Thanks to all for the replies. I found a paper on the TRADA site which gives a figure of 0.7t kg/m2 (where t = thickness in millimeters) for C5 grade. This makes a standard sheet come out at 25kg, which as C5 is denser than standard grade, seems to concur with the figures above.
PS My interest stems from not whishing to cream cracker my car by putting more than the advised 50kg load on to the roof bars!
It's not usually the downward pressure you have to worry about its the "lift" once the speed picks up. Get a set of ratchet straps, much better than messing about with string, rope and Knots, you can tell I wasn't a boy scout.
Also worth protecting the strap from the edges of the board, having had one wear through on the edge a a sheet a melamine faced chipboard, luckily it gave way as I was going over a speed hump so no damage.
I'd agree with that. I've had three 18mm hardwood ply and a bunch of timber on the roof before, no problems. The one time I did have problems was with one sheet, forgot the straps and had to scrounge some rope from the B&Q. Had an interesting time watching the sheet trying to take off all the way home.