The NEW Plywood - Oriented Strand Board

Q: Last weekend we drove through a local subdivision that is under construction. I noticed that the builder was using wood wall panels which looked like a bunch of pressed wood flakes (instead of normal plywood). Is this stuff any good?

A: What you may have seen is a manufactured product called "Oriented Strand Board" (or OSB for short). This is a new type of wood panel that will eventually completely replace plywood in residential and commercial construction. In answer to your question, it is as good as the equivalent plywood which would have been used in that condition.

OSB and similar new wood products were developed in response to changing resource availabilities and the desire by manufacturers to improve the efficiency of their operations. This adaptation also demanded a change in manufacturer's approaches to the design of their product, from one that focused on process to one that focused on results.

Fabrication Standards versus Performance Standards

For a long time, manufacturers operated under the guidelines of the United States Product Standard PS-1 for the Construction of Industrial Plywood, which specified how plywood panels were to be fabricated and graded. As a cooperative effort between the U.S. Department of Commerce and trade and industry groups, the establishment of these minimum standards offered the best protection to the consumer that plywood of a given grade had consistent quality from region to region. The drawbacks to this approach were it limited how much of the raw material the manufacturer could use in the construction of a piece of plywood and it restricted innovation.

Consider how plywood is made. A log is fed into the mill from the yard, the bark is removed and the bare log is placed on a lathe-type machine that will "peel" off thin sheets of wood - like you might peel an apple - until the remaining log gets so small the peel breaks due to the small radius of the log core. This core is jettisoned and the next large log takes its place. The sheets of wood produced in this process are sorted based on the number of knot holes, grain imperfections, etc. and sent to different areas of the factory. The best sheets become the outside ("face" sheet or "veneer" sheet) of the plywood; the part you see. Glue is applied then the first of the interior "plies" (strips of wood 12" to 3'-0" wide which come from the peelings closer to the log core) are laid down edge to edge with their grain running perpendicular to the face panel's grain. More glue. Another full panel, similar to the face panel but probably not of the same visual quality, is placed on top of these. More glue. More cross- strips, etc., etc., until the top face panel is placed. This collection of plies and still fluid adhesive is placed in a press and the press holds them together until the glue sets. The rough-edged panel then goes to the trimming area where it is cut to the appropriate size, grade stamped, stacked, strapped and readied for shipment.

Compare that to the way oriented strand board is made. Logs in a variety of sizes are fed into a lathe-like machine, the bark is removed, but this time the machine chews the log up completely, producing thin flakes of wood about half as large as your hand. These flakes are sifted to eliminate the very tiny particles, mixed thoroughly with a dust of waxes and heat-triggered resin glues, oriented with their long direction (the grain direction) parallel with a moving belt then sprinkled on a large (8'-0" x 24'-0") forming bed. 1" thick layers of fibers are placed on this bed in alternating directions (thus the name oriented strand board) until the desired thickness is achieved (to make a 5/8" board, you need approximately 5" of material), then the forming beds are stacked twelve deep in a huge thermal press. The press activates and compresses the loose materials, simultaneously causing the wax covered resin to activate and bond the adjacent wood fibers together. Think of it as reconstituting the wood. Following the pressing, the beds move one after the next to the final cutting area where the new OSB panels are dumped off and saws trim the larger panels down to size. Due to the uniformity of the materials and the process, grade stamping is applied by machine as part of the assembly line just before the panels are stacked, strapped and readied for shipping. Samples of the manufactured product are taken and tested to assure compliance with the minimum requirements (also known as Performance Standards) for the type of panel being made. The advantages of the latter method are:

1. More wood fibers are used per log - there is essentially no log core left following shredding.
2. The requirement for large diameter logs (old growth trees) is eliminated because the purpose of this process is not the generation of sheets of wood from large diameter trees, it is the production of uniform sized flakes/fibers.
3. The variety of trees which can be used is increased. Scrub or fast-growing trees like aspen can be used as easily as the more commercially viable species like Douglas Fir/Larch.
4. The uniformity of the manufacturing process and associated testing requirements allows the manufacturer to produce a panel that meets a specific set of performance criteria. Regional variations in materials can be compensated for without undue burden on the manufacturer. If a particular project calls for a special panel, appropriate materials and adhesives can be combined to make it.

Is OSB a superior product to plywood? No, not unless it is fabricated to possess superior strength. But its advantages coupled with the resource issue will certainly make it the preferred material in the coming years.

Scott McVicker, S.E.