Grading, defects and where they come from

A buyer of joinery or appearance timber opens a pack and judges the faces. Grading is the written, repeatable way of making that judgement before the pack leaves the mill. This course covers defects and visual and appearance grading. Strength classes and machine grading belong to the separate strength grading course.

Three kinds of grading that are easily confused

• Strength grading sorts structural timber into strength classes. EN 14081-1 covers strength graded structural timber, graded visually or by machine.
• Appearance grading sorts boards by what the faces may show. EN 1611-1 is listed for spruces, firs, pines and Douglas firs, EN 975-1 for oak and beech, and the Nordic countries use their own commercial rule sets.
• Round timber (log) classification sorts logs before sawing: the EN 1927 series for softwood and the EN 1316 series for hardwood.

These standards are paywalled, so they are described here only as the portal's articles describe them, from catalogue pages and secondary summaries. The portal's interpretation: a board can be top appearance grade and fail structural grading, or the reverse, so the two should never share one contract line.

What a grader looks at

Puuinfo (Finland) lists what visual inspectors check: knot count, location and quality, cracks, warping and distortion, and optionally growth ring thickness. A Polish paper (Krzosek et al., 2019) adds grain deviation, resin and bark pockets, rot, insect tunnels, wane, twist and mechanical damage.

Visual grading is a rule-based decision on what a person can see on the surface. That makes it transparent, and it sets its limit: the inside of a board, and slope of grain on a rough face, are hard to judge by eye.

Where grade-reducing features come from

Most are built into the tree: the mill can only recognise them early, route each log to the right product and avoid making them worse. This course uses three groups:

1. Growth features: knots, grain deviations, pith and juvenile wood, reaction wood, shake, resin pockets.
2. Biological damage: blue stain, mould, decay, discolouration.
3. Process features: wane and diagonal grain (sawing), checks, collapse, case hardening and warp (drying), sticker stain, mechanical damage.

A Lithuanian study (Baltrusaitis and Pranckeviciene, 2003) of about 4000 m3 of visually graded pine and spruce at one sawmill shows how the groups mix. Knots, distortion and fissures decided the grade in 62.4% of cases, and sawing accuracy and drying quality together in 24.1%. That is 62.4 + 24.1 = 86.5% (own calculation), leaving 13.5% that the summary does not attribute. The study used a British structural rule (BS 4978) at one mill, so it is not a general split, but sawing and drying clearly decide a large part of the grade.

The log is graded first

The log standards are paywalled and their limits are not quoted here, but the articles describe how they are used.

• The German framework for round timber (RVR) uses A (excellent, flawless or only insignificant defects), B (few or moderate defects), C (more frequent or stronger defects) and D (usable as stem wood but not meeting A to C).
• Poland uses a similar A to D ladder with quantified limits. In its state forest conifer document, internal and diffuse rot is allowed in no class and end-face blue stain is not allowed in class A.
• The letter schemes look alike but are not equivalent. A contract must name scheme, part and edition.

EN 1316 classes logs for an unknown end use, while EN 975-1 grades boards. A mill buys on one and sells on the other, and no open source gives a conversion formula. Log grade is a prediction of board quality, not a guarantee.

Stem form reaches the board

A harvester-data study from Estonia (Padari and Kangur, 2023) compared assortments with straight undamaged stems. Damaged or deformed stems gave 7 to 28 percent less sawlog volume for Scots pine and 5 to 30 percent less for Norway spruce, depending on site and age. Worked example. If 100 m3 of sawlogs would come from straight stems, damaged stems give about 72 to 93 m3 for pine and 70 to 95 m3 for spruce (own calculation). These are shifts in the assortment, not sawmill recovery.

The log end is the main information point in the yard: eccentric pith, rot, shake, resin and pith position show there.

Key points

• Strength grading, appearance grading and log classification are three different systems; do not mix them in one contract line.
• Visual grading is a rule-based decision on surface features and cannot see inside the board.
• Most grade-reducing features are natural; the mill's job is to recognise them early and not make them worse.
• In one Lithuanian mill study, knots, distortion and fissures decided 62.4% of grades and sawing plus drying 24.1%.
• Log classes (A to D schemes, EN 1927, EN 1316) predict board quality but do not convert to board grades.