This is most likely the standard size that you will find on pre made roof trusses.
Steel roof truss spacing.
Truss spacing for metal roof.
The typical standard for the spacing on the roof trusses is 24 inches.
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.
It all comes down to engineering.
Roof trusses were usually produced on site in the past.
The same concept holds true for roof trusses.
So the space between rafters can be greater without reducing the maximum span.
These standards are as follows.
Typically pole barn style buildings in my area have bays distance between trusses of 5 feet to 12 depending on pitch insulation and height.
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.
As most building these days are using those pre made trusses on the roof 24 inches become the main size for any roofing construction when it comes to its truss installation.
The manufacturers of the steel roof truss plates also referred to as gussets or gang nails provide the engineering design for pre fabricated wood trusses.