Why timber is graded: indicating properties and characteristic values

Selling a property, not just wood

A mill that sells structural timber is really selling a predictable mechanical property. A designer who specifies a joist or a rafter needs confidence that it will carry its load, and across millions of boards the only way to give that confidence is to sort the output into groups whose properties are known statistically. That sorting is strength grading.

Two facts shape everything in this course:

• Wood is highly variable, within one source and between sources. In the Gradewood data (Europe), national mean bending strengths of spruce differ by several MPa before any scatter inside a sample is considered.
• Strength cannot be measured on the board you are going to sell, because the only way to measure it is to break it.

So grading measures something non-destructive and relies on a statistical correlation with strength. That measurement is the indicating property. A trained person looking at the board (visual grading) or a machine measuring physical signals (machine grading) can provide it.

What a characteristic value promises

For normal construction timber, the grade-determining properties are bending strength, bending stiffness and density at a reference moisture content of 12%. Strength and density are 5th percentile values; stiffness is the mean.

Be careful with the meaning. A 5th percentile statement concerns a population, not each piece. If a population of boards is assigned to a class with a characteristic bending strength of 24 MPa, no more than about one board in twenty is expected to break below 24 MPa. It does not say every board reaches 24 MPa. (This reading is an interpretation, not a quotation from a standard.) For that reason one failed test piece does not by itself prove that a batch is mis-graded.

The class number carries the bending value: C24 means a 5th percentile bending strength of 24 MPa. Everything else in the class (stiffness, density, and in the standard other properties) is attached to that label.

Worked example: how many weak boards are expected?

A mill ships 2000 boards of one grade whose characteristic bending strength is 24 MPa.

• Fraction expected below the characteristic value: 5% = 0.05.
• Expected count: 2000 x 0.05 = 100 boards.

So about 100 of the 2000 boards may legitimately be weaker than 24 MPa while the grade is still correct. A design takes this into account; the producer's job is to keep the population statistics right.

The standards family (Europe)

European structural grading rests on a small family of standards, each with its own job. Full texts of these standards were not available to the portal's authors; what follows is taken from publisher previews and papers that cite them.

A strength class groups grades, species and sources with similar properties so they can be used interchangeably in design.

Checklist: can you explain it to a customer?

• Which property does the class number refer to? (Bending strength, 5th percentile.)
• At what moisture content are the values defined? (12% reference.)
• Which standard governs the product and its marking? (EN 14081-1.)
• Which route assigned the class: a visual grade listed in EN 1912, or machine settings?

Key points

• Grading sorts timber by a non-destructive indicating property that correlates with strength.
• Bending strength and density are 5th percentile values, stiffness is a mean; all refer to 12% moisture content.
• A class is a population statement: about 1 piece in 20 may fall below the characteristic strength.
• EN 14081-1 governs the product, EN 338 defines classes, EN 1912 maps visual grades to classes, and machine grading is graded directly into classes.