks to the roof.. Make sure you use tie down strapes and I don't recomend using one long strap instead of two because as you tighten it up it's going to be trying to pull the roof racks together.. Also don't have the straps too tight.. You need to be able the slide them left and right on the board with your fingers and if you can't IMO they are too tight and might result in board damage.. Don't worry to much about it wobbling up and own a little in the wind at high speeds..
Another sollution might be to use a board bag with a tie down loop at the front of the bag and use a short piece of rope from the nose to the front bumper.. I have a friend that does this.. btw.. You don't need one at both ends.
