How to Succeed at Air-Drying Lumber
If you're thinking about seasoning your own stock, there's a lot to know before you begin. But in the end, it's how you build the stack that really counts.
Many woodworkers prefer air-dried stock to the kiln-dried variety because they say it works easier and offers truer color. Then, there's the money savings.
Air-drying your own stock can save you at least 50 percent over kiln-dried boards from your lumber retailer. But, doing it yourself does require time, effort, know-how, and the room for stacking and storing. To help you avoid the mistakes that result in firewood, we contacted an experienced, hands-on expert.
Robert McGuffy has headed up the wood-drying sequence at the Anderson-Tully Company's Vicksburg, Mississippi, hardwood-processing facility for decades. At this complex, the largest of its kind in the U.S., Robert has the responsibility for air-drying, and then kiln-drying, about 70 million board feet of hardwood every year. And it's a mix that includes 65 species--all coming to him in varying degrees of wetness.
"Southern species, when you first cut 'em, are different in how much moisture they have," Robert says. "Take white ash, for instance. It has about a 60-percent moisture content. Cottonwood and willow will run 180 percent. Red oak runs 80-90 percent."
Editor's note: The degree of wetness in wood is called moisture content, and it's expressed as a percentage. But that percentage often can exceed 100 because it represents the ratio of the weight of the water in a piece of wood to the weight of the same wood when it is completely dry. For example, a piece of green wood weighs 50 lbs.; dry, it weighs 20 lbs. That means that the green wood contained 30 lbs. of moisture, or 30/20th of its dry weight was water. As a percentage (30 divided by 20 equals 1.5), that's a moisture content of 150 percent!
Such a variance, of course, means that each species--even where you live--requires either less or more time to dry down to the desired moisture content. At Anderson-Tully, the goal is to air-dry the boards to a 25-percent moisture content. Then, they're ready for the kiln where they'll be reduced to about eight percent.
Without a kiln, you should try to achieve an air-dry moisture content of 15-20 percent. Further moisture reduction occurs when you move the boards indoors where they'll eventually reach their equilibrium moisture content (EMC). (Note: The EMC equals a point where the wood neither takes on nor loses moisture due to the atmosphere.)
According to the U.S. Forest Service's Forest Products Laboratory in Madison, Wisconsin, it takes 1"-thick green boards from 45-60 days to air dry to 15-20 percent moisture content in sunny, temperate, not-too-humid weather. If you live where it's colder and damper, count on more time. Inside, the drying process can prove slower, taking three or four months before the wood reaches its EMC and can be worked. But achieving workable stock means starting with a proper stack.
Pick a storage spot for your boards that's in the open, but avoid low, damp, or boggy areas. And, keep the stack from under trees that can litter it with twigs and leaves. On the other hand, don't pick the sunniest spot in your yard--your boards might dry too rapidly. Keep wind direction in mind, too. Says the experienced Robert McGuffy: "The [prevailing] wind should blow through the side of the stack, not through an end. It'll dry much quicker going through the side, and you won't get end-checks."
At Anderson-Tully, Robert takes extra precaution so the green boards won't degrade in the drying process. "We dip them in an anti-stain sealer, and then put them on stickers [strips of wood that separate the board layers]," he says. "And we seal the ends."
Home woodworkers can do practically the same thing, notes Robert. "Paint the ends of all boards with latex paint [or a commercial sealant such as Sealtite 60 or Mobilicer-M]. Or, put double side-by-side stickers under them. The check won't go past that second stick. We make our stickers out of most any soft wood [meaning soft hardwood}: poplar, cottonwood, any low-grade lumber," Robert explains. "But I wouldn't make any out of walnut--that stains. The species of the stickers doesn't make that much difference, as long as you make them all the same size. Ours are ripped to 1-1/8" wide from boards dressed to 7/8" thick. If the thickness varies, even a little but, you'll have wavy boards."
Figure on cutting enough stickers per board course to lay them every 2' along the length of the boards. Determine the length of the stickers by estimating the width of the stack you intend to make. Once you cut the stickers, begin stacking the boards as shown below.
Drying your own wood can be great, if you follow this advice:
- Be sure to level the stack's foundation, but provide for a slight drainage slope. Put down a vapor barrier if the ground seems damp.
- Select only straight-grained, defect-free boards no thicker than 2" and less than 12" wide.
- Check the stack occasionally. Stains or mildew signal drying too slowly. Excessive checking means drying too fast.
- A moisture meter (about $100 at woodworking suppliers), as shown at the top of this page, is the most reliable means of determining moisture content. Check the wood every few weeks outdoors and after moving it indoors.
Written by Peter J. Stephano
Illustration: Brian Jensen
Photograph: Hethertington Photography