Drying The Lumber – Why is it necessary?

Wood is used for various purposes around the world. When we mill a tree into lumber, we have to reduce the moisture content in the wood before turning our boards into furniture. Whether the wood is used to build a small side table or to frame out a house, it must be dried before it can be used. Let’s discuss some of the reasons why it is necessary to dry the lumber before use.

What is Wood Moisture Content (MC)?

Trees contain a lot of water. Water is an essential element of the photosynthetic process and is used to transport nutrients and minerals throughout the tree in what we call the sap. The amount of water contained in a piece of wood is called moisture content (MC). In wood, the moisture content can be calculated by dividing the weight of the water present in the wood by the dry weight of the wood. MC is expressed in percentage. For instance, if we have a piece of wood that weighs 10 pounds with the water included, and it weighs 6 pounds without the water, that means there are 4 pounds of water in the piece of wood. When we divide the weight of the water by the weight of the dry wood, we get a moisture content of 66.6%.

What is Moisture Content Range?

The moisture content of freshly sawn wood can vary from 30% to over 100%. How can it be possible for a piece of wood to have a moisture content over 100%? When the moisture content is over 100%, it simply means the weight of the water in the wood weighs more than the weight of the dry wood, itself. MC varies from species to species and it also varies between trees of the same species. And even in boards sawn from the same tree.

What is Wood Equilibrium Moisture Content (EMC)?

Wood gains or loses moisture until it is in balance with the environment in which it is placed. When the wood is no longer losing or gaining moisture, it’s in equilibrium with its surroundings. This phenomenon is called equilibrium moisture content (EMC). The EMC is based on the relative humidity of the air and the temperature. For example, if the temperature is 70 degrees F and the relative humidity is 66%, the equilibrium moisture content is 12%. This means, the wood if given enough time, will eventually achieve a moisture content of 12%. If the MC of the wood is greater than 12%, it will begin to lose moisture. If its MC is lower than 12%, it will begin to gain moisture. The gaining or losing of moisture will continue until the wood reaches 12% moisture content, assuming the temperature and the relative humidity do not change before it gets there.

Wood Shrinkage & Warping

Wood begins to shrink in thickness and width when the moisture content starts to get below 30%. The extent of shrinkage varies by species, but it could range from 5-10% in width and 3-6% in thickness depending on species, and how the wood was sawn from the log. Damage to the joints can occur, and the product might fail if it is made before the shrinkage can take place. If the shrinkage is not excessive, the wood may warp even though the joints may not break. When lumber is dried, the shrinkage has already occurred, and as long as the wood is kept in conditions that will maintain a close equilibrium moisture content to the moisture content the lumber was dried to, there should be no issues due to shrinkage.

Wood can Gain Moisture Even After Proper Drying

Wood intended to be used inside of a home is usually dried to a moisture content of around 7% because that is typical of conditions found inside of a home. Wood that is used for construction lumber is usually dried to around 15% moisture content. However, it must be kept in mind that wood will also gain moisture, and when it does, it begins to swell. So if lumber is dried to 7% moisture content, then left outside in an uncontrolled environment, it will gain moisture until it reaches an equilibrium moisture content of the outside air. In North Carolina, the equilibrium moisture content in the outside air can range from around 10 – 15%, depending on the time of the year and the weather conditions. This means that the lumber that was properly dried can end up with a moisture content too high to use for products inside of a home.

Prevents Fungal Attack

Another reason to dry lumber is to prevent fungal attacks, which can discolor the wood or even cause it to rot. The fungus needs to have moisture in order to survive; thus, eliminating the moisture will kill any fungus that may already be present and will prevent fungal attacks in the future as long as the wood remains dry.

Kills Insects and Their Eggs

Properly drying the wood also kills any insects and eggs that may be present in the wood. Most insects require moisture content above 10% in order to survive in the wood. However, some termites and some powder post beetles are an exception to this. As long as the temperature of the wood gets over 135 degrees Fahrenheit during the drying process, any insects that are in the wood and the eggs that may be present will be killed. Air drying the wood will not kill insects or the eggs found in the wood because the temperatures are not high enough.

Paints and Other Finishes Problems

Problems with paint and finishes can also occur with moisture contents that are too high. Typically, moisture contents lower than 13% should minimize problems with paints and other finishes. Blistering and peeling may occur if the moisture content is too high.

Reduces Weight & Improves Strength of the Wood

There are several other reasons lumber needs to be dried; for instance, lumber that is to receive a preservative treatment must be dried. Drying also reduces the weight of the wood, and more can be loaded onto trucks to be shipped at a lower cost per board foot. Drying also improves the strength of the wood. Whatever the reasons are for drying the wood, we are there to get your lumber dried as we do everything from sawmilling to kiln drying and furniture.

Why Choose Wild Edge Woodcraft for Wood Drying Services?

The technique for drying wood is both an art and a science. It requires drying our material fast enough so it won’t mold or rot, yet not so fast that it stresses the wood, which results in checking, splitting, and all sorts of other undesirable defects. Drying or seasoning wood properly is the key to using it for furniture. The drying process takes place through air drying and through kiln drying. We dry lumber quickly without the quality being compromised, as we use iDRY vacuum kilns to dry the lumber to the correct moisture content and the highest quality. We specialize in drying thick lumber. With multiple vacuum kilns, we usually have the capacity to dry customers’ lumber quickly. We are here to save you time and deliver the best quality wood that suits your needs.