Stacking boards on cartop rack

[isobars]

With OEM (came with the vehicle) roof racks, the weakest link is the cheap widget that connects the rack to the roof. For loads taller and higher drag than one board nestled beneath and behind the slipstream, I've always drilled into the roof and bolted the front of the rack to steel subframe (or at least huge fender washers that cannot pull through the sheet metal) with actual ... you know ... BOLTS.

Overkill? Hell, yes! Considering the damage flying gear can do to your gear, other vehicles, their occupants, and your legal expenses, overkill is the only way to go. I enjoy the peace of mind I get from KNOWING that those 100+ mph relative windspeed gusts can NOT tear my stuff off my roof. A bud's
Nissan XTerra woof wacks took a big flying leaf across the highway when its OEM fasteners pulled out, and I've seen those chintzy pieces of metal -- little more than threaded pop rivets -- up close. A WS board buffeted by wind can greatly exceed the load ratings of OEM roof racks.