Components of a Roof – Oriented Strand Board (OSB): Part 3

Glove on Oriented Strand Board

In a previous blog topic, I identified what a sheathing or roof deck is, and identified that it is commonly plywood or oriented strand board (OSB).  I am going to delve a bit deeper; what is OSB and why is it superior to wood planks?  Also what makes OSB different than plywood?

OSB is relatively new to the building market, coming only into common construction uses, including sheathing or roof deck, since approximately 2000.

While plywood is made up of several plies that are veneered and glued together with wood grain oriented in specific directions, OSB is specifically engineered and made up of wood strands hot compressed with adhesive.  While those strands may look random, the directions of the strands are strictly controlled in the manufacturing process.  Plywood may have 5-7 layers; OSB may have 50 layers.  In the plywood blog I identified that the plies in different directions increase the strength of plywood vs. wood planks, with OSB this benefit is magnified somewhat as the engineering and manufacturing controls can result in product where the strands are oriented in many more directions.  For structural grade plywood, we already identified that there could be knots and defects, and this could result in voids internal to plywood sheet where you cannot inspect the product.  OSB is compressed and will have no voids; further demonstrated by OSB weight (a sheet of OSB will weigh more than the same size sheet of plywood).  Since OSB is a more highly processed and engineered product, it will be more consistent than plywood.

 As with structural grade plywood, since OSB used as sheathing and roof deck will be covered, appearance doesn’t matter.  Which is a good thing, OSB does not have an attractive finished surface.  It was engineered as a structural product though, and for that purpose it works admirably.  As a specifically engineered product serving a specific purpose, it is definitely superior to wood planks.

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