Did you know that wood moves, even after it’s cut down into boards? Working with lumber is somewhat of an art form, as the craftsman must work with the lumber and its movement to create a long-lasting, beautiful piece of furniture. Below we’ll discuss how and why wood moves, and what can be done to minimize it and make the movement work with a furniture piece.
Tree growth
A tree grows by drawing nutrients and water from the soil underneath it. To do this, trees use natural channels within the wood called capillaries. These capillaries are shaped like straws and run vertically throughout the tree.
Water and nutrients flow through the capillaries, to every leaf and branch on the tree. As water evaporates from the leaves, a negative pressure is created within the capillaries, creating a small vacuum. This vacuum allows the tree to draw water upwards from the soil, carrying it to the very top.
Moisture affects on lumber
You may be wondering why I just explained how a tree grows. Well, this knowledge will help understanding why wood moves, even after it’s been cut down. As you learned above, the capillaries are responsible for moving water throughout the tree.
Even after a tree has been milled, these capillaries still do their job of moving moisture. Even if the cells of a tree are dead, these capillaries only rely on air movement to transfer water. And it may not seem like there’s much water in a tree, but trees can be more than 50% water!
Newly milled lumber will begin to dry out as all the moisture in the tree begins to evaporate, by exiting through the capillaries. As it dries, the wood shrinks. If one side of the wood is drying faster than the other, the drying side begins to shrink, while the other side(s) don’t. This imbalance creates tension within the wood and it can begin to move in all sorts of undesirable ways; twisting, cupping, bowing, etc.
In addition to moving due to moisture, wood can also move because of stresses within the wood. The inner most part of a tree trunk is under immense pressure, as the outer layers press inward on it as the tree grows. When milling the tree into lumber, this pressure gets released. The wood can twist and bend in different ways, based on where the pressure is within the wood.
Kiln drying lumber
To reduce the amount of movement caused by moisture, most companies who sell lumber will dry their wood in a kiln before selling it. The kiln works like an oven that heats the wood. The temperature, humidity, and air circulation are carefully controlled. This helps remove moisture and relieve pressure slowly, as to not warp the lumber.
An alternative to kiln-drying lumber is air drying. In fact, you can air dry lumber at home. The biggest downside to air drying lumber is the amount of time it takes. While a kiln can dry an oak board in 30 days, it’s recommended to air dry lumber for one year per inch of board thickness.
It’s also important to properly prepare and store the lumber for air drying. To slowly release the moisture as a kiln does, the ends of the boards must be sealed. There are special sealants, made specifically for this purpose, or you can use latex paint as well. If the ends are not sealed, moisture will evaporate from the ends of the boards much quicker than the center, and they will crack.
If air drying lumber, the boards need to have the same amount of airflow on all sides. To do this, small boards (usually an inch thick) called stickers are placed between the stacked boards. The empty space between boards helps air move all around the boards as the moisture evaporates.
Direction of wood movement
Believe it or not, wood moves different amounts throughout different parts of the board. For instance, a standard 12 inch wide board will move the most along its width, while the thickness and length will move the least. When designing a piece of furniture, it’s important to account for these different rates of change and how they’ll exert pressure on surrounding joints.
Lumber also moves when it is placed in different environments. When a piece goes to a more humid climate, the wood swells. This swelling can cause moving parts to no longer fit or work correctly. The opposite holds true for drier climates. Wood will shrink in a drier climate, due to moisture loss. This can also lead to inoperable moving parts and gaps.
Ways to reduce wood movement
The best way to reduce the movement of lumber is to have it kiln dried. It’s also best to not use wood that was close to the center of the tree trunk, as there will be more internal pressure here, as mentioned above.
It’s never a good idea to try to restrain wood from moving. The forces exerted are much larger than would be expected, and the wood will eventually move the way it wants. This sudden release of force will more than likely damage the piece of furniture.
Designing for wood movement
Despite all the different ways that wood continually moves, it is possible to design a piece of furniture in a way that the movement and structure work together in harmony. When constructing a wide tabletop from many panels, it’s important to orient the grain on one panel opposite of the grain on the adjacent panel. This helps any movement balance out or offset each other.
It’s also important to not restrain the tabletop panels from expanding across their width. To do this, breadboard ends must be used. This style of joinery connects all the panels to the end piece, which is oriented perpendicular to the panels. Pins are made from wood dowels, and these are inserted into slots in the end piece, which allow the panels to expand and contract without breaking.
Attaching a finished tabletop to the base must also allow for movement. One of the most common ways to do this is to attach the top with z-clips or figure 8 clips. These types of clips hold the top firmly against the base but can swivel as the wood expand and contracts.
As you can see, wood moves a lot more than you probably realized. This movement creates challenges when designing a piece of furniture. If you have questions about what you’ve read or if you’d like us to take the guesswork out of your project, send us a message today and we would love to design and build a custom piece of furniture just for you!