Components of a Roof Series – Plywood vs. OSB - Part 4

Storm Damaged Roof

In previous blog topics, I identified what a sheathing or roof deck is, and identified that it is commonly plywood or oriented strand board (OSB).  I have further expanded this discussion to identify how plywood is manufactured, how OSB is manufactured, and why both are superior to wood planks that may have been used in distance past.  However one of the questions I regularly field is OSB vs. plywood, which is superior?

From a roofing contractor perspective, for new house construction the sheathing or roof deck is installed by the carpenters.  For replacement roofs, for jobs the roofing contractor tears off the old roof, we have an opportunity to inspect the sheathing or roof deck and replace is necessary.  Where we see damages to sheathing or roof deck, it is usually the result of a roof leak.

Some factors that enter into the homeowners or roofing contractor’s decision to use OSB vs. plywood for repairing damaged sheathing or roof deck include typically limited to cost.  As both are wood commodities the price of plywood and OSB are variable and change almost daily.  Sometimes plywood is cheaper, sometimes OSB is cheaper.  When the contractor built your house, you can bet this was the deciding factor.

 While some roofing companies will sell you on performance differences, the Engineered Wood Association identifies that plywood and OSB are structurally equivalent.  Among the general public, there is a perception that plywood is higher quality; probably based entirely on appearance as plywood is still recognizable as wood and has an appearance they are familiar with.  For this reason alone, the homeowners decision would drive plywood vs. OSB decision.

I will tell you that there are distinct differences in performance with prolonged exposure to water.  With prolonged exposure to water plywood is definitely the superior product.  When plywood gets wet, it swells consistently and will return to its original dimensions as it dries; not so with OSB.  When OSB gets wet, it can remain swollen as it dries out, resulting in sheathing or roof deck that is no longer smooth and this will telegraph through to the finished roof when using asphalt shingles (we previously identified that smooth sheathing or roof deck is critical).

 As a roofing contractor that has been in business over 25 years, I didn’t stay in business this long because my roofs leak.  Since my roofs don’t leak, the decision to install plywood vs. OSB is job specific and includes input from the homeowner on what they prefer and the finished cost.

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