The Pros and Cons of Knots in Wood
From the rustic charm of a log cabin to the elegance of fine furniture, wood is a material that exudes warmth and character. One of the unique features of wood is the presence of knots, which add to its distinct appeal but also present certain challenges.
Knots, the remnants of branches that were once part of the living tree, can be viewed as both a boon and a bane in woodworking and construction.
We’ll give you the quick pros and cons then dive into them further.
- Attractive and creates beauty
- Weakens the strength of the wood
- Not ideal to build with
Now more details about knots in wood.
Why Do Trees Have Knots?
Knots are imperfections from branches that cause living wood grain to grow around them. These imperfections are just part of what makes wood a beautiful material with which to work, but can also contribute to possible defects in structural strength for construction lumber. Dead branches drop off healthy, living trees all the time, and wood knots appear in the trunk where branches died.
The Impact of Knots on Wood Strength
Understanding the tensile and compressive strength of wood is integral to gauging its ability to withstand different forces. Tensile strength refers to the maximum amount of tensile (pulling) stress that a material can withstand before failure, such as breaking or permanent deformation.
On the other hand, compressive strength refers to the capacity of a material or structure to withstand loads that tend to reduce size.
The presence of knots in wood can significantly affect these strengths. Due to the interruption of the wood grain around knots, the tensile strength can be compromised, making the wood more susceptible to breakage under tension. This is particularly evident when the forces applied are perpendicular to the grain.
Similarly, the compressive strength can also be reduced due to the uneven grain direction and the higher density of knots compared to the surrounding wood.
Wood Knots in Art
Wood knots make beautiful wood displays. Ironically, it is the very curves and bends in the wood grain that makes knots so attractive to view. The wood grain weaves its way around the knot. If you have a closer look, notice how the grain pattern slopes up to 90 degrees to get around wood knots. Remarkable. This is why many consumers enjoy having decorative wood with knots.
While wood knots carry visual appeal, they adversely affect wood strength.
Wood strength in knots can be assessed in two ways: tensile strength and compressive strength. These measurements allow us to predict how much load (weight) wood can safely bear before collapsing. Tensile stress elongates or expands an object. Compressive stress shortens or compresses the material.
Since knots are defects that weaken lumber, this weakening effect is more serious when the lumber is subjected to forces perpendicular to the grain and/or tension than when under load along the grain and/or compression. Please note that the extent to which wood knots affect the strength of wood depends on their position, size, number, and condition.
Wood Knots in Building
Wood knots weaken wood strength. In fact, knots materially affect cracking (known is the US as “checks”; known in the UK as “shakes”), warping, and the ease of working the lumber. They are defects that lower the lumber’s value for structural purposes where strength is critical. Manufacturers account for the presence of wood knots in lumber grading and knots are classified according to their form, size, soundness, and the firmness with which they are held in place. Frankly, the knot lowers the wood grade.
Encased knots loosen during drying. The dried knot is smaller than the knothole and sometimes falls out during handling or machining as nothing can be done to prevent the loosening of dead knots during drying. Fewer dead knots will fall out if the final moisture content (MC) of the lumber is kept as high as possible before machining.
Consumers can benefit from understanding that wood knots are a perfectly natural indication of wood’s growth process. On the other hand, experienced builders understand the circumstances in which wood knots may have an impact on wood strength in structural framing.
Case Study: Knots in Wood in Practice
Consider a real-world example of a woodworker crafting a dining table. When selecting the lumber, they might opt for wood with visible knots to give the table a distinctive, rustic aesthetic appeal. This choice comes with challenges. The woodworker must be careful when cutting and shaping the sections of wood with knots to avoid cracking or breaking the wood. Extra care must be taken when sanding and finishing these areas to achieve a smooth surface.
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As Sales Manager for Wagner Meters, Ron has more than 35 years of experience with instrumentation and measurement systems in different industries. In previous positions, he has served as Regional Sales Manager, Product and Projects Manager, and Sales Manager for manufacturers involved in measurement instrumentation.
Last updated on June 13th, 2023