The National Design Specification for Wood Construction - the wood design code - provides wood material properties such as allowable bending stress and modulus of elasticity based in part on lumber grade.
When we analyze existing wood-framed or heavy-timber buildings, lumber grade is usually unknown. One way to make a reasonable determination of grade - and to avoid undervaluing high-quality existing construction - is to follow the procedures outlined in "A Grading Protocol for Structural Lumber and Timber in Historic Structures," published by the National Center for Preservation Technology and Training."
The protocol provides a way for design professionals to use physical features of wood members such as grain angle, knot size and location, and the presence of checking to estimate lumber grade for use in design and analysis.
The document, along with a Microsoft Access database file to help simplify the grade estimation process, are available for free from the National Center for Preservation Technology and Training.