Wood as a Building Material
... some soft woods are actually harder than hardwoods, such as southern yellow pine, which is a very popular choice for construction grade lumber. A tree is made mainly of the sapwood and heartwood, but there are other parts like the pith, cambium, and bark. Sapwood consists of the most recently formed layers located just inside the bark. It carries sap up and down the tree, and it shows annual growth rings. The heartwood is formed from dead sapwood cells, and extends from the pith to the sapwood. It is dense, dark, gives the tree strength, is resistant to decay, is difficult to impregnate with wood-preserving chemicals, and makes the best lumber. Lumber quality is determined by several things. The number of knots is one factor. The more knots the weaker the wood due to cross graining. Sometimes knots are desirable in the application of knotty pine, which looks good with a clear coat or stain on it. The age of the tree is another factor. The older the tree the more heartwood there is which leads to stronger lumber. Some common lumber defects are bark pockets, checks and splits, knocks, shakes, wanes, warps, compression wood, cross-grain, and dry rot. Bark pockets are small patches of bark embedded in dressed lumber. Checks and splits are tiny splits that develop along the fibers of the wood during drying. Compression wood refers to the internal fibers of wood that can be damaged by high winds, snow loads, or rough handling during logging. Cross-grain is when the wood fibers aren’t all facing the same direction parallel to the length of the piece. The greater the slope of the grain is, the weaker the piece of lumber. A knot is where a branch had grown from the tree. The knot itself isn’t weak, however the area around the knot due to the orientation of the grain is weak. Shakes are separations that occur between the annual rings. A wane is bark or lack of wood on the edges or corners of lumber. Warping is the bending from flat of lumber which occurs during seasoning or drying too fast. Dry-rot isn’t dry at all and is caused by a fungus encouraged by moisture. The growth rings in a tree tell how many years the tree has been growing. Each ring represents one year’s growth. The orientation of the growth rings in a piece of lumber will affect how it performs. As soon as the tree is cut it starts to shrink. Most of the shrinkage happens in the direction of the annual rings (tangentially), only half that much across the annual rings (radially), and not much at all along the grain. There are two methods of cutting wood; plain sawn or quarter sawn. The first method of plain sawing has the log squared and sawn lengthwise. This is the most common and least wasteful way of sawing lumber. Kiln drying is easier with this method of cutting but gives the wood a high tendency to warp and shrink. The quarter sawn method has the log sawn up so that the rings angle from 45 to 90 with the surface of the board. There is less a chance of warping, shrinking, cupping, twisting, or swelling with this method. This creates a more durable surface that holds paint better but is more expensive, and harder to dry than the plain sawn method. There are several classifications of lumber; boards, dimensional lumber, structural lumber, dressed lumber, and timber. First of all lumber is any wood cut into size and shape suitable as a building material. Boards are 1 to 1 ½“ thick, 2” to 12” wide, and are graded on appearance ...