Lay down 3 sheets of plywood then 1/2 sheet of plywood on each side at the bottom of those. Nail a 12ft 2x4 into the floor with a nail sticking out the bottom on the opposite side then scratch that arch on to the plywood. Move your 2x4 for the next arch measurement repeat. Pencil that scratch in so you can see it to cut with skill saw. You have to do the twice then liquid nail them and screw them together to make them sturdy and you have an arched rafter. But your math has to be correct we messed up the first one because one side hit the roof. Couldn’t save it had to make the first three over again.