The 5 Percents of Lumber

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The 5 Percents of Lumber

  • Wood, like other structural materials, exhibits an inherent variability in strength properties from piece to piece.
  • The design value is obtained by reducing the fifth percentile test result by a property reduction factor.
  • The grading of lumber is not an exact science; a reasonable amount of discrepancy is to be expected between individual pieces graded by different graders.
  • Cross-cutting stress-graded lumber into pieces shorter than the original piece is a common practice wherever lumber is used in construction.