Use a roof joist span table by finding the maximum span possible for different spacings, species, grades and cuts of wood. Different live and dead load amounts require different roof joist span tables. Most charts only list specific spans up to 26 feet, as longer spans are uncommon.
First identify the joist spacing you are using in the leftmost column of the chart. Common spacing distances are between 12 and 24 inches, with more closely spaced joists capable of supporting longer spans. In the next column to the right, select your species and grade of wood. SS-Grade Douglas fir supports the longest spans, but is the hardest to find and the most expensive.
Identify the cut of wood you are using for your joists. Larger boards support longer spans, but they are more expensive. The column under the cut size is the maximum span that species, grade and cut of wood can support. The chart may use a standard foot-and-inch form of designation, or it may use a single, dashed number. In the second case, the first number is the number of feet and the second number is the amount of inches. For instance, a 2-by-8 of number-2 hemlock has a maximum span of 18-2, meaning 18 feet and 2 inches.