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OSB Oriented Strand Board Sheathing:
This article defines and illustrates the properties of OSB or oriented strand board roof or wall sheathing.
This article series describes wood products used in construction including engineered lumber, OSB, and Plywood products.
Page top photo: OSB exterior sheathing being applied to a Poughkeepsie, New York garage during reconstruction and repairs in 2011.
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Definition & Characteristics of OSB - Oriented Strand Board
"OSB" or Oriented Strand Board wood products is similar to LVL and PSL, but OSB is produced in sheets (typically 4' x 8' in size) and is constructed of shorter strands of wood or even (in the case of 4' x 8' sheets) wood chips that may be roughly rectangular in shape.
Just above our photograph shows OSB subflooring. [Click to enlarge any image].
Developed in the 1980's, oriented strand board is an engineered wood product in which strands and flakes of wood are cut from straight, low-knot small-diameter logs, usually aspen or white birch.
The wood strands and flakes of OSB are glued (or "bonded") together with a phenol-formaldehyde resin binder (forming a waterproof glue), heated (to at least 120 degF), and are compressed and flattened using pressure.
Other chemical binders may be used in OSB products, and the OSB sheathing surface may have a wax coating to improve water resistance of the product.
Our photographs of OSB "plywood" below illustrate that the wood fragments glued together under heat and pressure are deliberately oriented randomly to produce greater strength than would be achieved if these small individual fragments were all oriented in the same direction
Our OSB sheathing photo above shows fire retardant treated plywood roof sheathing and below, OSB sheathing the roof of a multi-unit condominium building.
Notice that to meet fire codes, for one truss-bay adjacent to the firewall a fire retardant-treated FRT plywood was used in lieu of OSB.
In an OSB panel the two exterior surface layers of wood strands are oriented parallel to the long axis of the panel. In the interior OSB panel layer or "core layer" strands are oriented either randomly or across the short axis of the OSB panel. In overall thickness the ratio of face panels to core panel ranges from 40:60 to 60:40.
According to the Universite Laval and also Timberco, the dimensions of wood strands used in OSB are specified in industry standards; most producers of OSB use a combination of strands that are up to 6" long and 1" wide or from another source, 19-40 mm in width and 90 to 100mm in length.
OSB is a modern wood product that developed from earlier 1970's "waferboard" product. In 1990 the Structural Board Association was formed. By 1996 there were 38 OSB producers in North America.
But unlike waferboard whose composite wood chips were place randomly, an oriented strand board product is made from wood chips that are deliberately oriented with respect to one another to provide greater strength.
As a result, modern OSB products are rated at the same strength as plywood products. OSB roofing panels are available with a perforated (breathable) foil radiant barrier affixed to the pane's interior surface.
Type Variations of OSB Specialty OSB Sheathing Products
At below-left you can see an Oriented Strand Board whose composing wood fragments are longer than the more rectangular wood chips from our OSB pictures above. Still the strands are oriented randomly for strength.
Below is a cedar OSB product used in construction of cedar-lined closets, a less costly alternative to solid wood cedar planks traditionally used for that purpose.
Mold Growth on OSB Oriented Strand Board Products
Our moldy OSB roof and attic gable end wall sheathing photo (left) illustrates that as with most other wood products, if left exposed in a damp or wet environment, some species of mold will grow readily on this material.
OSB, even OSB manufactured with waxes and preservatives intended to protect it from moisture, will in our experience ultimately show delamination or feathering or "frizzing" when left exposed to the weather.
In my OPINION superficial weathering like that shown above (at a VA hospital in New York) probably doesn't have a significant impact on the OSB sheathing's structural strength, but left exposed to moisure or weather for sufficent time we expect OSB, like other wood products, to become softened, damaged, and ultimately vulnerable to failure.
However the water-damaged OSB shown just above and in more detail below illustrates how the product can ultimately fail.
Failures may occur as loss of connections to the structure if the OSB is softened around nails or construction screws.
Failures may occur as point loads or fractures if water damaged OSB is, for example, in use as roof sheathing in an area subject to snow loads.
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