In other words you would space a roof joist 15 inches apart from another roof joist but never 17 inches apart.
How far apart are studs in a flat roof.
Ceiling joists are usually installed either 16 or 24 inches on center which means roughly 16 or 24 inches apart.
If the roof is not completely square you can decrease the space between joists but never increase the space.
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.
How to find a ceiling joist.
Measure from the ends.
Other relevant factors include the type of wood being used for the rafters the size of the rafters and the load that the rafters need to support.
The spacing between rafters in a given building is part of the engineering calculations that determine the required structure of the roof.
For heavy roofs or in areas with substantial snowfall or winds call your local building department for code requirements.