Building A Home? Should You Use OSB Or Plywood Sheathing?

by Engineer Designer on November 9, 2011

The Building Profession is traditionally slow to change and adapt to new methods and materials. One example is the use of OSB sheathing verses the "good-ol" CDX plywood. I still find, in 2011, some builders who have never used nor plan to use OSB. When asked why, their answers seem to boil down to a few false ideas. Without further evaluation many dismiss OSB because they have seen the result of water damage to particle board in cabinet countertops. They remain convinced that OSB is similar and will come to pieces in high moisture. There is also, to some, a question of strength. How could a bunch of wood fibers that are pressed together be as good as the old standby, CDX plywood? I was a bit skeptical myself when I first came across OSB as an option - several years ago. However when I took a look at the product I had to admit it was quite well worth the try. I\'m going to go over a few myths that I have heard regarding OSB - but first let\'s cover a few definitions: Sheathing is used to stiffen a group of parallel framing members to keep them from racking. It is sometimes called a diaphragm. Plywood is a flat sheet of wood that is quite strong because of the way it is made. It consists of layers of wood (thin veneers) which are laminated together in alternating grain orientations. Since wood has a weak direction and a strong direction (relative to its grain pattern) these layers contain wood turned in different directions to gain strength in all directions. CDX plywood is a designation indicating a rough finish and a glue that is okay for exterior use. OSB (meaning Oriented Strand Board) is produced by taking small fibers from smaller, low diameter trees and bonding them with a resin, heat and pressure. These panels achieve excellent strength in all directions. Myths: Myth 1 OSB lacks the strength of CDX plywood. This is untrue. All building codes and dictating standards make no differentiation between OSB and plywood. For sheathing, they are considered structurally equivalent. Myth 2 - OSB will fall apart like particle board. This is untrue. Many have observed kitchen cabinets (many are largely made of covered particle board) which expand and even crumble after exposure to moisture. This would be quite unacceptable for a structural member of a home or building. However particle board is different than OSB in that it is not produced for exterior use. The truth is OSB is designed for longer exposure to exterior higher moisture conditions than even CDX. Myth 3 Plywood is just better than OSB and its what I have always used. I would never suggest anyone changing against their own certainty however my experience is that such considerations do not stand up to evaluation. If one looks one will likely conclude that plywood is not \"better\" than OSB. Factually OSB is made from younger and smaller fibers and are actually faster to product when one includes the growth of the wood. There is great consistency in the product and it is more stable, in dimension, than plywood when exposed to moisture. OSB has proven itself in the industry and is used in about 70% of homes in the U.S. I now specify it in my own designs and on my own projects unless the owner really insists on plywood. When this occurs I always make available the facts but will go with plywood if he or she chooses. Oh, and when folks compare the prices of OSB and Plywood (OSB sometimes costing half as much) this is usually the deciding factor. I believe that all of us should be open to newer and better ways to build our homes and buildings. OSB is one of those methods that I readily endorse.