Timber grading

Timber grading is the process of evaluating and categorizing timber based on its physical characteristics, strength, and suitability for specific applications.^[1][2] This classification ensures that timber meets industry standards and is appropriate for its intended use in construction, furniture making, and other applications.^[3]

Principles shown as for the grading of wood by ultrasound.

Diagnostic grading of squared oak timber using a certified test, the Sylvatest.

Grading methods

Timber grading (or wood grading) is primarily conducted through two methods:

Visual grading

Visual grading involves the manual inspection of timber by trained graders who assess characteristics such as knots, grain patterns, and defects. This method is widely used due to its simplicity and cost-effectiveness.^[4]

Machine grading

Grading a wood element by Timber Grader, a certified device.

Machine grading utilizes mechanical devices to assess the strength and stiffness of timber. This method provides more consistent and objective results compared to visual grading.^[5]

Standards and applications

Different regions in the globe have established standards to ensure uniformity in timber grading:

• In Europe, the EN 14081-1 standard specifies requirements for strength-graded structural timber with rectangular cross-sections.^[6]
• The EN 338 standard defines strength classes for structural timber, facilitating the selection of appropriate timber for construction purposes.^[7]
• In the United States, the National Hardwood Lumber Association (NHLA) provides grading rules for hardwood lumber, focusing on the size and number of clear cuttings that can be obtained from a board.^[8]

Properly graded timber ensures safety, durability, and performance in various applications, such as:

• Construction: Structural timber must meet specific strength requirements to ensure the integrity of buildings and infrastructure.
• Furniture making: High-grade timber is preferred for furniture due to its aesthetic appeal and workability.
• Packaging: Lower-grade timber may be used for pallets and crates where structural strength is less critical.

Strength classes

In Europe, strength grading classifies the structural performance of individual timber boards. In accordance with the standards outlined in standards BS EN 338 and EN 14081, timber is subjected to various assessment methods to determine its mechanical properties. Based on these results, a strength class is assigned, providing an at-a-glance indication of the timber’s load-bearing capability. This classification system helps ensure that the right type of timber is selected for specific structural applications. Timber strength classes are categorized based on the type of wood — hardwoods or softwoods. For hardwoods, the classification begins with the letter ‘D’, representing their origin from deciduous trees. The number following the ‘D’ indicates the strength level, with higher numbers denoting greater strength. Available hardwood strength classes include D24, D30, D40, D50, D60, and D70. Softwoods, which come from coniferous trees, are labeled with a ‘C’. The numbering system follows the same principle as hardwoods: the higher the number, the stronger the timber. Common softwood grades include C14, C16, C18, and C24, or (rarely) higher.^[9]

See also

• Lumber
• Woodworking
• Structural engineering
• Building code