What Knots Are and Why They Form
Knots are the cross-sections of branches enclosed within the growing trunk. When a tree produces a branch, subsequent annual rings of the trunk enclose the branch base. The trunk wood fibres divert around and through the branch base, creating the cross-grain pattern visible as a knot in sawn timber.
Knots are a natural feature of how trees grow, not a manufacturing defect. But their presence introduces localised grain deviation, stress concentration, and potential discontinuities in the wood structure that directly affect mechanical performance.
Types of Knots
Tight knots (sound or intergrown knots) are firmly attached to the surrounding wood throughout their perimeter. They formed in a living branch and are integral to the wood structure — they cannot be removed without leaving a hole. Dead knots (encased or black knots) form when a branch dies before being enclosed by trunk growth. Because the dead branch is not actively bonded to surrounding tissue, it may be held only loosely and can fall out during drying — leaving a knot hole, which is structurally more serious.
Spike knots appear when a board is cut along the length of a branch rather than across it, producing an elongated oval exposure. They affect a greater area over a longer board length than a round knot of equivalent diameter.
- Tight knot: firmly attached, formed in living branch — graded by size and location
- Dead knot: loosely held, may fall out — treated as a knot hole in most grading rules
- Knot hole: missing knot — more serious than an equivalent tight knot
- Spike knot: elongated from long-axis sawing — larger affected area
How Knots Affect Structural Performance
The structural impact of a knot depends on its size relative to the board cross-section, its location (edge, face, or centre), whether it is tight or loose, and the type of stress being applied. In a bending member, a knot on the tension face (lower edge of a downward-loaded beam) is more serious than one on the compression face. Tension fibres carry full bending stress in direct tension — any interruption creates a stress concentration that can initiate fracture. Compression fibres are less sensitive to localised defects.
A knot in the middle third of the span — where bending moment is highest — is more serious than the same knot near the supports. Structural grading rules apply different limits to the critical middle third and the end thirds of the piece.
Knot Limits in Structural Grading
Structural grading expresses knot limits as a knot area ratio (KAR) — knot diameter divided by board width. A 40mm knot in a 150mm board gives KAR 0.27; the same knot in a 75mm board gives KAR 0.53. Higher structural grades permit lower KARs. European C24 grade limits KAR to 1/4 (0.25) in the critical zone. This is why knot limits are always expressed relative to board width, not as an absolute diameter.
For appearance-grade furniture and joinery, knot size, type, and frequency determine the yield of clear cuttings available from each board. Lower appearance grades are often adequate for furniture production — the yield of clear cuttings from a No. 1 Common board may be nearly as high as from a top grade, at significantly lower cost.