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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.
Pros
- Attractive and creates beauty
Cons
- 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.
Science
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.
Free Download – 6 Reasons Your Wood Project Failed
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 December 13th, 2024
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I have just had a decking done and there seems to be a lot of knots some which have fallen out already. Should I be worried as it was only put down in April?
You see this when wood begins to dry out, and the knots shrink and fall out. Regarding your statement about…’a lot of knots’, that is obviously a function of the type (and maybe grade) of wood used.
Hi Ron I am building a large patio cover in southern ca that will be attached to my home, and I will be using Douglas fur, and most will be 6x and 4x lumber. I have been trying to decide between 3 types of lumber, and I would really appreciate the advice of someone so well versed in wood.
Type 1 is #1 KDAT FOHC S4S Not incised,
Type 2 is #1 kiln dried Doug fur select for appearance, S4s Exposed non treated
Type 3 is Select Structural GDF Select for appearance, FOHC Exposed
I want to build something that will not rot, and not worry about termites, but also want something that looks really high quality. I want to minimize the risks of cheeks, and other problems associated with moisture in wood as well. Any advice would be highly appreciated
Art, I am not an expert on treated lumber or rot resistance, but I would choose the most important aspect needed (structural integrity or rot resistance), and pick the best product; maybe a mix, example: the posts might have more of a structural need than the any roof member that will not take a lot of weight.
Hope that helps.
Just bought a large shed and it’s been put up this morning. It has loads of knots in the wood. See pics.
I’m wondering if I should not accept it as what good us a shed that going to leak when the knots drop out.
Too many to patch up.
What shall I do. It wasn’t a cheap shed. Three grand including base.
Some pics. Would appreciate your advice.please.
Paul
Hi Ron,
I have a large maple tree relatively close to the house with lots of knots beginning at the base and at least 7’ up. Does this compromise the strength of the tree? Thanks
I recently bought a log home built in 2014. I have noticed that the knots in the unfinished beams have a “whiskery” look. Could this be the knot still trying to grow? Or could it be a mold?
Thanks!
Barb, can you please send a photo. You can email me.
Does knots or branches can be used in plywood how to avoid them in plywood how to deal with them ?
Remember that wood gains moisture from its surroundings, so maybe your wood knots are growing because your environment is humid. If you have a child that spill water a lot or it rains a lot in your area then maybe that is why your wood knots are ‘growing’ because they are expanding. I think a simple fix is to make sure you fill the knots with knotting.
Hi Ron,
Can you somehow download photos Vs just text? This will be the easiest way for me to ask the question I have regarding what I’ve noticed about my my roof attic trusses and am curious to learn if what I’m seeing is just a knot or something that poses concern. I feel it is definitely the latter but need expert advise to be sure.
Knots are seldom due to “imperfections” in the wood — they are due to the branching of the tree.
You’re right, of course! At the same time, they’re typically considered one type of “imperfection” in the finished lumber. Thanks for the comment.
Ron,
As a woodworker, many projects that I make force me to cut around knots in boards because customers see solid knots as defects. I throw out these scraps. Some people feel that knots add character to wood projects. Loose knots, well they aren’t welcome in any projects. They get tossed.
My desire is show that useless loose knots can actually impact our climate through unique program . I ask you and your audience to help me seek out loose knots, big or small, half or whole to support a environmental sustainability program. Please provide me with any information you may have on gathering these defects, these flaws, these potential castaways that will provide a significant benefit to our environment.
Awesome wood knots! This wood could be used to make different kinds of arts and displays. I hope I can reach you out so you can discuss everything about woods.
Such an informative article, thank you for putting this topic. I have some questions related to this subject I hope I can reach you somehow, it would be a great help.
It’s great that wood knots made beautiful wood displays. My brother wants to use some wood in a project that he’s working on. I’ll share this information with him so that he can look into his options for getting lumber with knots.
Thank-you Ron. That was about the only explanation that i could come up with as well. i appreciate the reply!
Hi Ron,
I am a structural engineer with a fair bit of experience in timber design & evaluation. I was recently asked to consult on a matter regarding a friends’s a new (2 year old) CEDAR log home built in Maine, where the exposed knots on the exterior wall are rotting out.
My friend sent me pictures and he is able to easily dig-out with screwdriver a significant number of the knots in the logs, 1″ to 2″ deep, and in some cases, and is having to chasing the rot laterally (along the grain) by 2″-3″ under the surface of the log.
There seems to be no discernible pattern to the rotted knots in terms of location, and not all, but A LOT, are rotted. As many as 100 on a 24′ x 20′ high exterior wall.
Soon after the house was finished, the logs were stained and then weather-sealed, but again the house is only 2 years old.
In my 20+ years of building evaluation, I’ve never seen this.
Any thoughts on the cause and possible remedy?
Thank-you.
Tim, as you know, cedar is typically not prone to do this, that is why they use cedar. Very puzzling to me, although I don’t have a ton of experience with rot in cedar. I could speculate that the moisture content may have been too high when it was stained and weather-sealed, but just speculation.
Hi
I am looking to but a cherry wood salad bowl. I found #2 bowls for $40 less than #1bowls. Is it safe to eat from a bowl with a knot or a hole or other defect. Will bacteria get in there? I appreciate your help
Terri
Hi Terri,
Sorry, we’re not able to give you any information on the sanitary nature of a bowl with knots. This is not our area of expertise.
I ought a heap shed a few month ago it was a complete waste of money I had an expensive shed built today which I was assured would be good quality it is amass of knots the chap building it assured me that it was known as live knots and they would not fallout ever is this a sales pitch or what iam in my mid eighties and I feel vulnerable when purchasing large items the firm I ordered from outsourced the order after assuring me it would be atop quality job signed Mary
Hi Ron, we are getting a new deck. We asked for high quality pine lumber. I noticed the lumber yesterday after the contractor left, and it had a lot of knots in the wood. Is high quality wood full of knots? He is finishing up the framing today.
‘High quality’ is a subjective term. In the world of lumber, we use the term ‘grade’, and the grading of lumber is very ‘objective’ rather than ‘subjective’. Lumber is graded for structural integrity and/or visual characteristics. Example: the higher (more expensive) grades of lumber do not typically have many knots, and even knot size is graded for in the sawmill.
There are certified grading agencies that are routinely in mills checking the lumber.
What species of pine is this, and is this actual pine decking, or just pine lumber?
Feel free to give me a call at (800) 634-9961 ext. 225.
Hi, Ron. Thanks for the useful article. I just had my floors redone. I had chosen a wood type from a sample, and what I have been given is very different in that it has a lot of knots. The thing is, these knots seem to be getting larger and streaking more than when the floors were completed about a month ago. Have you heard of this happening? (Perhaps worth noting is that this is the second time this flooring company did my floors – the first time a few months ago resulted in buckling and cupping that ended up being due to their not letting moisture dry up before putting the wood down on the plywood, so they had to pull everything up and re-do it.)
This seems very unusual, for a knot to grow. Most often, as you read, the knot will shrink with time, and might even fall out of the board in extreme cases. Maybe you are still getting some moisture ingress into the wood.
1. What type of subfloor do you have, and was the subfloor tested for moisture prior to installation?
2. Was the wood flooring itself tested for moisture with an accurate, calibrated moisture meter prior to the installation? If so, what was the average moisture content? What is the relative humidity in the room?
3. Was the flooring acclimated in the room prior to installation, and if so, were most of the faces of the wood flooring exposed to the room’s environment?
how do you measure the size of a knot in timber flooring? Is there a correct way to do this?
For lumber grading purposes, there are procedures for correctly measuring knot size. Where are you located, and please provide more explanation?
Thank you.
Such a useful article.Wood is not an easy material to work with.