Knots and grain deviations

Knots and grain deviations decide more grades than any other feature. This lesson separates what a knot is, why position matters as much as size, and why grain is the feature the eye judges worst.

Sound and dead knots

A knot is the embedded base of a branch. In a board it appears as a section through that branch, and its shape depends on how the saw cut meets the branch axis.

• Sound (live, intergrown) knots come from branches that were alive while the stem grew around them. 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 dead stub, often with bark or resin between, so the knot can loosen and drop out as the board dries.

In general forestry knowledge, which the portal's article flags as not checked in its cited pages, dead knots concentrate in the outer part of the log and sound knots nearer the pith. Checks commonly start at knots in drying.

Swedish Wood adds that spruce often has small pin knots between the branch whorls, while pine tends to oval knots and lacks pin knots.

Position matters as much as size

Grant, Anton and Lind (1984) studied structural radiata pine from New South Wales, Australia. They separated face knots, margin knots (on the edge of the wide face), margin-and-face knots and arris knots (at the corner). Size was expressed as the knot area ratio (KAR), the share of the cross-section taken by knots. Group mean KAR ranged from 0.11 for arris knots to 0.48 for margin-and-face knots in 35 x 70 mm timber. The study found that the visual rules of the time graded margin and margin-and-face knots two grades too high and face knots one grade too high. That is about 40 years old and Australian, not a description of current European rules, but it shows why rules look at where a knot sits.

A finite-element study of Norway spruce beams (Saad and Lengyel, 2023, Hungary) ranks the mechanisms. A knot at mid-span cut load-bearing capacity by about 39 percent, and in a separate comparison the largest loss, about 69 percent, occurred with the knot at the lowermost, tension-side position. Knots in the compression zone had only a marginal effect. Worked example. A beam that carries 100 units without the knot would carry about 61 with the mid-span knot (100 - 39) and about 31 with the knot in the lowest position (100 - 69). These are model results, useful for ranking, not design values.

For appearance products size, count, soundness and position decide the grade.

How rules express knots

Rules describe knots in different ways, so figures cannot be compared across rules. DIN 4074-1 (lesson 5) uses ratios of the cross-section: 3/5, 2/5 and 1/5 for S7, S10 and S13. Stapel notes that INSTA 142 is the only standard in his comparison with absolute knot values in millimetres. The UK supplier summary of EN 975-1 oak grades (lesson 6) disregards small sound knots and allows a stated number up to a stated size. A contract that says only "knots up to 20 mm" leaves open how knots are measured, on which face and where.

Grain deviations

Cross grain means fibres not parallel to the board axis. It is diagonal when a tapered or crooked log was sawn parallel to the bark rather than to the pith (a sawing issue), or spiral when fibres wind around the stem (a growth feature). DIN 4074-1 limits slope of grain to 16%, 12% and 7% for S7, S10 and S13.

Spiral grain matters mainly because it causes twist on drying. In Swedish progeny trials of Norway spruce (28 or 29 years old, measured near breast height), Hallingback, Jansson and Hannrup (2008) found a mean grain angle under bark of 1.76 degrees, but about 7 percent of progeny trees had 4 degrees or more. Worked example. In 1,000 trees like those, about 70 would be in the 4 degree group (own calculation). The average looks harmless while the tail does the damage. Grain angle peaked near ring 10 at about 3 degrees, so spiral grain is not simply strongest at the pith.

Practical rules

These are working rules from the mechanisms, not published thresholds:

• Judge knot position as seriously as size.
• Expect dead knots to loosen in the kiln; for appearance products, grade after drying.
• Do not trust the eye on spiral grain. Run twist trials on samples from each supplier region.

Key points

• Sound knots are held in the wood; dead knots can loosen and fall out as the board dries.
• Position changes the damage: in one spruce model a mid-span knot cut capacity by about 39% and a lowermost tension-side knot by about 69%.
• Rules express knots as ratios, millimetres or counts; figures from different rules are not comparable.
• Spiral grain in spruce is small on average (1.76 degrees in one Swedish dataset) but about 7% of trees had 4 degrees or more.
• Grade appearance products after drying, because dead knots can drop out.