Warped, twisted, bent, crooked, cupped……just a few words that we use to describe wood that has distorted from that nice, flat, and straight board we purchased just a couple of weeks ago. Warped wood is one of the most frustrating things to deal with when building anything, be it shelving, tables, benches….ANYTHING. Trying to overcome a bend, twist, or bowed board can often creating more problems. If you’ve ever been a victim of distorted lumber, then continue reading. This article will tell you why it moves, how it moves, and how best to deal with movement – in a practical manner and hopefully clearly illustrated & good pictures. I will give you some great tips/list on how to purchase cheap lumber at the big box store that won’t warp as much as most boards people buy.
Why does wood move and warp?
Wood is a ‘breathing’ thing. It is made up of fibers that are basically shaped like straws that run the length of the grain. These fibers suck up and expel moisture all the time when exposed to air. Think of it like a collection straws that behave like a sponge. When dry, the ‘sponge’ is small and compact. When it contains moisture, it will be larger. But, it doesn’t move the same amount in each direction.
In fact, the dimensions in wood can change small amounts daily with the difference in humidity. But as the seasons change the US Forestry service has shown that the dimensional changes can be substantial. Below I will show you the key factors to consider when buying wood to ensure that any movement is either minimized, or is at least more uniform.
How wood moves by grain direction – an Illustrated Visual Guide
Wood will expand and contract in three directions;
- Longitudinal, or along the length of the grain
- Radially, or across the growth rings
- Tangentially, or with the growth rings
These concepts are illustrated in the image below.
The change in longitudinal dimension is very small, less than 1% of the overall length. While the change radially is more, but not too significant. But, the change tangentially in the growth ring lengths is the most pronounced. More than double the radial change, per the USDA Forestry Service.
If the growth rings were uniform thickness, and perpendicular to the width of a board, you would still have movement. However, the movement would be uniform in the thickness of the board, which would be the most forgiving in any project. But, you are unlikely to encounter that in any practical situation, as trees grow different rates each year. Also most boards are never perfectly cut in this manner.
Why does it bow, cup, or twist?
Wood will expand and contract based on the growth rings. And the growth rings are not uniform, as they are larger/smaller depending on how much the tree grew each year. But, they all will hold moisture.
So, the growth rings that are the longest on the end grain will contain the most amount of wood fibers. These will expand (or contract) more than the growth rings at the top corners or bottom-middle of the end grain. This is what can cause a significant amount of bending. This is the largest reason for cupping.
Bowing / Crooking
Lumber will bow when there are uneven growth rings on the sides of the boards. So, if the trunk is not perfectly straight (aka – almost every piece of lumber you’ve ever seen). Now, the degree of ‘bow’ will depend on how bent, or what angle the board was sawn. But, if there are fewer growth rings, or they are of different thicknesses on one side, you will likely receive a bow or crook.
A board that is twisted is one of the most frustrating problems to deal with. This is when two opposite corners that are diagonal from each other are raised, while the other two are firmly on the table. Almost every board that exhibits this situation has the center growth ring in the end grain (or close to it) or close to it in my experience. Aka it was the center of the trunk or limb. The end result is kind of like some giant came and manhandled the board as you would ring out a rag (OK, not that bad – but you get the idea).
Some projects you can live with a good deal of warped lumber. But if you are doing rustic sign that is framed, or a framed shiplap the twisted boards can ruin the whole project. Twisted slats or frame would prevent the piece sitting flush against the wall.
Other confounding factors contributing to warp
One further reason that I often notice in lumber is when there is a significant knot on a board, you can often notice a significant warp or change of direction, where the knot is located. So you can have a perfectly straight board that all of a sudden veers off in one direction, but both sections will be straight and not curved. You just need to find this when buying lumber, and discard boards that have really large knots.
It all starts with the lumber yard. How a tree is sawed into lumber will determine what kinds of distortions can happen, as well as how the tree had grown. But in general, there are three ways a tree is sawed into boards.
1 – Flat sawn (most common and least stable, nearly all construction grade lumber, cheap)
2 – Quarter Sawn (premium lumber, pretty stable, hardwood section at big box stores, expensive)
3 – Rift sawn (least common, most stable, most waste when sawing, which means highest cost)
Flat Sawn Lumber
Think of this when a log is laid flat and the entire tree is cut up into even slices. The slices nearest the top/bottom are the narrowest boards, while the middle of the cross section has the widest boards. This method results in noticeable curves in the end grain, which represents the different growth rings. Each of these growth rings have a different profile, density of fibers. Each growth ring will expand/contract with moisture at different rates, which will result in distortions in your board over time.
The bigger the difference in growth ring profile, the more type of lumber will move in strange ways. It is thus the most difficult to deal with, but is also the cheapest to buy (more on that later).
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Quarter Sawn Lumber
Quarter Sawn Lumber divides everything up into different sections, trying to cut boards perpendicular to growth rings. It does a good job accomplishing this, but near each ‘edge’ of the section there can be some curvature of the growth rings within the end grain. But, lumber sawn in this manner will have much greater stability compared with flat sawn lumber described above. Boards that are quarter sawn will still move, but much more uniformly than flat sawn.
However, this method does produce some waste, as the entire tree isn’t utilized. Therefore the price of the lumber will increase. This is often only available at sawmills and specialty hardwood stores. That is, unless you do it yourself.
Rift Sawn Lumber
You are unlikely to ever come across this type of method unless you commission it yourself, or saw it yourself. But it divides the tree up into pie slices, and generates much waste. However, boards that are rift sawn have the greatest stability, as the growth rings are nearly all perpendicular to the width of the board. This is by far the most ‘stable’ lumber, in that it will expand/contract uniformly. So the lumber will still move, but it will do so equally in all directions (or mostly equal). It will therefore have the lowest risk of distortions, and will be very nice to work with.
But – rift sawn lumber will also be the most costly as much of the tree cannot be used as lumber. As you can see below, you will get the fewest boards per log, hence the expensive price. That doesn’t mean you can’t get rift sawn lumber at flat sawn prices….you just have to be picky (next section).
How to buy cheap lumber that won’t warp….much
So, I buy most of my general lumber from the big box stores. If a project is going to get stained strange colors, or get painted – I will purchase my lumber cheaply. But I do it in a smart way, that can be time consuming. Certain steps I won’t go into detail on, or skip – aka big knots, damaged boards. This only concerns selecting boards that should move in a mostly uniform manner.
By following this simple list as best as I can, I have bought multiple boards for various projects that were straight but didn’t get around to using them soon. And two months later…….they were still straight! They only had minimal cupping or other deformities. So this process has been working very well for me, and saves me a lot of time. By doing these steps when buying lumber, I successfully discard many boards quickly, without even sighting down the length of them.
Steps to buy straight lumber
- I find the type I’m interested in, and begin to feel it. If the wood feels moist to the touch, I will leave the store. Even though it is ‘kiln’ dried, if I think that the lumber has regained significant moisture content, then I know that big changes could be coming in the dimensions. Leave, go somewhere else, or come back in a week. Sighting a wet board for straightness is a fools errand when dealing with cheap and possibly knotty wood.
- I will sort through dozens of boards until I find what I want. And what I want is in the end grain – I will get on one side of the pile, and look at board after board. When I see end grain that is mostly perpendicular to the width, or parallel to the width – I then pull that board out.
- I then check the opposite end grain. If the opposite end grain looks similar to the other end, I will then pull that board. If the end grain on each end of the board look similar, then I can be sure that this board is fairly uniform end-to-end. So, most movement should be somewhat more uniform.
- Now, I will sight the length of the board. I’m mainly looking for significant bowing, or twisting. If the board looks mostly straight – I buy it.
What to be wary of when buying lumber
- Cupping is not as large of a concern, as you can generally deal with that quite easily with a hand plane. As long as it is less than 1/8″, I’m not worried about it.
- Bowing – when an 8′ board has a 2″ bow – that is a problem. If I need all 8′ of length, then I know that to join it with a plane will reduce the overall width substantially. So I will pass on that board and let someone else buy it.
- Twist – If a board has 1/4″-1/2″” twist or more, where one corner sits up that amount from the corner it is next to – that is a red flag. If you need this board to be straight, you will be fighting it forever. Stay away.
- If the board has the ‘pith’ or heart/center of the log/limb. Any board that you can see a full circle growth ring – don’t buy it.
- If the boards don’t feel dry – or any dampness – don’t buy them. They are going to change substantially.
- Big knots throughout the board. Large knots mean significant limbs that were offshoot from the main board. This can cause strange grain patterns around the knots, which can lead to bowing or twisting. This is a judgement call, as there is no ‘right’ answer if buying cheap lumber.
- Another note about knots – they often won’t move with the rest of the board. So you might wind up with the knot sinking into the board, or rising slightly above it. There are ways to handle this, but you should be aware of this process.
How to keep your boards straight after purchase
Sometimes the best defense is a good offense, so the saying goes. And it is no different in woodworking. When you get the boards home, if you are pleased with how straight they are, the best thing you can do is to use them right away, and apply a finish to them to prevent moisture from entering (as long as you think it is dry).
Alternatively, purchasing a moisture meter can help ensure this, as you can check the moisture content of newly bought lumber against some that you already have in your shop. If there is a big difference, you may want to let the new lumber acclimate for a while, then join it using a hand plane or joiner before use. I do this on most lumber I buy, as there is generally some light cupping on all flat sawn boards.
Again, then apply finish to them – whether it is latex paint, polyurethane, wax, linseed oil after using them in your project. Just make it difficult for moisture to enter. So, make sure you coat the whole board at least once, and the end grain multiple times if you can do so. This is because most of the moisture will enter/exit the board from the end grain, as that is where the end of the wood fiber straws are.
So that’s it! This is a comprehensive, but practical overview how lumber will move. I hope you liked my handy tips on buying cheap lumber from a big box store, and maybe learned a couple of things. Check out our other DIY articles for some cool, unique projects you probably won’t see anywhere else. If you want to stay up to date on large updates to our site, sign up for our email list – we promise not to spam you!
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