A knot is a blemish in timber: the point where a side branch grew from the trunk. In woodworking, a knot appears as a round, dark discolouration on the timber surface. The Dutch word “kwast” also means paintbrush, but in construction the term refers to the timber defect.
How does a knot form?
As a tree grows, the lower branches are enclosed by new wood tissue. The base of that 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** | Firmly intergrown with the surrounding timber | Limited |
| **Dead knot** | Not intergrown, may fall out | Significant (weak point) |
| **Through knot** | Runs through the full thickness of the board | Depends on size |
| **Black knot** | Dark, often black, remnant of a dead branch | May fall out |
Knots and timber grading
Timber is graded according to the number and size of knots:
- A grade — few to no knots, clean appearance
- B grade — a few small knots permitted
- C grade — more knots, suitable for structural use
- Scaffold board / packing timber — many knots, ungraded
Structural implications
- Knots interrupt the grain and reduce bending strength
- In structural members (beams, trusses), large knots are avoided
- For decorative work, a knot can be desirable (rustic effect)
Treating a knot
- Glue a loose knot in place — use wood glue and clamp
- Fill a knot — remove the loose knot and fill the hole with wood filler or epoxy
- Prevent resin bleed — apply a shellac primer to resinous knots before painting
Related terms
- Hardwood
- Softwood
- Spruce
- Timber grading
- Douglas fir
- Structural timber