To read a lumber span chart, find the desired joist size in the column on the left. Next, find the row that contains the desired spacing figure for the given joist size. Each joist size and spacing has a row that shows the required elasticity rating for various span widths. The table's bottom section contains the required strength ratings.
After you find the desired spacing for the joists, move across the row from the spacing figure to see if the desired span is permitted. From the span figure's column, move down to the bottom row in the same column to find the Fb value. The Fb value is also known as the strength rating.
Look for joist sizes that can span a distance equal to or greater than the project's requirements. Once the sizes have been calculated, look for the type of wood that meets the required strength and flexibility values. Further tables are used to calculate the best wood for the job. Different grades of various woods are suitable for different load-bearing tasks.
Finally, check that the chosen wood can withstand enough weight at the ends where it is attached to other beams and supporting walls. Span tables for joists and rafters provide this information. When sizing rafters, remember that the rafter's span is measured horizontally and may be less than its actual length.