Why Does Lumber Need to be Dried?
After lumber is sawn from the log it must be dried before it can be put into use. The forest products industry spends a lot of time and money to dry lumber and this video explains the reasons why.
- [Scott] People use wood for many different applications around the world.
Whether the wood is used to build a kitchen cabinet, or to frame out a house, it must be dried before it can be used.
Let's take a look at some of the reasons why.
Trees contain a lot of water.
Every part of a tree, from the leaves down to the roots, has some amount of water in it.
Water is a critical component of the photosynthetic process and it is used to transport sugars, nutrients, hormones, and minerals throughout the tree in what we call sap.
The amount of water contained in a piece of wood is called moisture content.
In wood, the moisture content is expressed as a weight of the water present in the wood divided by the dry weight of the wood.
Moisture content is expressed in percent.
So if we have a piece of wood that weighs six pounds with the water included, and it weighs three and a half pounds without the water, that means there's two and a half 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 71.4%.
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.
Moisture content varies from species to species.
And it also varies between trees of the same species.
And even in boards sawn from the same tree.
Wood will gain or lose moisture until it is balance with the environment in which it is located.
When the wood is no longer losing or gaining moisture equilibrium with its surroundings.
This concept is called equilibrium moisture content.
The equilibrium moisture content is based on the temperature and the relative humidity of the air.
For example, if the temperature is 70 degrees and the relative humidity is 68% the equilibrium moisture content is 12 and a half percent.
This means the wood if given enough time, will eventually achieve a moisture content of 12 and a half percent.
If the moisture content of the wood is greater than 12 and a half percent, it will begin losing moisture.
If its moisture content is lower than 12 and a half percent it will begin to gain moisture.
The gaining or losing of moisture will continue to occur until the wood reaches 12 and a half percent moisture content.
Assuming the temperature, and the relative humidity, does not change before it gets there.
Why does it matter if wood gains or loses moisture?
The answer is dimensional stability and it is one of the main reasons wood must be dried.
Wood begins to shrink in width, and thickness when the moisture content starts to get below 30%.
The amount of shrinkage varies by species.
But it could range from five to 10 percent in width and three to six percent in thickness depending on species, and how the wood was sawn from the log.
If a product is made before the shrinkage takes place damage to joints can occur and the product will fail.
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 closed equilibrium moisture content to the moisture content the lumber was dried to there should be no issues due to shrinkage.
Wood intended to be used inside of a home is usually dried to a moisture content around seven percent because that is typical of conditions found inside of home.
Wood that is used for construction lumber is usually dried to around 15% moisture content.
The picture shown here is a table made from properly dried red oak to a moisture content of seven percent.
The wood was placed in an unheated carport under roof, for about a year before the table was made.
If we take a closer look at the table we can see that there is some warp taking place.
When a straight edge is placed across the tabletop the amount of warp can be clearly seen.
We can also see how the warping of the tabletop has affected the drawer.
In the winter time when the EMC is low inside the house the drawer cannot be opened.
In the summer time when the EMC in the house increases the warped wood relaxes enough to allow the drawer to be opened.
Problem is caused because the moisture content of the lumber was too high when it was used to build the tabletop.
And the wood began to shrink when placed inside the home.
This American chestnut picture frame, is another example of the wood having too high of a moisture content when the frame was built.
After the frame was built it began to shrink causing the joint to crack from the inner corner of the frame.
Keep in mind that wood will also gain moisture.
And when it does, it begins to swell.
So if lumber is dried to seven percent moisture content then left outside in an uncontrolled environment, as was the case with the lumber used to build the table, it will gain moisture until it reaches equilibrium moisture content of the outside air.
In Pennsylvania the equilibrium moisture content in the outside air can range from 11 to 14% depending on the time of the year and the weather conditions.
So lumber that was properly dried can end up with a moisture content too high to use for products inside of a home.
The moisture content of the wood used in a red oak table was probably around 12% moisture content when it was built.
Even though it was initially dried to seven percent and then the lumber dried to around five or six percent moisture content once it was in the house.
The fact that wood swells when it gains moisture explains why the drawer can be open after the wood has gained some moisture.
In the case of the picture frame had it gained moisture and swelled rather than losing moisture and shrinking, the opening in the joint would have been on the outside corner rather than the inside corner.
Another reason lumber is dried is to prevent fungal attack, which can discolor the wood or even cause it to rot.
The fungus needs to have moisture in order to survive.
So 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.
Properly drying the wood also kills any insects and eggs that may be in the wood.
Most insects require moisture content above 10% in order to survive in the wood.
The exception to this is some termites and some powder post beetles.
As long as the temperature of the wood gets over 135 degrees during the drying process any insects that are in the wood and the eggs that may be present will be killed.
Air drying wood will not kill insects or the eggs found in the wood because the temperatures are not high enough.
Problems with paint and finishes can also occur with moisture contents that are too high.
Typically, moisture contents lower than 14% should minimize problems with paints and other finishes, but if the moisture content is too high, blistering and peeling may occur.
There are several other reasons lumber needs to be dried.
For example, lumber that is to receive a preservative treatment must be dried.
Drying also reduces the weight and more lumber can be loaded onto trucks to be shipped at a lower cost per board foot.
Drying also improves the strength properties of the wood.
Whatever the reasons are for drying the wood it isn't cheap and the lumber industry spends a lot of time and money to dry their lumber to the correct moisture content and the highest quality.
Now you know why.
Frequently Asked Questions