Understanding The Many Sides Of Plywood

sure has changed through the years. The that was available when I was beginning my career in the construction industry consisted of thin layers (sheets) of wood, alternating the grain of each layer at a right angle to that of the previous layer. This layering is what gave the its strength. Layers of became known as ply’s, 3 ply had 3 layers, 5 ply had 5 layers. Another type of was called multi-ply, which consisted of many thinner layers. The types of adhesives used in bonding all ’s together classified it as interior use, exterior use, or even marine use.

Grading of plywood signifies it applicable uses as indicated by a letter system, A,B,C,D. Each stamp will have two letters one for each side.  A grade of A signifies the side is sanded smooth with no gaps, knots or voids, while B grade will have football shaped “patches replacing those knots, gaps and voids. The next grade, C grade allows gaps, knots and voids and D grade allows more gaps, knots and voids. The exterior stamp is CDX. One side is C grade the other D and X signifies exterior use

A new type of plywood product was introduced in 1949, called wafer board. It was an engineered wood product that did not require large trees for its manufacture.

Wafer board consisted of small strips of wood, approximately 2” long and 1” to 2” wide randomly placed together with adhesives to form 4×8 sheets. I personally used this product in the 1970’s as shelving and a non structural sheathing. In the mid to late 1970’s this wafer board morphed into OSB (oriented strand board).

OSB is made up of strands of wood approximately 1” x 6”. These stands are orientated into specific positions relative to each other, and glued together.  Their orientation is the feature that makes OSB different from wafer board and adds strength.

In 1980 OSB production was about 751 million square feet. By 1990 production had skyrocketed to 7.6 billion square feet, and by 2001 OSB had surpassed traditional plywood in production and sales. In 2005 OSB production was at 25 billion square feet.

OSB like other lumber products is graded, specifying its use. There are 3 grades in the United States, sheathing, structural 1 sheathing, and single floor. Sheathing is for use in construction applications, such as covering material for roofs, subfloors and walls. The next grade structural 1 sheathing is a sheathing panel that meets additional requirements for cross-panel strength and stiffness. Single floor grade is for use as a combination subfloor and underlayment. Next to the grade will be panel thickness and span rating, which specifies the allowance for roof and floor spacing.

The above information will give you the basics of the different types of plywood and grading. There is a lot more information available about plywood including specialty types and their uses. Visit http://www.apawood.org/plywood for all you need to know about plywood.

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