A knot is a natural defect in timber where a side branch grew out of the trunk. In woodworking, a knot appears as a round, dark discolouration on the surface of the wood. The term refers specifically to this natural feature in the timber, not to a tied knot.
How does a knot form?
As a tree grows, the lower branches become enclosed by new wood tissue. The base of the branch remains as a hard, dark core within the timber. When the log is sawn into boards, this core appears as a round mark on the surface.
Types of knots
| Type | Description | Effect on strength |
|---|---|---|
| **Live knot (tight knot)** | Firmly intergrown with the surrounding wood | Limited |
| **Dead knot (loose knot)** | Not intergrown, can fall out | Significant (weak spot) |
| **Through knot** | Runs through the full thickness of the board | Depends on size |
| **Decayed knot** | Dark, often black, remnant of a dead branch | Can fall out |
Knots and timber grading
Timber is graded based on the number and size of knots:
- A grade — few to no knots, clean appearance
- B grade — some small knots permitted
- C grade — more knots, suitable for structural use
- Pallet / packaging grade — many knots, ungraded
Structural consequences
- Knots interrupt the wood grain and reduce bending strength
- In load-bearing elements (beams, trusses) large knots are avoided
- For visible work, a knot can be desirable for a rustic effect
Treating knots
- Glue a loose knot — use wood glue and clamp it in place
- Fill a knot hole — remove the loose knot and fill the gap with wood filler or epoxy
- Prevent resin bleed — apply a shellac-based primer over resinous knots before painting
Related terms
- Hardwood
- Softwood
- Spruce
- Timber grading
- Douglas fir
- Structural timber