Need to be smarter than I am?

I am making an imaginary roof on a house in the south, and need to know the basics. The roof is plywood, and use nails to hole it on the beams. If I were in construction, and I was inspecting it, my plywood would be how thick, nails how long? The shingle, and those nails, I can make a guess on. Trusses hold the force of the wind, and beams hold the load?