ENGINEERED LUMBER

The most influential engineered wood products of the 20th cen-tury may be classified
quite simply as wood composites—recombinations of wood and wood fibers that
overcome many of wood’s natural limitations and extend its usefulness. Surpris-ingly,
however, in this century of rapid scientific progress, most of the notable new lumber
products have been rather modest steps forward—chemical improvements on essentially
mechani-cal 19th-century inventions.
Plywood, the trade name adopted by the Veneers Manufac-turers Association in 1919,
is a perfect example of a product that became truly viable only in the 20th century. The
industrial process of cutting thin layers of wood veneer by either peeling logs or slicing
them, along with the concept of adhering layers of veneer together, was first introduced
in France around 1830. Furniture makers such as Thomas Sheraton and the Steinway
company began using laminated wood veneers in the mid-19th century, and in 1884 a
factory in Reval, Estonia, began manufac-turing three-ply birch seats for bentwood
chairs. By 1870, a practical version of the rotary veneer lathe had been developed in the
United States. However, the development of a structural wood veneer panel that could be
used in everything from air-plane fuselages to wall sheathing depended on the discovery
of reliable, waterproof adhesives. That did not take place until after 1933, when German
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companies began manufacturing a new type of synthetic, heat-activated resin glue.
Previous to the 1930s, plywood had been manufactured with a variety of other types of
adhesives, such as blood albumin glue, casein glue (made from milk curd), and soybean
glue, but its application was rather limited by its adhesives’ vulnerability to moisture and
light. Once these limitations were removed, plywood quickly replaced dimensional
lumber as the most efficient material for flooring, wall sheathing, roofing, and concrete
forms.
Like plywood, the origins of glue-laminated timber lie in the 19th century. First used
in 1893 in Basel, Switzerland, glu-lam timbers are composed of many small, dry boards,
laminated together with glue and/or metal fasteners, to form extremely deep, long, stable
timbers. At first, glu-lam timbers could be used only indoors, where they would not be
exposed to harmful moisture or radiation. After the 1930s, however, new adhesives made
it possible to use glu-lam timbers practically anywhere. Glu-lam timbers have several
advantages over long single timbers cut from old-growth logs. First, they can be made
from much smaller logs grown in rotation. Second, because they are made from smaller
selected planks, their composition is highly predict-able. Third, they can be manufactured
to practically any size or shape. Glu-lam timbers have many architectural applications,
but their one weakness is their reliance on the strength and longevity of the adhesive.
Numerous new products have been developed on the plywood and glu-lam themes
since the 1970s. One is a composite wood joist manufactured in an I-beam cross section.
The web-bing is usually made from long sheets of plywood or oriented strand board, and
the flanges are made either from a parallellaminated plywood product called micro-lam
or from a material called Para-lam. The joists are much stronger and more stable than
traditional wood joists and are perfectly uniform. Naturally, they rely completely on the
strength of their glue bonds. Paralam is a type of glu-lam timber, but rather than being
made up of 2-by lumber, it is made of thousands of long, thin strips of wood
approximately one-eighth by one-half inch in cross section and up to a few feet in length.
In the manufacture of a Para-lam timber, a long bundle of spaghetti-like strands is coated
in glue, compressed as it is squeezed through gigantic rollers, dried by microwave, and
chopped off to convenient lengths. It can be manufactured to virtually any length or cross
section. In the United States, Para-lam is superseding traditional glu-lam beams in many
applications.
Not all engineered lumber requires chemical adhesives for its manufacture. One
product in particular, which was an essential part of America’s war arsenal during World
War II, was a com-pressed particle or fiber panel, often called Masonite or Hardbord. It
was manufactured throughout the 20th century out of many types of agricultural and
lumber waste and by many differ-ent processes. Ordinarily, sawdust or wood chips were
finely ground, boiled into a slurry, and then strained, pressed, and dried into hard sheets.
Fiberboard relied on the natural bonds between the wood fibers themselves for its
strength. Because it could be manufactured in practically limitless quantities out of
extremely low cost materials, it became popular for housing projects of all types both
before and after World War II. Partly for this reason, fiberboard has come to be
synonymous with temporary, cheap construction. Its extremely limited insulating
properties have also been far exceeded by fiberglass and rigidfoam insulation, and as an
interior finish it has been superseded in both cost and simplicity by gypsum wallboard.
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Oriented strand board (OSB) is a recent variation on the fiberboard-panel theme and
has been on the market since the 1980s. In OSB, small, flat chips of softwood
approximately one to four inches in size are gouged out of waste scraps from saw-mills
and low-quality logs. The chips are mixed into a gooey resin and then are laid and
pressed flat in a hard matrix of resin and wood chips. Sheets of OSB are ordinarily cut to
four- by eight-foot panels and come in a variety of thicknesses. Particle-board is
manufactured in a very similar way out of sawdust. Both of these products are finding
wider use. Oriented strand board is an extremely inexpensive sheathing material,
although it is weaker and more moisture sensitive than plywood, and painted
particleboard takes an extremely hard, smooth finish for interior detailing and exterior
siding.
The engineered lumber products described here have been designed to decrease the
cost of housing and the use of scarce timber resources while increasing the reliability of
structural designs and the palette of architectural options. Unfortunately, many
“innovative” products, such as finger-jointed studs, cobble together pieces of poor-quality
lumber to create flimsy replacements for an already cheap existing product. Many
engineered wood products also require careful handling and can maximize their strength
and efficiency only if they are installed perfectly. Because the engineered timbers of the
20th century are composites designed to combine small, cheap, plentiful strips, scraps,
and planks into larger units, nearly all of them also rely on chemical adhesives, which can
release toxic gases and deteriorate under certain conditions. Despite these drawbacks,
there is no doubt that engineered lumber is the rational, ingenious, and optimal solution to
many of the environmental, economic, and political dilemmas that Western nations faced
throughout the 20th century and will certainly face in the 21st.