I am trying to learn some of the basic engineering calculations that apply to timber frames. I am not an engineer, but I'm beginning to grasp some of the principles. I'm stumped on a common rafter issue. That is, what type of calculation is used to determine the maximum span between ties? For example, if I have a 16 x 20 stand-alone common rafter structure, would it be appropriate to simply put posts and ties at the ends, 20 feet apart? I'd think not, as the middle would surely bow out due to the thrust of the rafters. What type of calculation is used to determine the maximum spacing between ties? Thanks to anyone who can point me in the right direction. I understand, of course, that any calculations I do will need to be checked and approved by a qualified engineer.