Reviewed October 1993
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Drying, (air seasoning) wood can be complex. The purpose of this guide is to outline some of the important points in drying small quantities of wood. People who buy green lumber from one of the many small sawmills in Missouri are often interested in drying and possibly surfacing the lumber. Sawmill operators and dealers who wish to approach the subject on a larger, more professional scale are referred to USDA Agriculture Handbook Number 402. "Air Drying of Lumber."
There are many reasons for drying wood. Four main reasons include:
The objective of air drying wood is to remove the water in wood by exposing all surfaces of each piece of wood to circulating air. In Missouri, wood can be air dried to a minimum of about 15 percent moisture content, provided the drying time is sufficiently long. It is also necessary to support the wood during drying to prevent the lumber from warping during the drying process. Lumber is piled in a special way to maximize the surface exposure of each piece of lumber to the air and at the same time to support each piece so it will dry straight and without unnecessary warping.
The first consideration is to prepare a strong foundation, 1 to 2 feet above the ground, on which to pile the lumber. The ground beneath the foundation should be kept free of vegetation or debris that would hinder air circulation under the pile.
Your lumber probably will be cut in random lengths and widths. For best results, pile each course so that each board within a layer is well supported and does not protrude at either end of the pile (Figure 1).
This system of piling is called "box piling" and has proven to be the best method of piling random length lumber. The outside boards of each tier are full length. This is important to tie the pile together and make it less subject to tilting or falling over. Leave spaces between adjacent boards approximately equal to the thickness of the boards.
Figure 1
Plan view of a tier of boards, illustrating the system of alternating short lengths for box piling. Unsupported ends of boards placed on the inside of the pile will dry with less defect than if allowed to extend over the end of the pile.
An adequate supply of wooden sticks (spacers) will be needed to separate each layer (Figures 1 and 2). It is very important that the sticks be uniform in thickness. Sticks usually are cut 3/4-inch thick. Note that the stickers are carefully aligned vertically (Figure 2) so that each layer of lumber will be supported from the base of the pile. If the stickers are not properly aligned, forces will be created in the drying lumber that will result in permanent kinking of the lumber.
Figure 2
Diagram of essential features of good lumber stacking for proper seasoning.
Finally, cover the pile with old boards, plywood, corrugated metal or any materials that will protect the top layers of lumber from sun and rain. It is also a good idea to weight the top by placing heavy objects such as concrete blocks or stones on the roof. This will reduce warping in the top tiers of the pile as well as secure the roof on the pile.
In warm weather (April through October), 1-inch lumber can be dried to 15 or 20 percent moisture content in 45 to 60 days (2-inch lumber in 60 to 90 days). In the winter months, lumber will require twice as long to dry. Lumber at 15 percent to 20 percent moisture content is adequate for building unheated structures such as garages or barns. If the wood is to be used inside a heated structure, further drying in a commercial kiln is necessary (6 percent to 8 percent moisture content for indoor use in Missouri.)
G5550, reviewed October 1993