The terms "wood" "lumber," and "timber" are often spoken of or written in ways to suggest that their meanings are alike or nearly so. But in the Builder's language, the terms have distinct, separate meanings. Wood is the hard, fibrous substance that forms the major part of the trunk and branches of a tree. Lumber is wood that has been cut and surfaced for use in construction work. Timber is lumber that is 5 inches or more in both thickness and width.
Seasoning of lumber is the result of removing moisture from the small and large cells of wood - drying. The advantages of seasoning lumber are to reduce its weight; increase its strength and resistance to decay; and decrease shrinkage, which tends to avoid checking and warping after lumber is placed. A seldom used and rather slow method of seasoning lumber is air-drying in a shed or stacking in the open until dry. A faster method, known as kiln drying, has lumber placed in a large oven or kiln and dried with heat, supplied by gas- or oil-fired burners. Lumber is considered dry enough for most uses when its moisture content has been reduced to about 12 or 15 percent. As a Builder, you will learn to judge the dryness of lumber by its color, weight, smell, and feel. Also, after the lumber is cut, you will be able to judge the moisture content by looking at the shavings and chips.
A defect in lumber is any flaw that tends to affect the strength, durability, or utility value of the lumber. A blemish is a flaw that mars only the appearance of lumber. However, a blemish that affects the utility value of lumber is also considered to be a defect; for example, a tight knot that mars the appearance of lumber intended for fine cabinet work. Various flaws apparent in lumber are listed in table 3-2.
Table 3-2. - Wood Defects and Blemishes
|Bark Pocket||Patch of bark over which the tree has grown, and has entirely or almost entirely enclosed|
|Check||Separation along the lengthwise grain, caused by too rapid or nonuniform drying|
|Cross Grain||Grain does not run parallel to or spiral around the lengthwise axis|
|Decay||Deterioration caused by various fungi|
|Knot||Root section of a branch that may appear on a surface in cross section or lengthwise. A cross-sectional knot maybe loose or tight. A lengthwise knot is called a spike knot|
|Pitch Pocket||Deposit of solid or liquid pitch enclosed in the wood|
|Shake||Separation along the lengthwise grain that exists before the tree is cut. A heart shake moves outward from the center of the tree and is caused by decay at the center of the trunk. A wind shake follows the circular lines of the annual rings; its cause is not definitely known|
|Wane||Flaw in an edge or corner of a board or timber. It is caused by the presence of bark or lack of wood in that part|
|Warp||Twist or curve caused by shrinkage that develops in a once flat or straight board|
|Blue Stain||A blemish caused by a mold fungus; it does not weaken the wood|