Selecting a suitable wood for any building use, no matter in what section of the country it is to be used, affords an opportunity for an extremely wide choice of species and grades. How to restrict that choice in a practical manner for a particular job is the question before the architect, engineer, or builder.
The suitableness of a wood depends first on the use requirements, and then on the mechanical and physical properties such as strength, hardness, durability, and ability to hold paint, of the species and grades involved.
The two general divisions of woods are the hardwoods, from the broad-leaved trees such as oak, maple, birch, gum, and sycamore, and the so-called softwoods from the conifers or needle-leaved trees such as the pines, firs, cedars, spruces, and hemlocks.
This latter group furnishes the lumber for practically all house framing, sheathing, subflooring, siding, shingles, and window sash and frames, while hardwoods and also softwoods are used for finished flooring, woodwork and trim, paneling, and doors. In addition, oak and chestnut are used occasionally for exposed beams and trusses as in clubhouses, churches, and similar structures.
From the standpoint of volume used, by far the most important woods for building are the yellow pines from the South and Douglas fir from the Northwest. Practically all the softwoods, however, are used to some extent in house construction.
It is generally impossible to choose the "best" wood for any service, but one of the most suitable usually can be selected if we are familiar, not only with species characteristics, but also with the structure of wood.
Wood, like any form of plant life, is composed of minute cells. A tree grows in height and diameter through the formation of new cells, under the bark and at the tips of branches and roots. As new cells are formed in the outer portion of the tree, some of the inner cells mature or "die" and become filled with water and perhaps gum or resin or mineral matter. Only an outside layer of the wood around the tree contains live cells and constitutes what is known as "sapwood," while the inner portion, made up of matured cells, is the heartwood. This heartwood, in practically every species, is more durable, that is, more resistant against decay, than the sapwood. This explains why the heartwood is frequently specified for use in exposed locations.
Contrary, however, to what was once a popular notion, sapwood is just as strong as heartwood of the same species, other things being equal.
The growth which a tree makes occurs during the spring and summer months in the form of annual rings. In the spring the growth is rapid and the new cells are large with thin walls.. In the summer growth is retarded and the new cells are smaller with thicker walls. The light spring wood and dark summer wood thus formed each year together make up an annual ring. The summer wood, having a thicker cell structure, has more wood fiber per unit of volume than the spring wood, and hence is denser and stronger.
Consequently the higher the percentage of summer wood in a piece of wood, the stronger the piece. In such woods as Southern yellow pine and Douglas fir, the percentage of summer wood is referred to as the density of the wood and is a definite grading requirement in certain structural timber grades.