Another question to ask is what are the fasteners going into, a soft, soft wood or oak?
A couple of years ago I built a plank on frame using wide pine, 2-1/5" full thickness, wall plank house. These planks had a single spline and pegs along the edges. The frame was oak and pine, I used 5" GRK lag screws. It went well, I think installing 3x6 on a roof deck would be easier, you shouldn't have any problem.
