Why natural defects decide log value, and how knots work

Most value lost between the log yard and the packing line is lost to a short list of natural features: knots, reaction wood, spiral grain, pith, shake, wane, rot and stain. None of them is a processing fault. They are built into the tree, so the sawmill can only recognise them early, route each log to the right product, and avoid making them worse.

Knot types

A knot is the embedded base of a branch. In a board it appears as a section through that branch.

• Sound (live, intergrown) knots come from branches that were alive while the stem grew around them. Fibres of stem and branch are joined, so the knot is firmly held.
• Dead (loose, encased) knots come from branches that died while the tree kept growing. The stem wood grows around the stub without joining it, so the knot can loosen and drop out as the board dries.

Graders also classify knots by position in the piece: face knots, margin knots (on the edge of the wide face), margin-and-face knots and arris knots (at the corner). The same knot does very different damage depending on where it sits. The position terms come from an Australian study of structural radiata pine (Grant and co-authors, 1984), so treat the old visual rules in that study as Australian and historic, not as current European practice.

Size: the knot area ratio

Size is often expressed as the knot area ratio (KAR), the share of the cross-section taken by knots. In the Australian radiata study the group mean KAR ranged from 0.11 for arris knots to 0.48 for margin-and-face knots in 35 x 70 mm timber, and bending strength decreased as KAR increased.

Position matters as much as size

A knot removes load-carrying clear wood and bends the grain around itself, which creates local slope of grain and stress concentration. A finite-element study of Norway spruce beams (Saad and Lengyel, 2023, Hungary, Europe) found:

These are model results for particular beam geometries. The two percentages come from separate comparisons and must not be added or chained. Use them as a ranking of mechanisms, not as design values.

Stiffness (MOE) is usually reduced by knots less than strength. In the Australian dataset measured MOE was the main predictor of bending strength, which is the logic of machine grading: stiffness integrates knots, density and grain.

Sawing and drying effects

Knots are denser and harder than the surrounding wood and often resinous, so they dull knives and can tear grain when planing. Dead knots can fall out in the kiln, and checks commonly start at knots.

Worked example: converting a percentage loss into a capacity

Suppose a clear spruce beam of a given section carries 20 kN in a model. With a mid-span knot the model suggests a loss of about 39 percent. Remaining capacity:

capacity = 20 x (1 - 0.39) = 20 x 0.61 = 12.2 kN

The loss is 7.8 kN. The point of the calculation is the habit: always turn "loss of x percent" into "remaining share = 1 - x", never subtract the percentage from the original in the wrong unit. Remember that the real loss for your timber depends on beam size, species and grading rule.

Checklist: reading knots at the yard and the sorting line

• Is the knot sound or dead? Dead knots will loosen on drying, so appearance products should be graded after drying.
• Where does it sit: edge, corner, tension side, or compression side? An edge or tension-side knot in a structural piece is worse than a larger knot near the neutral axis.
• How large is it relative to the cross-section (KAR)?
• Is the product structural (stiffness and strength matter) or appearance (visible knots matter)?

Key points

• Knots, reaction wood, spiral grain, pith, shake, wane, rot and stain cause most natural grade loss; they are features of the tree, not processing faults.
• Sound knots are intergrown; dead knots are encased and can fall out when the board dries.
• Knot damage depends on position as well as size: in a Norway spruce model a mid-span knot cut capacity by about 39 percent and a tension-side knot by about 69 percent (separate comparisons, Europe).
• Knots reduce stiffness less than strength; machine grading by stiffness integrates knots, density and grain.
• The old Australian visual-rule findings (radiata pine, 1984) are not current European rules.