Knot (lumber defect)
Also called: sound knot, unsound knot, knot hole
A knot is the portion of a branch embedded in a board. Knots weaken wood by disrupting the grain, so pallet lumber grades limit their size and number, with stricter limits for unsound (loose or decayed) knots.
In depth: Pallet lumber grades: what Standard, Utility and Economy mean
Knots are the most common natural defect in pallet stock. A sound knot is solid and firmly grown into the surrounding wood. An unsound knot is loose, decayed or has fallen out, leaving a hole.
Limits in the Uniform Standard (2012), Table 1
Limits are stated as a fraction of the piece's cross-section.
| Defect | Reusable (about PDS Standard) | Single-use (about PDS Economy) |
|---|---|---|
| Sound knots | Up to 1/2 of the cross-section; no more than 2 maximum-size knots per 6 in | Up to 7/8; 1 maximum-size knot per half length |
| Unsound knots and holes | Up to 1/4 | Up to 1/3 |
Why it matters
Grain bends around a knot, so a board with a large knot is weaker in bending, and it tends to break at the knot. On a stringer, a knot near the bottom edge or next to a notch matters most, because that is where bending stress is highest. Knots also deflect nails and cause splitting at joints. A lower lumber grade allows larger knots, which is one reason the same design is rated lower in a lower grade.
On a spec sheet
Knots are controlled by grade. PDS spec sheets state it as part of the lumber mix, for example "Standard &BTR".
Related
- GlossaryLumber grade (pallet lumber)A lumber grade is a quality class that limits defects such as knots, wane, splits and decay in pallet parts. PDS uses Select, Premium, Standard, Utility and Economy, often as '&BTR' mixes such as Standard & Better.
- GlossaryWane (lumber defect)Wane is bark, or the absence of wood, on the edge or corner of a sawn piece, where the cut reached the outside of the log. Pallet lumber grades limit how wide and deep it can be.
- GlossarySplit (lumber defect)A split is a lengthwise separation of wood fibers that runs through a board, usually from an end or a nail. Pallet lumber grades limit split length, and splits at joints are a main cause of pallet repair.
- GuidesPallet lumber grades: what Standard, Utility and Economy meanPallet lumber is graded by the defects each part may contain: knots, wane, splits, decay and slope of grain. The Uniform Standard for Wood Pallets sets minimum quality for reusable and single-use pallets, and PDS names grades from Select to Economy. Specify a minimum grade per component.
- StandardsUniform Standard for Wood Pallets (Woodpack Global, formerly NWPCA)The Uniform Standard for Wood Pallets is the North American industry standard for specifying, building and repairing wood pallets: terms, lumber quality, tolerances, fasteners, notches, repair and performance. Woodpack Global (formerly NWPCA) publishes it; it is voluntary and was revised in 2025.
More in Materials
- GlossaryBark (on pallet lumber)Bark is the outer layer of a tree trunk. ISPM 15 requires wood packaging to be made from debarked wood, allowing only small remnants: pieces under 3 cm wide of any length, or wider pieces with a surface area under 50 cm².
- GlossaryCant (pallet lumber)A cant is a log that has been sawn flat on two or four sides. In the pallet industry it usually means the low-grade center of a hardwood log, left after grade lumber is cut from the outside, which is resawn into pallet parts.
- GlossaryComposite block (pallet)A composite block is a pallet block made from wood chips, particles or fibers bonded with resin under heat and pressure. It does not split like solid wood and, as processed wood, is exempt from ISPM 15 on its own.
- GlossaryCorrugated fiberboard (pallets and slip sheets)Corrugated fiberboard is a board made of a fluted paper medium glued between flat paper liners. It is used for boxes, and in pallet work for corrugated paper pallets, slip sheets, tier sheets and pallet runners.
- GlossaryCut stock (pallet components)Cut stock is lumber already sawn to the finished thickness, width and length of pallet parts, such as 5/8 × 3-1/2 × 40 in deckboards, ready for nailing. It is also called pallet stock or shook.
- GlossaryDouglas-fir (pallet lumber)Douglas-fir is a dense, strong softwood from western North America, also grown in Europe. It is species class 11 (Douglas-fir–larch) in the Uniform Standard and PDS, and class 33 in the European species list.
Last reviewed 2026-09-24. 219 words.