Roofs are framed with rafters that run from the peak of the rood to the eaves. These rafters are covered in a sheathing that consists of either plywood or oriented strand board. Plywood is typically used for sheathing because it’s water resistant, provides great structural stability and is low cost. However, oriented strand board is another favorite. It costs less than plywood and provides the same resistance and stability.
OSB and plywood share many similarities as far as structural performance. When it comes to sheathing either makes a good choice. So what makes a professional choose one over the other? Here are a few reasons.
Both OSB and plywood are made from very different components. Plywood is composed of thin sheets of veneer cross-laminated and bonded together with a hot-press. Plywood panels have odd number of layers so that the panel can balance around its center. This makes it less likely that the plywood will swell, shrink or warp.
OSB, however, is made up of thin wood strands that are ground from logs. The strands from OSB are dried, mixed with glue and wax and formed into large, thick mats and finally hot-pressed together forming panels. These strands are aligned and positioned perpendicular to each other through alternating layers. This gives OSB the ability to support more weight than plywood.
When it comes to density OSB wins. While plywood can be about 5 to 7 plies thick, OSB can be composed of as many as 50 strand layers compressed together. Thickness often depends on how spaced apart the rafters are. But, most of the time plywood is about ½-⅝ inch thick when sheathed on a roof. Whereas, OSB is usually about 7/16 inches thick. Because of its thickness, OSB is 2 times stronger in shear value than plywood. Also, because the two-knot holes overlap you never have to worry about soft spots in the panel like you do with plywood. The downside of OSB is that it is significantly heavier than plywood, although this does not affect its performance.
While all wood expands when wet, the biggest difference between OSB and plywood is how they react when they get wet. OSB expands slightly faster around the edges of the panel, which then continue to stay swollen even after they’ve dried. Whereas, when plywood gets wet it swells consistently across the panel, then shrinks as it dries. OSB takes much longer to dry out and tends to retain moisture, which can cause it to degrade a lot faster than plywood.
Plywood has a slight advantage with stiffness. Because of the way it is manufactured OSB is consistently less stiff, with about a 7% difference from plywood. Georgia Pacific says “plywood panel bending stiffness is 10% greater than OSB at equal joist spacing.”
As stated previously OSB is less expensive than plywood. OSB sells for about $3 to $5 per sheet less than plywood, which saves an average of $100 per household. However, because OSB costs much lower than plywood it is thought that OSB is a less well-rounded product.
Because OSB tends to absorb moisture slowly, it in turn dries slowly as well. This is not ideal when installing roofs because of the risk of rain. If it rains, snows or ices the moisture that is locked in the panel can lead it to rot. This makes living in areas with high precipitation and humidity difficult with OSB if roof maintenance isn’t kept up. If shingles shift and expose the OSB underneath deterioration can and will occur. Plywood, on the other hand, has good moisture resistance and fast drying times. This is a plus during construction time.
Whether you decide to go with plywood or OSB it’s best to weigh the pros and cons of each. Take into account the weather in your area, how high or large your roof actually is (OSB is cut in larger panels than plywood) and durability when choosing a sheathing material. While OSB is more versatile and consistent, plywood has a good, long-standing reputation and is moisture resistant and durable. Whatever choice you make, keep in mind that nothing can withstand nature’s elements if your roof is not properly and routinely maintained.