Association of Home Appliance Manufacturers
Lumber Fundamentals
If you intend to become a dedicated do-it-yourselfer, you should take the time to study and understand the markings on lumber and plywood. The success of any building project depends on choosing the right materials to use, and lumber and plywood are marked to provide you with important information about the intended end use of the wood. By understanding lumber markings you can be s, sure you are using the best and most economical grade of lumber for your project, so you will get the best results while saving money by not buying material that is a better grade than you need.

Lumber is identified according to the species of tree from which it is produced, i.e., redwood, spruce, pine, oak, etc. It is further divided into softwoods and hardwoods. These terms do not refer to the actual hardness or density of the particular lumber. By definition, softwoods come from trees that have needles, such as pine or fir; hardwoods come from trees that have leaves, such as oak or maple.

Lumber is further identified according to grade. Characteristics such as appearance, grain direction and density, natural and manufacturing defects, and strength are taken into account in grading the lumber. Lumber is sight-graded (visually) board-by-board by inspectors at the lumber mills, who separate and stamp the lumber according to its particular characteristics of strength and appearance.

Lumber is also identified according to its moisture content. For example, a fresh-cut tree may have a 50 percent moisture content. This moisture will be lost naturally as the lumber is milled and dries out, but moisture loss is accelerated by the mill by use of lumber dryers or kilns. Wet lumber that has a high moisture content is unsuitable for building, because it will shrink, warp and split as it dries out, therefore, it is important to reduce the moisture content before using the wood in building projects. This step ensures that the lumber will not shrink or warp too much after the house, furniture, or other project is finished.

In addition to warping and shrinking, high moisture content in lumber can also result in rusted or corroded fasteners and cracking, flaking, or peeling of any paint or finish coat. Choose lumber that is low in moisture content to ensure best results.

When wood moisture content has been reduced to 15 percent, usually the level that is referred to as "kiln-dried," the wood has undergone about one-half its potential shrinkage. To be useful, dimension lumber must be stamped S-DRY (Surface-dry), meaning it has a moisture content of 19 percent or less. Lumber that is stamped GRIN (green) has a moisture content over 19 percent. Carpenters often joke of GRIN lumber that you first have to pick the leaves off it before you can begin nailing.

Many people misunderstand the term "settle," as in "the house will crack as it settles." The term does not mean that the footings will sink away; settling is the term used to refer to the materials in the house losing moisture and shrinking, which often results in cracks in the walls or gaps in the wood trim. Projects built with green or wet lumber will literally pull themselves apart as the wood loses moisture and shrinks.

Kiln-dried lumber has a moisture content of 15 percent or less, which ensures that framing done with this lumber will stay straight and strong, without excessive shrinkage or warping.

Because it also serves for decorative and ornamental purposes and will be in view throughout the life of the building, finish lumber used for trim, moldings, cabinetry, flooring, and furniture should have an even lower moisture content than framing lumber. Interior trim and flooring lumber should have a moisture content between 7 percent and 10 percent.

Lumber moisture content is critical to the success of a building project, and we cannot emphasize this point too strongly. Many builders and cabinetmakers who are considered master craftsmen owe much of their reputation for quality to their ability to chose superior materials for the job.

In addition to checking the lumber markings for moisture content before buying, one can also take other steps to be sure that lumber and other wood products have a stable moisture content, i.e., that they are dry enough to use after they are delivered.

Lumber mills often wax the ends of framing lumber to prevent the lumber from absorbing airborne humidity or moisture. Still, lumber that is warehoused outdoors, or in unheated lumber sheds, can pick up moisture from the air during humid weather. For building projects such as applying trim, or for cabinet or furniture building, buy your material from a supplier that has an indoor warehouse where temperature and humidity are controlled. Find a lumber yard or home center that caters to professional carpenters to be sure you are getting quality wood materials.

Next, buy wood products for your projects well ahead of schedule, and store them in the area where they will be used. At room temperatures and normal moisture levels, the wood products will quickly acclimate, meaning they will reach a moisture content level to match their environment. Conditioning building materials for even a few days can make a difference in how the wood products perform in place.

Written by Gary Branson
Reprinted with permission. Copyright HouseNet, Inc.

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