I aways use roof rack pads to avoid damage to my board and deck pad.. and those annoying black marks that get on your deck pad from the racks.. and also avoid the chance of crushing the board rails from over tightening the straps.
My first pads were just foam blocks that would blow off if there was any wind and I'd have to chase them down the road after taking my board off the roof at the beach.. Even loading up to go home was a hassle because I'd place the pads on the racks but before I could lift my board on they'd blow off again.. Even in no wind when a big truck would wiz past they would be off down the road again.. That kept pissing me off.. So I cut my pads so they are a firm push on fit.. I was happy now and these pads have lasted me for years.
Cutting them is a lot trickier than you might think and I've been asked by a few friends now to make them some.. and I have.. The other day while I made some for another friend I took some pics so that people can make their own in the future..

First you need to find yourself some suitable foam about the right size.. Not the squeaky esky foam or the soft spongy foam.. This piece is from a noodle that they use to protect the boats from the wharf and I found it floating in the water one day while paddling.. I made these pads for my friends new Golf and her racks were a little more narrow than my Rola racks.. First I cut the block in half.. shown by the red line.
Next I do the two side cuts on an angle and note that you can not do the bottom cut yet shown by the green line.. That's the tricky part.
You need to do that bottom (green line) cut in two parts.. The first one like this.
... and the second one like this.. It's not easy.
This is what it should look like.. if you're nice a neat.
There is no need to have these pads very wide.. There's no need to have them as wide as the board and mine are about an inch short of the length of a hack saw blade.. If your board has a slightly domed deck only the center touches anyway.
You can now crank down the tie down straps because the pads will have some give.. before the rails give.. and you deck grip should remain mark free..
I often think about going into production making these and selling them.. Maybe I could then quit work and become a millionaire..

DJ