What Every Woodworker Should Know About Air Drying Wood
New to woodworking? Or maybe you’re an expert in the field who has decided to start drying your own lumber.
You may be wondering whether to air dry your wood or choose an alternative.
Air drying wood can be time-consuming because it depends on weather and air conditions. It’s not like using a dry kiln or dehumidifier kiln which provides a controlled environment and heat to speed up the process.
We’ll walk through air drying and discuss some details to help you decide the best option for your wood and business. Here’s a rundown of the topics:
- What is air drying?
- What are the pros and cons of air drying wood?
- When to use air-dried lumber
- How to air dry
Let’s jump in!
What is air drying?
Air drying is the process of stacking green lumber and allowing it to dry while exposed to the natural elements (usually outdoors). Wood dried this way will reach the equilibrium moisture content of its surroundings.
It’s different from kiln drying in a few ways:
- It’s dependent on the weather rather than being in a controlled environment
- It doesn’t require a heat source
- It’s free
- It takes much longer
We’ll look at that last point next.
How long does it take?
Air drying has a long drying rate. It can take several months to a year since the wood is at the mercy of the weather. Some woodworkers cite the rule of thumb that wood will air dry one inch of thickness per year, but this number doesn’t consider many other factors.
Those factors include:
- Climate—temperature, relative humidity levels, wind/air movement, and sunlight
- Location—whether the wood is in a marshy area or a dry, elevated area with good drainage
- Wood species—different species of wood dry in different amounts of time
- Wood thickness—the thicker the wood, the longer it’ll take
- The grain pattern—quartersawn wood dries more slowly than flatsawn wood
- How the wood is stacked—the stack determines the amount of air circulation
A number of woodworking and lumber resources have charts with estimated drying times.
For example, Daniel L. Cassens, Forest Products Marketing and Utilization Specialist, provides a table with approximate times for drying green 1-inch thick lumber to 20% moisture content.
According to this table, when drying conditions are ideal in the spring and summer, sugar maple wood can air dry in about 50 days.
Otherwise, it can take up to 200 days in wetter, colder conditions.
The U.S. Department of Agriculture’s Forest Service puts out a booklet called Air Drying of Lumber that contains charts, too. A separate booklet also by the Forest Service, Estimates of Air Drying Times for Several Hardwoods and Softwoods, divides its charts based on major cities throughout the United States.
How much does it cost?
Air drying wood doesn’t necessarily cost anything upfront. After all, it’s free to stack the wood on your property.
But if you’re an owner of a woodworking business, you’ll have to consider the cost of the time required to dry the wood and the impact on your project deadlines. You may also find that the space needed to store wood for so long isn’t worth it.
With these factors in mind, let’s discuss the pros and cons.
What are the pros and cons of air drying wood?
Air drying your own wood can save you money, but the downsides—the time, storage space needed, and more—may outweigh that benefit.
As we’ve already discussed, air drying is a long process that could potentially hinder you from meeting your deadlines and being able to work at your desired rate.
It also requires a lot of space, and space that will be occupied for all of that time.
And in the end, your wood may still not be at the equilibrium moisture content (EMC) necessary for many indoor projects. Usually, air drying will bring the wood down to the moisture content of the outdoor environment, which can still be many points off from the needed number for indoors—about 6 to 9%.
Thus, even after all that time, you may have to pay for some kiln drying.
Air drying has a few risks for wood, too.
Chemical stains are possible as components in the wood, such as sugar and amino acids, react with outdoor factors. These are most common if the wood sits for a while and isn’t immediately stacked for drying after being cut.1
Insects, fungi, mold, and decay are all potential issues too. The USDA regulations for heat sterilization require wood to be heated at 133° F for 30 minutes to be sterilized—something that won’t happen without a kiln.
Fungi stains can turn the wood blue or bluish-black.
And termites and powder post beetles can also wreak havoc. Powder post beetles bore holes into the wood and leave piles of powder behind for an unsightly mess.2
Mold and decay can develop in damp environments when the wood stack doesn’t have plenty of airflow.
And finally, the wood itself can end up with drying defects due to changing weather conditions and shrinkage. The result can be anything from end checks and cracks to splitting and honeycombing.
Now, if you’re still determined to air dry your wood, these risks don’t have to stop you. There are times when air drying is appropriate and many ways to prevent these risks. We’ll look next at when you should use air-dried lumber, followed by some best practices for the process.
When to use air-dried lumber
Air-dried lumber is preferable for outdoor projects, such as barns or farm buildings, since it usually ends up with a moisture content appropriate for outdoor conditions.
Very few people air dry wood in the commercial wood business because of all the variables—weather, time, space, and more. And partly because air drying doesn’t usually result in a moisture content suitable for indoor projects (6–9%). That means the wood still has to be kiln dried or brought inside to finish drying.
If time is not an issue for you, one option is to air dry your lumber to a moisture content of 15–20% and then finish it off with a kiln. This will result in a much shorter kiln drying time.
Now, on to some steps for doing it yourself.
How to air dry wood
Air drying isn’t a particularly involved process. Once you’ve stacked your wood well, you can forget about it for a few months, aside from checking moisture content once in a while.
Here are the before, during, and after steps:
- Prepare the wood
- Prepare your base
- Stack the wood
- Cover the wood
- Add weight
- Monitor the wood with a moisture meter
- Complete the drying process
Prepare the wood
Stacking your wood as soon as possible after cutting will help to prevent damage. So be prepared to work quickly.
After cutting the tree, cut the wood as thin as possible—about 1 to 2 inches, ideally. And try to keep your cuts as even as possible.
End grain coating should happen as soon as the lumber has been sawn. This involves placing some kind of seal, whether leftover latex paint, log paint, or a commercial end sealer (such as AnchorSeal) on the ends of the wood. Doing so helps prevent checking and splitting defects that can result because wood dries fastest from its ends.
Then, it’s time to get your stacking space ready.
Prepare your base
The base where you stack your wood should be as level as possible—a measure that will help keep your wood from warping.
Create a foundation that is at least 12 inches off the ground, using something like concrete blocks.
Next up: stacking.
Stack the pieces of wood
Wood will dry the best if it’s stacked with boards of the same length and thickness. But if your boards are different lengths, you can use a technique called box piling.
Box piling involves putting longer boards on the outside of each layer with shorter boards on the inside. Put thicker boards at the bottom of the stack and thinner ones on top.5
Between each layer of wood, place spacers, called stickers, to allow for airflow. Stickers are usually about 1 inch thick and 1 ½ inches wide.
Here are a few sticker tips to avoid warping or sticker shock (staining of the wood from the stickers):
- Make sure the stickers are exactly the same height so that the wood lies completely flat.
- Line the stickers right above each other in the stack.
- Don’t use green wood for your stickers.
- Use stickers made of wood that’s similar to the wood you’re drying.6
- Place the stickers no more than 24 inches apart.
Cover the wood
When you’re finished making your lumber stack, place a cover on top to protect it. This is particularly important if you’re not able to stack the wood in a shed, garage, or shop. A cover will prevent discoloration and degrade caused by sunlight, rain, and snow.
It could be a sheet of plywood, some corrugated metal roofing, or even a tarp. But make sure the pile cover overhangs the wood about 6 inches.
If you use a tarp, don’t let it cover the sides of the stack because that’ll prevent air from getting through.7
Place a weight, such as concrete blocks, on top of the stack to keep the wood flat and prevent warping. The Wisconsin Department of Natural Resources recommends 50 lbs of weight per square foot.
Or you could even use ratchet straps to provide pressure. Then, all you have to do is tighten the straps a little each week.
Monitor the wood with a moisture meter
Throughout the drying process, use a pinless wood moisture meter to get an idea of where your wood is at and how quickly it is drying. Compare this number with the equilibrium moisture content of your area (determined by the temperature and relative humidity).
If the number stops going down for a couple of months, your wood may have dried the most that it’ll dry in the current climate conditions.
This number may or may not be as low as you were hoping for. That’s where the following step comes in.
Complete the drying process
If you’re in a fairly humid region, your wood may only dry down to about 12–15% moisture content—which may not be enough if you plan to use the wood indoors. In that case, bring the wood inside to finish drying there or put the wood into a kiln for a little while.
Finishing the wood in a kiln also has the extra benefit of sterilizing it against insects, decay, and mold.
Air drying is not for everyone
Air drying is a process for those who are patient—and for those who aren’t on tight business deadlines. You may have other reasons for not being willing to wait, too.
If that’s you, you’ll probably want to consider an alternative—kiln drying. Learn more about drying wood with a kiln by reading our page on the topic. Perhaps you’ll even want to build your own kiln!
- Air Drying of Lumber, USDA Forest Service, p. 43. (↑)
- Ibid., p. 44 (↑)
- “Air- and Shed-Drying Lumber,” Oregon State University. (↑)
- “Drying Small Quantities of Hardwood Lumber—Understanding the Effects of Moisture on Wood,” Forestry & Natural Resources: Marketing and Utilization. (↑)
- “Air-drying Hardwood Lumber,” University of Missouri, Extension. (↑)
- “Air-Drying Wood Slabs,” Woodworkers Guild of America. (↑)
- Ibid. (↑)