The spacing between rafters in a given building is part of the engineering calculations that determine the required structure of the roof.
How far apart are studs in a flat roof.
Maximum rafter spans for a patio roof in areas with mild climates patio roofs are generally designed for loads of 30 psf pounds per square foot.
In other words you would space a roof joist 15 inches apart from another roof joist but never 17 inches apart.
When starting to space out the roof joists always start at the end of the roof.
For heavy roofs or in areas with substantial snowfall or winds call your local building department for code requirements.
If the roof is not completely square you can decrease the space between joists but never increase the space.
How to find a ceiling joist.
The size height and spacing of studs shall be in accordance with table 23 i r 3 except that utility grade studs shall not be spaced more than 16 inches 406 mm on center or support more than a roof and a ceiling or exceed 8 feet 2438 mm in height for exterior walls and load bearing walls or 10 feet 3048 mm for interior non bearing walls.
Measure from the ends.