What is the maximum allowable span for untreated structural lumber used in moderate climates?

The correct answer is based on established building codes and guidelines for structural lumber. In moderate climates, untreated lumber, such as those from species like Douglas fir or Southern pine, generally can span up to 16 feet when considering common factors like load, spacing, and type of lumber used. This span allows adequate structural support while maintaining safety and stability for residential constructions.

It's important to recognize that spans can vary based on specific conditions such as the load-bearing requirements, the type of wood, and the grading of the lumber. Therefore, the maximum allowable span of 16 feet ensures that the structure remains durable and safe under typical conditions encountered in moderate climates. Using lumber spans that exceed this limit may compromise structural integrity and is often not compliant with building codes, which are designed to prevent failure under various load conditions.