WOOD BEAM SIZINGTM   Span

BEAM SPAN

The beam span is the distance measured along the beam between the center line of the end supports. The beam itself will always be longer, in order to bear sufficiently on each end post or wall.

In this example, required bearing would be 3-inches at each end - 2 jack studs. (Bearing is never less than 1-1/2 inches).

In this example, the final cut length would be 6-inches longer than the clear span.

RAFTER SPAN

For roof rafter structural calculations, the span is the horizontal length in plan, from ridge to inside of exterior wall.

With WBS Basic, the overhang is fixed at 24-inches. The ridge board is assumed to be 2-by lumber.

The length is calculated as the minimum cut board length required to layout the rafter.

For one way to increase the rafter span, see braced rafter construction,
IRC figure R-802.5.1