This is traced back to building codes and structural design standards.
How far are roof trusses apart.
I ve had roof truss manufacturers try to convince me it is impossible to place wood trusses at spacings of over every 4 feet.
With traditional stick frame construction the trusses are usually spaced 2 feet apart.
Typical roof truss spacing means that they are usually about 24 inches from centre which means at the centre of one truss there will be a two foot gap or twenty four inch gap until the centre of the next truss.
Most are spaced between 18 26 inches apart.
With this design change roof trusses could be placed 12 feet apart making it possible for roofs to support the loads to which they would be subjected.
Putting the trusses into place.
The same concept holds true for roof trusses.
Common spacing for trusses on this type of building is often between 2 and 4 feet but trusses built with advanced designs may allow spacing as great as 8 or 12 feet.
Roof trusses were usually produced on site in the past.
Engineers have since determined however that the physical design of a post frame building will allow for standard spacing as great as 12 feet from the center when using high quality metal connector plates and lumber.
The trusses should be a little longer than the exact width to allow for overhang and you will need sufficient trusses to mount across the length.
It was perkins who pioneered roof purlins being placed on edge.