Definition
Span is the horizontal distance between supports for a structural member such as a beam, joist, rafter, or truss. Clear span is the distance between the inside faces of supports, while effective span includes some bearing width. Maximum span depends on member size, material species/grade, spacing, and load requirements. Exceeding maximum spans causes excessive deflection (sagging) or structural failure.
Formula
Span tables provide maximum spans based on member size and loading. For example, a 2x10 southern pine joist at 16 inches OC spans approximately 16 feet for residential floors (40 psf live, 10 psf dead). For the same joist at 24 inches OC, the maximum span reduces to approximately 14 feet due to increased load per joist.
Example
Span Table Example
For a floor joist application: 2x8 southern pine at 16 inches OC spans up to 12 feet 7 inches; 2x10 spans up to 16 feet; 2x12 spans up to 19 feet 9 inches. These spans assume #2 grade lumber, residential loading, and normal deflection criteria (L/360). For decks, use more conservative spans due to higher loading requirements.