EN 338 Structural Timber - Strength Classes

The European standard “EN 338 Structural lumber – Strength classes” establishes a strength class system for general use in design codes. This standard gives the characteristic strength and rigidity properties and density values ​​for each class referred to in the EN 14081-1 standard. This standard applies to all softwood and hardwood lumber intended for structural use.

This standard includes additional strength classes primarily for structural timber subjected to tensile loads.

For the purposes of this standard, a population refers to timber with relevant characteristics. A population is defined by parameters such as species or combination of species, source, and strength grade.

The EN 338 standard provides a series of strength classes, each indicated by a number representing the edge bending or tensile strength value in N/mm².

• For softwood species based on edge bending tests, the characteristic strength, stiffness, and density values ​​of the strength classes are given in the table titled "Strength classes for softwood based on edge bending tests - Strength, stiffness, and density values".
• For softwood species based on tensile tests, the characteristic strength, stiffness, and density values ​​of the strength classes are given in the table titled "Strength classes for softwood according to tensile tests – Strength, hardness and density values".
• For hardwood species based on edge bending tests, the characteristic strength, stiffness, and density values ​​of the strength classes are given in the table titled "Strength classes for hardwoods based on edge bending tests - Strength, stiffness, and density values".

The most commonly used strength classes in Europe are shown in bold letters in the tables.

The EN 338 standard, a system for classifying structural timber according to its mechanical properties, ensures consistency, safety, and reliability in timber used for load-bearing applications in construction throughout Europe.

The EN 338 standard defines a range of strength classes for both softwood (coniferous) and hardwood (deciduous) lumber. This standard provides characteristic values ​​for the following properties:

• Strength properties (e.g., bending, tensile, compressive, shear).
• Rigidity properties (elastic moduli).
• Density.

These values ​​are used in structural design codes such as Eurocode 5 (EN 1995-1-1 standard), allowing engineers to specify a strength class (e.g., C24) rather than specifying individual species characteristics. This standard applies to rectangular solid lumber and refers to the EN 14081-1 standard for lumber classification and CE marking.

The latest update to this standard includes the following:

• New classifications for softwoods based on tensile tests.
• Extended classes for hardwoods.
• Characteristic values ​​set for specific features.

According to this standard, the strength classes are as follows:

• C: For coniferous/softwoods (e.g., C16, C24, C30 are the most common).
• D: For deciduous/hardwoods (e.g., D30, D50, D70 for higher strength species like oak).

Higher numbers indicate greater strength and hardness. Common grades range from C14 to C50 for softwoods and D18 to D70 for hardwoods.

These tables typically list the following characteristics:

• Bending strength.
• Parallel/perpendicular tensile strength.
• Parallel compressive strength.
• Average parallel modulus of elasticity
• Density

For example, a popular grade, C24, has a characteristic flexural strength of 24 MPa and an average hardness of approximately 11 GPa.

The EN 338 standard is important for architects, engineers, and builders who specify lumber for beams, floors, frames, or truss girders. It integrates with the following regulations:

• EN 1995 standard for design (Eurocode 5).
• EN 14081 standard for classifying timber marking.

EN 338 is a fundamental European standard that defines strength classes and provides characteristic values ​​for mechanical properties such as bending strength, stiffness (modulus of elasticity), compression, tensile, shear, and density.

This standard is considered important for the following reasons:

• It ensures structural safety: Property values ​​are based on statistical lower limits (5th percentile) and provide a safety margin against the risk of failure in load-bearing elements such as beams, slabs, and frames. Without standardization, the varying properties of lumber from different types or sources can lead to unpredictable performance and risks in buildings.
• Eurocode 5 simplifies design and integration: Engineers use strength classes directly in calculations according to Eurocode 5 (EN 1995-1-1), the primary design code for timber structures in Europe. Instead of testing each batch or type, designers specify a class (e.g., C24) and apply its predefined values, making design faster, more consistent, and harmonized across EU countries.
• It facilitates harmonized trade and CE marking: This standard enables the classification of timber of various types, origins, and producers into common classes, ensuring the free movement of goods in the European market. Timber classified according to this standard (visually or mechanically) and bearing the mandatory CE marking meets the requirements for structural use.
• It supports the efficient and sustainable use of timber: It encourages optimal material selection: higher grades for demanding applications, lower grades for general use. It promotes sustainable forestry by standardizing various species (including hardwoods in updates), reducing waste, and supporting timber as a renewable, low-carbon alternative to steel/concrete. Modern mass timber construction, high-rise buildings, and innovative designs rely on reliable classification.
• Visual and mechanical classification methods: The EN 338 standard supports both visual classification (inspection of knots and defects) and mechanical classification (non-destructive testing for hardness/density), increasing accuracy and efficiency.

Essentially, the EN 338 standard harmonizes practices and promotes sustainability in construction by supporting safe, efficient and innovative wood engineering across Europe.

In short, this standard provides a robust and standardized framework that supports modern timber engineering, balancing performance, safety, and sustainability.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its customers, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services to businesses in accordance with the "EN 338 Structural timber - Strength classes" standard.