The Owner Built Home - Chapter 6: Properties of Wood

This chapter, excerpted from "The Owner-Build Home," examines the properties of wood as a building material.
By Ken Kern
January/February 1972
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Ken's hand-drawn illustrations are found throughout the book. This one covers wood seasoning and wood grain.
ILLUSTRATION: KEN KERN
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Ken Kern, author of The Owner-Built Home and Homestead, is an amazing fellow and everyone interested in decentralist, back-to-the-land, rational living should know of his work. Back in 1948 he began collecting information on low-cost, simple, and natural construction materials and techniques. He combed the world for ideas, tried them, and started writing about his experiments. We're excerpting chapters from Owner-Built Home and Owner-Built Homestead. Here he considers the properties of wood. — MOTHER EARTH NEWS


Realistic builders estimate actual carpentry costs at $6 per hour, allowing for the wasted time of workmen. Consider, for instance, the time it takes to drive a nail (perhaps bending it, pulling it, getting another, and so forth). Multiply this by the 65,000 wire nails that it ordinarily takes to hold up a small house! Or take into account the one-third saving in the costs of stud lumber and stud erection by spacing on 24-inch centers instead of on the code-enforced 16-inch centers. The University of Illinois Small Homes Council has found that a 40-cent stud actually costs $5 by the time it is "in place." Professor Albert Dietz of MIT recently stated: "Practically all small houses built today use too many studs. You can't say they are over-engineered, because they aren't engineered at all. They are just overbuilt." From an engineering standpoint, for the vertical loads imposed and for building spans of up to 32 feet, the 2 X 4 stud is adequate when spaced on 8-foot centers.

As for "in place" nailing costs of wood frame houses, it is known that threaded nails can replace twice as many cut flooring nails on a weight basis alone. This is without regard to the superior performance of the threaded nail; and the actual purchase-cost saving of 28% doesn't include corresponding labor-savings either. The permissible load for a single, smooth nail, driven parallel to the wood grain, is from 80 to 90 pounds. A properly threaded nail of the same size will provide as much as ten times (800) this holding power. And a toothed-ring or split-ring connector will provide twenty times (up to 1900) the holding power of a threaded nail.

All the above is by way of introducing this chapter on wood with the fact that framing methods can be unduly complex, inefficient, and costly, or they can be simple, effective, and inexpensive. The material itself—wood—can be a perfect choice in performing a simple building task. Wood possesses high tensile and compressive strength, and the high strength-to-weight ratio of some woods gives relative immunity to vibration. We certainly should not discount the possibilities of building with wood merely because it is so grossly misused in current building practices. But before the owner-builder attempts to construct his house out of wood, he would do well to understand fully the structural properties and behavior characteristics of this versatile material. Builders too often choose a high-strength wood siding where what is really called for is a wood having good paint-receiving and weather resisting properties, and ability to stay in place. Joists are chosen for high bending strength, whereas stiffness is needed more than strength, and the concern should be for dryness, ability to stay in place, and minimum tendency toward shrinkage.

Wood can best be thought of as a reinforced plastic. Its main constituents (cellulose and lignin) are also the major ingredients of commercial plastic. Lignin is the adhesive which gives strength and rigidity to the wood. Cellulose is nature's "strong" material. It is made up of long-chain molecules which line up with the long axis of the tree. This explains why wood splits along the grain, but must be chopped or sawed across the grain. It also explains why wood shrinks much less in length than in width when it dries. Moisture in the wood is not held inside the molecular chains, but rather between them. When evaporation occurs, the chains contract very little, but draw together in closer contact. Shrinkage is therefore lateral rather than longitudinal. While the tree is living both the cells and cell walls are filled with water, but as soon as the tree is felled, water within the cells ("free water") begins to evaporate. Shrinkage in the wood occurs only after the free water has fully evaporated.

Investigations at Virginia Polytechnic Institute have disclosed the fact that nail-holding powers decrease by three-fourths when driven into green or only partially seasoned lumber. During seasoning the wood shrinks away from the nail shank, thus reducing friction between the nail and the surrounding wood. It has also been found that moist wood in direct contact with a nail deteriorates as a result of chemical digestion of the cellulose due to the formation of iron oxides on the nail in the presence of wood acids. A certain unsheathed house-frame, assembled with green lumber, lost during seasoning more than one-third of its initial resistance to racking.

Besides the increase in strength and nail-holding power, air-dried or kiln-dried lumber holds paint and preservatives better and is less liable to attack from fungi and insects. It is fungi which cause decay in wood. Since wood will not rot when dry, the term "dry rot" actually refers to rot that leaves the wood, when dry, in a brown, often crumbly, condition that is suggestive of extreme dryness. Dry rot is caused by certain fungi which, by means of special water-conditioning strands, are able to carry water from the soil into the building.

Before the purchase of any major amount of lumber, the owner-builder should make a rough check as to moisture content. First select a half-dozen flat or plain-sawed boards from the lumber pile and cut a sample from each. The sample should measure 1 inch along the grain and be cut so as to include the entire width of the board. It should be cut at least 1 foot from the end of the board. Trim the sample so that it will measure exactly 6 inches in width, and place in a warm, dry place for 48 hours or longer. The 6 inch dimension is then measured to determine shrinkage. A shrinkage of over one-fourth inch shows the wood to be unsatisfactory for framing; a shrinkage of over one-eighth inch means that the wood is unsatisfactory for interior trim or finish.

As many advantages as there are to the use of seasoned lumber, its use will not be widely practiced so long as the cost remains relatively higher than green lumber. A person can maintain quality in construction and yet make substantial savings by buying less expensive (green) lumber, using threaded nails in erecting the framework, and then allowing the house shell to season prior to the application of interior finishing.

If the house frame is not sufficiently seasoned, "nail popping" will most certainly result after the installation of interior panels, for in the seasoning process wood shrinks as it dries out and swells as it absorbs moisture, while the nails penetrating the lumber cannot appreciably change in length. This results in a backing-out of the nail. Some carpenters use longer nails to prevent nail-popping, presumably figuring that greater holding power comes with longer length or heavier gauge. On the contrary, nail-popping has been found to be directly proportional to both lumber shrinkage and depth of nail shank penetration. A good rule to remember when buying nails is that the most effective nail is the shortest nail providing sufficient holding power, since it offers maximum resistance to nail-popping. Elaborate house framing studies by the Housing and Home Finance Agency have disclosed that nothing larger than the 3-inch-long ten-penny nail need be used in attaching two 2-inch members in a simple lap splice.

Another would-be solution of the popping problem is to use cement-coated, plain-shank nails. True, the withdrawal resistance is improved by three-fourths immediately after driving, but the withdrawal resistance of cement-coated nails still decreases as much as fifty percent during seasoning.

From the standpoint of nail-popping and green lumber framing alone, I have come to the opinion that the owner-builder can ill afford the use of plainshank common wire nails in his wood-framed house. The joining of wood has been for centuries a bottle-neck in utilizing the material to its fullest extent. In the past, contact areas of the jointed members had to be large, and this governed member-sizes more than the actual working stresses did. But today we have synthetic resin-adhesives and hardened, properly threaded nails to provide joints that are even stronger than most of the woods to be assembled. A special nail has been designed for every specific wood framing operation, be it cement-coated cut, etched, barbed, knurled, twisted, fluted or threaded

Threaded-type nails have remarkable holding qualities, as wood fibers penetrate the grooves made by the threads and this fishhook-like action keeps the nails in place. The slender, hardened, high carbon steel grooved nail is more than one gauge smaller in diameter than the common wire nail. Being more slender, it can be driven faster and with less energy. It is also less likely to split the wood, so that these nails can be spaced closet together and near the edge or end of the wood.

About one percent of the cost of a wood house is for nails. If threaded nails (which have twice the withdrawal resistance) were used throughout, the cost of a $5000 house would be increased by $20. But this additional expenditure would be more than offset by increased strength and performance; the unsheathed framing would provide from 4 to 6 times greater lateral thrust resistance. The wood floor would not squeak or spring, cup or buckle. Siding would not "creep" or "pop." With threaded nails, times greater holding power is given to asbestos shingles, fifty percent increased holding power to plaster board.

The problem of wood-joining has been but partly solved by the development of improved nailing. Two other improvements in fastening, since World War II, have contributed to making wood usable as a continuous material; first, the development of mechanical connectors, and second the invention of strong glues.

Metal plates and gussets are on the market for use in strengthening wood joints, particularly against a direct pull. They are made of heavy steel and have special designs of teeth stamped into them. Held in place merely by nails, they make one nail do the work of several. Bent, pre-drilled, steel U-grip hangers can often be used to advantage in locating and supporting 2 inch beams and joists. Framing anchors are also advantageously used at joints between plate and rafters, and where studs attach to sills. Where toenailing prevents sidewise movement, anchors give protection against uplift.

Shear-plates, toothed ring and split ring timber connectors are some of the best devices for use at joints that are fixed and permanently bolted. Toothed rings and shear plates are installed by forcing them into wood so that half the depth of the connector is embedded in each of the two lapping members. Split ring connectors are installed in pre-cut grooves, half of the ring in each of the two lapping members. A bolt passing through the lapping members and through the center of the ring completes this type of pin joint.

The gluing of wood may be compared to the welding of metal, as it makes a continuous member without cutting out any part of it. Gluing timbers is not a recent innovation, but with the development of plastics, much more durable and waterproof glues have been developed. Common casein glue is still one of the best, least expensive, and more available glues. It is especially suited to rough lumber where joints cannot be made completely tight. Only water is added to the dry casein powder, but, once it has set, it is completely waterproof.

As with the correct selection of joining devices, the proper selection of lumber is a prerequisite to successful framing and finishing. Being plentiful and therefore relatively economical in many parts of the world, wood is also easily worked—cut, drilled, or nailed—with simple tools. The choice among species, varieties, qualities, and grades of wood is sometimes staggering, and a proper choice must be made for each specific building operation. The following information is intended to assist in making proper choices (classification is made relative only to the more important structural and behavioral characteristics).

Hardness in wood is the property that makes the surface difficult to dent, scratch, or cut. We look for hardness in woods to be used for flooring and furniture making, but avoid this property where frequent cutting and nailing is required, since hard wood is more likely to split in nailing and is generally more difficult to handle, as well as usually being more expensive.

Weight is a reliable index of the structural properties of wood. That is, a heavy piece of dry wood is stronger than one lighter in weight.

Shrinking and swelling take place in wood as it absorbs moisture and then dries out. About one-half of the shrinkage is "taken out" of wood through air drying and about two-thirds through kiln drying. One fact which is especially important to know before selecting flooring boards is that a board will shrink about one-half as much at right angles to the annual rings (edge-grained). Edge-grained wood of a species having high shrinkage will prove as satisfactory as flat-grained wood of a kind with inherently lower shrinkage.

Warping of wood is closely allied with shrinkage. Lumber that is cross-grained or that is cut from near the central core of the tree tends to warp when it shrinks. Quarter-sawed dry wood warps the least.

Paint-holding depends upon a number of factors, such as the kind of paint and the circumstances of its application, as well as the type of wood. But, in general, paint holds better on edge-grained wood than on flat-sawed. And the bark side of flat-grained boards is more satisfactory to paint than the pith side.

Nail-holding properties are usually greater in the denser and harder woods, but (already mentioned) nails eventually lose about three-fourths of their full holding power when driven into wet wood. Blunt-pointed nails have less tendency to split wood than do sharp-pointed ones.

Decay in wood is caused by moisture and changes in moisture content. Wood is inherently vulnerable to moist and stagnant air; it is susceptible to attack by fungi and insects. Moisture in wood is also directly responsible for plaster cracks, air leakage, pulling of fastenings, vibration of floors, pealing of paint, and the warping and sticking of doors and windows. It is a wise owner-builder who insists on dry lumber, especially for interior use (for the exterior use of green lumber, consider again my pre vious comments in this chapter on its use).

Bending strength is a measure of the horizontal load-carrying capacity of wood. Rafters, girders, and floor joists all require high bending strengths. A small increase in the height or depth of a beam (or horizontal member) produces a much greater increase in bending strength than it does in volume, and an increase in the width of a beam increases the volume and bending strength proportionally. That is, an increase of 1 inch in the height of a 10 inch beam will increase its volume 10%, whereas the bending strength of the same beam set on edge is increased a good 21%. But an increase of 1 inch in the width of a beam 10 inches wide will increase both volume and bending strength 10%, equally. For maximum bending strength the slope of wood grain should not exceed an average of 1 to 12. Both the elastic limit and the ultimate strength of wood are higher under short-time loading than under long-time loading, so that wood is able to withstand considerable overloads for short periods or smaller overloads for longer periods.

Stiffness is a measure of the resistance to bending or deflection under a load. A 10 inch joist has only about one-fourth more wood in it than an 8 inch joist, but set on edge in a building it is more than twice as stiff. In studding too, stiffness is more important than actual breaking strength, as it is deflection that must be reduced to a minimum in order to avoid plaster-cracks in ceilings and vibrations in floors.

Toughness is a measure of the capacity of wood to withstand suddenly applied loads. Woods high in shock resistance are adapted to withstand repeated shocks, jars, jolts, and blows. They give more warning of failure than do non-tough woods, an important factor in beams and girders.

Wear resistance is higher with edge-grained wood than with flat-grained; better on the sap-side than on the heart-side; more even with clear wood than with wood containing knots.

Knowing the physical properties and the methods of attaching wood members is only the first or preliminary thought-stage in building a house of wood. After this basic understanding we must choose wisely some system of wood construction, and the structural system that may be used depends, in turn, on a host of factors—from general engineering principles affecting roof and wall and floor design to such commonplace considerations as available labor skills and equipment. These are the factors which will be considered in chapter 7, on Wood Framing and Structure.

BIBLIOGRAPHY (books listed in order of importance)

Laboratory Findings on Holding Power of Nails: Stern, Virginia Polytechnic Institute.
Selection of Lumber: U. S. Dept. of Agriculture, Farmers Bulletin 1756.
Trees:
Agricultural Yearbook 1949, U. S. Dept. of Agriculture.
Materials for Architecture: Caleb Hornbostel, Reinhold, 1961.
Wood Construction: National Committee on Wood Utilization Washington, D.C


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