Performance Considerations of OSB Vs Plywood in Residential Roof Construction

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    When building or buying a home you want to make sure that you understand the materials used in its construction, the construction methods used for those materials and how this relates to the quality of the resulting project and your overall safety.

    For this reason there has been a number of research labs that have inspected the results of hurricanes and other natural disasters along with normal wear on building materials to come up with recommendations on the use of certain materials throughout our homes.

    One of the products that you are likely to see wide use in residential sheathing, subfloors, and roofing materials is OSB Plywood. This product is used by contractors and builders for one primary reason the cost. OSB on average will cost ten to twenty percent less than standard building grade plywood. In some cases the cost savings is even larger.

    Unfortunately there are some huge drawbacks with the use of OSB and the contributory factors do include the method used to install the product but also the makeup of the product itself.

    The first large inspections of OSB came after a series of hurricanes that hit the east coast of the US in the early 2000’s. Inspectors saw a large number of roofs built with OSB failing while the standard plywood roofs were withstanding similar conditions.

    There are other considerations than disaster conditions.  Not every home in a hurricane prone area will see damage but standard deterioration of OSB may cause problems for home owners which result in larger cost and more frequent repairs.

    One of the main problems with OSB is that any swelling that occurs due to moisture will remain in the structure of the material for life.

    On the other hand Plywood when confronted with moisture has been shown to be more forgiving because it swells uniformly and when moisture is no longer a problem the material will return to a uniform state.

    Basically what happens is as your home is built it takes many days to complete the wood framing. If OSB is used the material can swell at the edges and along nailing lines causing the material to fail. The failure may not be immediate and may happen at a later time after shingles and siding are applied.

    That means you can find a home that has no serious visible problems in a standard inspection but when stress is placed on the structure it can fail more easily.

    This is important to understand the original deterioration during building can cause problems instantly or many years later and the damage may not be visible when inspected through a standard home inspection at purchase.

    Additionally the problem has been stated by proponents of OSB that improper nailing and gluing of their product can cause catastrophic failure.

    When OSB is used in Roofing inspections after hurricanes found that center H bracing was not inserted and the number of and spacing of fasteners was not up to recommendation.

    Additionally edge swelling on roof surfaces due to moisture can cause visible lines at joints that echo through to the shingles causing visual appearance problems and sometimes early failure of shingle materials.

    Proponents of OSB say that it takes much longer for OSB to be fully saturated and that since the use of glue in manufacture reduces this problem over Plywood that can be fully saturated in days the use of OSB is better.

    Mold and rott is also a more dramatic problem for OSB products but this damage is not normally tied to the initial time needed for building the structure.  It is more common in areas where there is prolonged exposure to moisture and or direct water contact.

     

    Final Note about OSB vs Plywood in Residential Home Construction

    Personally I have used both materials in building many homes. The use of construction Plywood is becoming very limited and I can see advantages of both materials.

    Plywood will delaminate quickly in rain and OSB will deteriorate substantially when it has water contact.

    I think the most important thing to consider is that framing must be completed quickly and wood can not be exposed to moisture no matter the type.

    The faster you can get a roof and siding on a home the better the outcome. However you do not want to rush so fast that you miss your nailing patterns or forget recommended practices.

    As for roofing material I would rather install standard plywood over OSB for a few reasons. I believe it is a better product in this area because it is more structurally sound when you need to get up on the roof for repairs or other reasons. OSB just feels weak under your feet even when it is installed correctly.

    On the other hand for subfloors over joists OSB if installed correctly is a good product.

    My recommendation is that you protect the structure from moisture while it is being built. Tarp it with plastic if necessary or make sure you have a large enough crew and siding and roofers ready when framing is completed.

    Normally it takes only a few hours to wrap a home in tyvek and roof it so that should be immediate once the roof decking is on.

    As for its safety rating in hurricane or tornado areas I can not suggest one product over another and you should ask your local building official or structural engineer what they suggest.

     

     

     

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