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.


| January/February 1972



properties of wood - log cross section and grain diagrams

Ken's hand-drawn illustrations are found throughout the book. This one covers wood seasoning and wood grain.


ILLUSTRATION: KEN KERN

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.





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