Glossary
Knot
Also called: sound knot, unsound knot, knot hole
Reviewed
A knot is the remains of a branch embedded in a board, visible as a round or oval area of cross grain. The Uniform Standard limits sound knots to one half of the cross-section on reusable pallets and seven eighths on single-use pallets, with tighter limits for unsound knots and holes.
Knot limits in the Uniform Standard (2012, Table 1)
| Defect | Reusable | Single-use |
|---|---|---|
| Sound knots | up to 1/2 of the cross-section; at most 2 maximum-size knots per 6 in | up to 7/8; 1 per half length |
| Unsound knots and holes | up to 1/4 | up to 1/3 |
A sound knot is solid and tight in the board. An unsound knot is loose, decayed or has fallen out, leaving a hole.
Why knots matter
Grain bends around a knot, so a knot reduces bending strength, especially in the tension (bottom) face of a board or stringer. A large knot at a nail line also leaves less sound wood to hold the nail. For stringers, knots near the notch are the most critical, since the notch already reduces the section.
Grades and knots
The PDS component grades (Select, Premium, Standard, Utility, Economy) and their "&BTR" mixes are defined largely by limits on knots and other defects. The reusable column above roughly matches PDS Standard grade. See lumber grades.
Questions
- Do knots weaken pallets?
- Yes, they reduce bending strength, which is why the Uniform Standard limits their size and number by use class.
Sources
- Uniform Standard for Wood Pallets (NWPCA, now Woodpack Global), 2012 edition