What is the maximum allowable span for SPF #2 ceiling joists creating an uninhabitable attic when spaced at 24" on center?

The maximum allowable span for SPF (Spruce-Pine-Fir) #2 ceiling joists when creating an uninhabitable attic and spaced at 24 inches on center is 14 feet, 9 inches. This span is determined based on the structural properties of the SPF wood species and the designated spacing. Joists need to possess enough strength to support the intended loads while allowing for safe deflection limits.

In this context, the allowable span refers to the distance a joist can span without requiring additional support. For uninhabitable attics, which have less stringent load requirements than spaces designed for occupancy, the maximum span can be extended. However, it must still adhere to building codes and structural engineering principles to ensure safety and integrity. The selection of 14 feet, 9 inches reflects a balance between material strength, spacing, expected load, and applicable building codes.

This span is a result of standardized tables used in construction that indicate the maximum allowable spans for various lumber types, grades, and spacing configurations, ensuring compliance with safety standards in construction practices.