Reading a visual rule: softwood appearance grades

Grading rules look different from country to country, but they work the same way: a list of features, a limit for each feature, and a rule for combining them. This lesson shows how to read one, then looks at the Nordic and European softwood appearance rule sets.

A rule is a table of limits

DIN 4074-1 is the German visual rule for softwood. The portal could read it only through an Austrian school textbook excerpt (HPT, undated) that cites "ÖNORM DIN 4074-1" without an edition year, and the DIN catalogue pages did not confirm the current edition. It grades strength of squared timber, not appearance, but shows the structure of a rule:

Limits tighten step by step with the grade, some features are excluded at every grade (rot, ring shake), and the table does not say how to combine them.

Worked example. A piece has knots over 1/4 of the section (0.25), slope of grain 10%, ring width 5 mm, wane 1/5 and no pith. Against S13 the knot limit (1/5) is exceeded, so it fails S13. Against S10 every listed feature is inside its limit (knots 2/5, slope 12%, ring width 6 mm, wane 1/3), and so is S7. The reading used here is that a piece reaches a grade only when every feature is inside its limit. That is the portal's reading of the table; check how your own rule combines features.

Other national rules

• Nordic INSTA 142: classes T0 to T3, approved as corresponding to EN 338 classes C14, C18, C24 and C30 (Puuinfo). Its knot limits are in millimetres, but the numbers were not found in an open source.

Softwood appearance grades

RISE (Sweden) describes the "Green Book", the "Blue Book" (Nordic Timber), the "Grey Book" (an update of Nordic Timber), EN 1611-1 and the "Black Book" (Commercial Grading of Timber, 2020), and notes that most sawmills base their own sorting on one or more of them. A Swedish Wood publication describes two of them:

• Green Book: six grades, I (highest) to VI. Grades I to IV are usually sold together as "U/S, unsorted", V is "fifths" and VI "sixths".
• Blue Book (Nordic Timber): grades A, B and C, with A subdivided A1 to A4. A is for joinery and visible boarding, B commonly for building, C for uses such as packing.

The Swedish handbook Grading of Sawn Timber (2020) lists seven classes, I to VII. EN 1611-1 defines appearance grades for European spruces, firs, pines and Douglas firs, dry and green (EVS listing), but the grades are in the paid text, which was not read. The consequence is practical: a contract must name the rule set and the edition, because "grade A" or "U/S" means different things in different books. The portal found no open source that converts between them.

Dimensions and measuring

Swedish Wood cites EN 1313-1 for tolerances: +3/-1 mm up to 100 mm in thickness and width, and +4/-2 mm above 100 mm, referenced to 20% moisture content, with average dimensions not below target. Thickness and width are measured at least 150 mm from each end, recording the smallest value.

Worked example. A board ordered as 50 x 150 mm: thickness is 50 mm (not above 100), so the range is 49 to 53 mm; width is 150 mm (above 100), so it is 148 to 154 mm.

When to grade

Dead knots can loosen in the kiln, so the portal's rule of thumb is to grade appearance products after drying. A moisture report under EN 14298 does not assess checks, warp or stain and cannot replace grading.

Keeping graders consistent

Human grading varies: at a Latvian veneer mill, seven line scalers agreed with a control scaler in 79% of shortlog cases, individuals ranging from 69% to 88% (another product, so only an illustration). Reference boards and audits against a senior grader are the portal's suggestions.

Key points

• A visual rule lists features and limits per grade; check how your rule combines them before applying it.
• Limits scale with grade, but some features, such as rot and ring shake in the DIN excerpt, are excluded at every grade.
• Nordic and European softwood appearance rule sets (Green, Blue, Grey and Black Books, EN 1611-1) differ; name the rule set and edition in the contract.
• EN 1313-1 tolerances are +3/-1 mm up to 100 mm and +4/-2 mm above, at 20% MC.
• Grade appearance products after drying and check grader consistency against references.