Lumber Grades In Wood Trusses
The grading rules usually permit some percentage of visually graded lumber to be below grade – I believe it is 5%. ANSI/TPI 1 requires that all lumber used in trusses be of the grade specified by the truss design. A strict interpretation of ANSI/TPI 1 would require that the fabricator cull the 5% that is off grade. What are your thoughts on this?
The 5% tolerance is to account for variations in lumber grader judgement and for human error. It is not there to intentionally assign inaccurate grades to lumber. Truss manufacturers buy graded lumber – it is not a reasonable expectation to have them cull through lumber or essentially “re-grade” the lumber they buy. They rely on the grade to be accurate.
The fact that the lumber is marked with the grade stamp specified on the truss design is enough to meet the intent of provision 3.4.1 of the Truss Plate Institute's ANSI/TPI 1: “Truss lumber shall be the size, species and grade specified on the truss design drawing.” However, any grade-allowed lumber defects such as wane or knots that occur in plated areas must fall within the quality tolerances for ineffective teeth outlined in section 3.7.4.