Thankyou Mr Ryan. Could I offer an exmple for complete clarity?

Rafters spaced at 2ft intervals span 14ft-2ins diagonally at 45deg. Assumed roof load is 40 lbs/sq-ft (projected horizontally) giving a total of 800lbs continuous on a 10ft horizontal span. The formulae suggest 3x4+1/2 (douglas), a section which would then be appropriate to span 14ft-2ins on the diagonal?

Thankyou for the book reccomendation.