BECAUSE WOOD is a part of every home and because anybody who can pull a saw or lift a hammer can work with wood, a general knowledge of its properties and uses will come in handy to every person who lives in a home.

The first point in selection of material for satisfactory performance depends on the use of the right wood for the right purpose. The man—or woman—who intends to work with wood has to determine what kind of service he expects to get from the wood. Will it be strength or hardness, or stiffness or resistance to decay, or beauty or some other property?

The use requirements usually do involve a combination of two or more, and selection involves finding the wood that has the best combination of the desired properties.

The species that have certain special properties that cost more (such as resistance to decay) should not be used unless those properties are definitely needed. For example: People often go to considerable expense to get highly decay-resistant wood for diving boards; they reason that the constant wetting and drying of the board demands it. Actually, however, the most important item in a diving board is strength, for most boards fail mechanically in a year or two if they are in continuous use, as at public beaches. The proper wood for this use, then, would be comparatively inexpensive and strong, selected with little regard to decay resistance.

Similarly, it will be wasteful to pay a premium price for wood with a beautiful grain pattern, like walnut or mahogany, for use in furniture that is to be painted. An inexpensive wood with equal or better painting characteristics but with little figure (yellow-poplar, for instance) would be a more logical choice. There is no economy in paying a high price for wood with a property that is not used.

Thus, wise use of wood in the home requires consideration of the properties needed and a basic knowledge of the main properties of the commercial woods. The final choice of the wood may also be affected by the skill of the worker and the availability of the desired species. The farmer, for example, who wants to use wood growing on his own land has a limited selection and may not be able to choose the ideal wood for a given purpose, but with the actual demands clearly determined, he can make the most satisfactory and economical selection from the wood that he has.

The characteristics vary among species, among the individual trees of the same species, and even among pieces of wood taken from different heights of the same tree. Thus the published values for the different properties are averages and do not hold for every individual piece of wood.

One should also understand that wood does not have the same strength properties in all directions. Strength depends on the direction of the grain. When tension—pull—is applied parallel to or along the grain, for example, wood may be 300 times as strong as when the tension is applied at right angles to the grain.

The terms "hardwood" and "softwood" are used to distinguish between two general classes of wood and not to indicate the properties of the included species. Hardwood is the name given to the group of trees that are broad-leaved. Softwood is the name given to trees that have needlelike or scalelike leaves and are mostly evergreen (cypress, larch, and tamarack being exceptions).

The hardwoods are not necessarily high in relative hardness; some woods classed as softwoods are actually harder than some classed as hardwoods. The softwoods are used principally in construction; the hardwoods furnish most of the wood for implements, furniture, and other industrial uses.

The weight of wood in itself has an important bearing upon the selection of a species for many uses. Weight also serves as a reliable index of the strength properties of dry wood and affords an accurate comparison between the strength properties of possible species when the degree of dryness and the actual sizes are the same. Generally speaking, the heavier a piece of dry wood, the stronger it is, regardless of the species.

Changes in temperature have little effect upon wood; they cause such small variations in size that for ordinary farm and home uses the effect of temperature can be overlooked.

Changes in moisture content, on the other hand, have a considerable effect on wood, which swells as it takes up moisture and shrinks as it dries. Difficulties may be encountered if this property is disregarded. When proper precautions are taken, however, most of the trouble due to swelling and shrinking can be avoided. The shrinking or swelling in the width of a flat-grained board is nearly twice that of a quarter-sawn, or edge-grained, board of the same width; the shrinkage or swelling lengthwise of the grain in both is negligible.

One can compensate for high shrinkage, if only that kind of wood is available, by using edge-grained pieces, which will prove as satisfactory as flat-grained stock of species that have lower shrinkage values. Much trouble can also be avoided by using only wood that has been dried to approximately the moisture content that the finished piece will have in service. Thorough air drying will take out about half and thorough kiln drying about two-thirds of the shrinkage of wood. That is enough for the ordinary uses.

Warping, which is the result of uneven shrinking or swelling, may occur in wood that is plain-sawed, or cross-grained, or improperly dried. It can be reduced to a minimum by the use of edge-grained, properly dried material.

Woods that are comparatively free from warping include: Northern and Atlantic white-cedar, eastern and western redcedar, cherry, chestnut, northern white pine, ponderosa pine, sugar pine, western white pine, yellow-poplar, redwood, walnut, and the eastern, Engelmann, and Sitka spruce.

THE STRENGTH PROPERTIES of wood that most concern the woodworker include bending strength, compression strength, stiffness, and toughness.

Bending strength is a measure of the load-carrying capacity of the members that are ordinarily used in a horizontal position and rest on supports.

High bending strength is required in barn rafters, girders, stringers, wagon tongues, and scaffold platforms. If the only available wood is low in bending strength compared with better-suited species, the deficiency can be overcome by increasing the size of the member used. An increase of 10 percent in the height of a beam increases its bending strength by 21 percent. Both the volume and bending strength of a beam, however, increase in direct proportion as the width is increased. Woods high in bending strength for farm and home building include ash, beech, yellow birch, cherry, Douglas-fir, rock elm, hickory, the western larch, locust, hard maple, oak, southern yellow pine, and walnut.

Compression strength of wood is the measure of its ability to resist a load applied in such a direction that it tends to crush the member, as in a post or column. Good compression strength is essential for members used to support houses, garages, barns, storage bins, and the like, because they hold up a load. It is not important in such items as fence posts.

Low compression strength can be compensated for in some instances by the use of proportionately larger members. In the construction of small buildings, then, the size requirements of posts where the length is less than 11 times the smallest dimension are determined by bearing area, stiffness, and stability rather than by actual compression strength. Because these requirements necessitate the use of posts large enough to carry greater actual compressive loads than are ever placed upon them, no particular consideration need be given to the compression strength endwise in selecting a wood for small houses. Where exceptionally heavy loads are involved, as in supports for bins or root cellars, the compression strength of the members should be considered. If the length is greater than 11 times the smallest dimension, the stiffness of the member becomes the controlling factor, and the compression strength can be disregarded. Of the woods used in farm and home building, those high in compression strength include white ash, eastern redcedar, cherry, Douglas-fir, hickory, western larch, locust, hard maple, southern yellow pine, redwood, and walnut.

Stiffness is a measure of the resistance to bending or deflection under a load. It assumes importance in floor joists of houses and in studding, where it is more important than the actual breaking strength. Lack of stiffness in these members will result in plaster cracks in ceilings and vibration of floors. Stiffness is important also in shelving, ladder rails, beams, ax handles, and long, slender columns. Construction practices can compensate for the lack of stiffness, on the one hand, or nullify the advantages of using wood with high stiffness on the other. Increasing the size of a member will increase its stiffness, but the use of wood that is not fully dry at the time of installation will result in a loss in stiffness of the structure as a whole, because the wood, as it dries, may shrink or split, so that the fastenings, bracing, and bridging will not hold so well. Woods high in comparative stiffness that are used in farm and home building include white ash, beech, yellow birch, cherry, Douglas-fir, rock elm, western hemlock, hickory, western larch, locust, hard maple, oak, southern yellow pine, the Sitka spruce, and walnut. Defects, such as knots, checks, and shakes have little effect upon stiffness. In light building construction, therefore, material of the sound, though knotty, grades may be used to good advantage for joists and studs because stiffness is more important than breaking strength in those items.

Toughness is a measure of the capacity to withstand suddenly applied loads. Tough woods, therefore, can withstand repeated shocks or blows, such as are given ax handles, wheel spokes, and wagon tongues. Because they are high in comparative toughness, the following woods are used in farm and home building when toughness is desired: Ash, beech, yellow birch, elm, hackberry, hickory, locust, hard maple, oak, and walnut. Of those woods, hickory is used most often if toughness is the main requirement.

NAILS, screws, and bolts for joining his work are a primary concern of the home woodworker, although a variety of timber connectors have been developed.

Because the strength of a unit depends on the fastenings, they merit careful consideration. The denser and harder the wood, the greater is its inherent nail-holding power. This resistance to withdrawal increases almost directly with the diameter of the nail. Thus, if the diameter of the nail is doubled, the holding power is doubled, providing the nail does not split the wood when it is driven. Nails have been treated in various ways in an effort to increase their holding power. Among such nails that are in common use, the cement-coated nail has a higher holding power than the common nail in well-seasoned wood, and the barbed nail a lower value.

The moisture content of the wood at the time of nailing strongly affects the holding power of nails driven into it. The best guarantee of good joints and high nail-holding power is to use well-seasoned wood. Nails driven into wet wood lose as much as three-fourths of their full holding power when the wood becomes dry, and such a practice can result in the loosening of siding, barn boards, fence pickets, and the like. If one has to use unseasoned wood, it is best to use barbed nails in it.

The holding power of nails is greatly reduced if the wood splits; even a slight amount of splitting results in a considerable loss in holding power. The heavy, dense woods such as maple, oak, and hickory, split more in nailing than do the lightweight woods, such as basswood, spruce, and the true firs. Woods of uneven texture, such as southern yellow pine and Douglas-fir, split more than do the uniform-textured woods, such as eastern white pine, sugar pine, or ponderosa, pine. Splitting due to nailing can be reduced by using smaller nails, but the number of nails must be correspondingly increased to obtain the same holding power. Blunt-pointed nails have a smaller tendency to split wood than do sharp-pointed nails, but blunt-pointed nails have lower holding power. The danger of splitting can be reduced by staggering the nails or by boring holes for the blunt-pointed nails.

THE SURFACE CHARACTERISTICS of the wood affect its appearance and its strength and so should be considered when wood is selected for specific uses.

If maximum strength or fine appearance is desired, the material should be chosen from the select grades, from which most knots, pitch pockets, and the like are eliminated. The common grades, which include those defects in greater or lesser amounts depending upon the wood, should be used if appearance or high strength is not of primary importance or if knots or other defects are desired for architectural effects, as in knotty pine trim.

A knot is the part of a branch or limb that has become embedded in the body of a tree and subsequently has been cut through in the process of lumber manufacture. There are various types of knots, but the distinction that the woodworker should know is the one between an encased knot and an inter-grown knot. An encased knot is one whose rings of annual growth are not grown into those of the surrounding wood. An intergrown knot is one whose rings are completely intergrown with those of the surrounding wood. Because the grain of knots is at a considerable angle to the grain of the surrounding wood, the knots in a flat-sawn board shrink at a faster rate than the remainder of the wood. If, as with encased knots, the knots are not an integral part of the wood, they may become loosened even to the extent of falling out of the board.

Knots also affect both the appearance and the strength of a piece of wood. Except for knotty finish, they are considered objectionable from the standpoint of appearance. They reduce the strength of lumber according to their number, size, quality, and position in a piece. Strength is reduced more by an intergrown knot than by an encased knot, or even a knot hole, because the wood fibers are more distorted.

Where painting is to be done, wood that contains pitch—which is an accumulation of resin in the wood cells—should be avoided because it does not easily retain paint or varnish. The select grades of lumber allow only a small amount of pitch. Pitch pockets have a slight weakening effect on lumber, but their chief disadvantage is that the liquid pitch sometimes runs out of the board in use. Woods that tend to have pitch pockets can usually be detected by visual examination.

WOOD DETERIORATES in use like any other material. Iron and steel may rust upon exposure; wood may deteriorate through the action of fungi in damp places. The best way to prevent decay in the wood used in homes and farms is to use only dry wood in the original work and to keep it always dry.

Most of the wood used in homes does not come in contact with moisture enough to cause concern. A number of the danger points, however, call for definite precautions. Wood posts in basements should rest on concrete footings that rise about 3 inches above the flood. The same precautions should be taken where wood stairs rest on the basement floor. Points to watch outside the house include steps, siding, posts, and framework of porches that are in contact with the ground; basement window frames and siding that are near drain pipes; fence posts; and floors that are laid close to the ground over unventilated areas.

Untreated wood should be kept at least 18 inches above the ground level. When that is not practical, one should use heartwood of a decay-resistant species (sapwood of all species has low decay resistance) or wood that has been given a good preservative treatment. Wood that has been pressure-treated with a preservative gives the best service, but the life of fence posts and similar items can be extended by preservative treatment in a bath, a process the farmer or home owner can do himself.

Proper care of that kind and proper selection and use will give further evidence of the reasons why wood has been one of the foremost building materials for thousands of years.

Charles F. Brannan

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