Product rules: CE marking, strength classes and grading

Product rules ask a different question from legality: does the board perform as declared? For structural timber the answer is a strength class backed by a CE mark.

The standards family

The Commission describes the Construction Products Regulation (CPR) as providing harmonised rules for marketing construction products, with CE marking tied to the declaration of performance, assessment and verification systems, notified bodies and market surveillance.

• EN 14081-1 covers strength-graded structural timber with rectangular cross-sections, graded visually or by machine. Part 2 covers type testing and part 3 factory production control for machine grading.
• EN 338 defines the strength classes (C14 to C50 for softwood).
• EN 1912 lists which national visual grades, species and sources belong to which class.

A guidance note for Ireland (2016) says that most structural timber has an assessment level of 2+: the producer runs and documents a factory production control (FPC) system assessed by a notified body, with regular surveillance, and prepares a declaration of performance (DoP) before applying the CE mark. A generic DoP can be placed on a website and should stay accessible for at least 10 years. The 2+ level was not confirmed from an opened official source; confirm it with your notified body.

CE marking concerns product performance, not the legality of the wood.

The new framework

The European Parliament Observatory summarises Regulation (EU) 2024/3110: in force 7 January 2025, applying from 8 January 2026 with some provisions earlier and others from 8 January 2027, and a 15-year transition to 8 January 2040 for the older framework. Our interpretation: ask your notified body which declaration format applies on the day you ship.

What a class promises

Strength class numbers match the characteristic bending strength. Strength and density are 5th percentile values; stiffness is the mean. Selected softwood classes:

These come from a design-office reproduction of the standard (values for C50 density disagreed between sources); take design values from the current standard.

The 5th percentile means that in a population graded as C24, no more than about one board in twenty is expected to have a bending strength below 24 MPa. It is a statement about the population, not about every board.

Visual and machine grading

In visual grading a trained grader assigns grades from knots, grain deviation, cracks, wane and so on. Nordic INSTA 142 grades T0, T1, T2 and T3 are approved as equivalent to C14, C18, C24 and C30 (Puuinfo; Swedish Wood agrees). In Finland C14 to C30 may be sorted visually or by machine, while C35 to C50 are machine-graded only. Machine grading measures indicating properties such as dynamic modulus of elasticity at line speed.

Different rules give different yields from similar wood. In one compilation of Norway spruce, the share reaching C24 ranged from 27% (INSTA 142) to 61% (BS 4978). In two Polish pine samples (n = 150 and n = 210), visual grading rejected about 40% while machine grading rejected about 1%. These are sample results, not industry averages. Strength depends on origin as well: in the Gradewood data, mean spruce bending strength ranged from 34.8 MPa (Slovakia) to 43.7 MPa (Slovenia). Yields are confidential at industrial level, so use your own grading data.

For dry-graded timber, EN 14081-1 limits mean moisture content to 20% at grading with no individual reading above 24%; characteristic values refer to 12% moisture content.

Checklist before selling a strength class

1. Is the grader trained or approved, and does your FPC system cover this size and species?
2. Does the grading rule (visual standard or machine setting) cover your species, origin and cross-section?
3. Is the class written in the DoP and on the label, with the notified body certificate number where required?
4. Are the CE documents consistent with invoice wording?
5. For GB, has the buyer confirmed what marking they accept?

Worked example

A buyer wants "C24 spruce". You offer visually graded boards under INSTA 142 grade T2. Per the articles, T2 corresponds to C24, so the claim can be consistent, provided your FPC and CE documentation cover that rule. If you offered Polish KS pine, note that Polish KW, KS and KG grades were not included in EN 1912 in the cited papers; the equivalence used in research is a convention, and the 2024 edition was not checked.

Key points

• CE marking proves declared product performance, not legal harvest.
• Class numbers show 5th percentile bending strength; stiffness is a mean.
• INSTA T0-T3 correspond to C14, C18, C24 and C30.
• Visual and machine methods yield different class mixes; yield figures in studies are sample-specific.
• Ask your notified body which declaration format and assessment level apply.