Whether you're building a shed roof or adding a dormer to your second story, it is important to choose the proper size rafter. This Sizing Guide for Roof Rafters will help determine what you need for your small project.
There are several variables to consider when determining the proper rafter size. These factors include:
There are also factors such as deflection, elasticity and wood species that come in to play. Consult local building codes for snow load capacity and reference the Span Tables for Joists and Rafters published by the American Wood Council (AWC)
As a general rule, roofs that have a high human occupancy rate should be framed with rafters every 16." Structures such as sheds or barns can be frames with rafters every 24." Again, make sure to consult local codes.
If your project had a rafter length of 8' or less, a safe and strong method is to use 2x8 framed 16" on center. It is always better to have a large rafter, rather than be concerned with a sagging roof.