 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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 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.  Roofing company workers 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 plywood and why is it superior to wood planks? Which do roofing companies prefer to use?
Have you ever seen a Martial Arts demonstration that included breaking wood planks? Perhaps using feet, hands, or even their head? Have you ever inspected the planks themselves? It is easier to split a board with the grain than it is across the grain. Had those boards been cut differently, the Martial Arts demonstration might look somewhat different. Wood structures and wood furniture take advantage of this strength as the any loads or forces acting on the wood support the load itself. However for sheathing or roof decking, what is the direction of the force acting on the wood planks? Plywood is made up of many thin “plies” of wood veneer that are glued together. Each “ply” is laid out so that the grain is oriented in a different direction. This is called “cross-graining”, and among other things it contributes to dimensional stability, reduced potential to split when nailed close to the edges, and makes the strength of the panel consistent across multiple directions. The thin layers result in a product that is stronger than a single layer of wood, even if the single layer of wood may be thicker. These factors taken together are why plywood is superior to wood planks as a sheathing or roof deck material. Plywood is graded based on appearance and defects in the plies. For sheathing and roof deck plywood, since it will be covered by roofing materials to the outside and not visible to the inside, the important issue is structural and not appearance. There is no need to purchase Grade A (surface veneers free from all defects) and instead we use CDX (face has knots, defects, not appearance grade, not sanded). CDX is some pretty rough appearance plywood, but for a structural purpose it is more than sufficient. The important factor for sheathing or roof deck uses is that the plywood be exterior grade. Now then, next time I see a Martial Arts demonstration that involves breaking wood planks, I want to slip in a plank or two made of plywood. That will result in quite the headache… |
Bobby WilliamsWe are an experienced Lees Summit roofing company with over 15 years experience in repair, service and installation. Archives
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