The Strength of OSB vs. Plywood
Plywood once held a monopoly as the material of choice for sheathing and roofing -- until oriented strand board (OSB) was introduced in the late 1970s. Today, carpenters, builders and homeowners can choose between the two materials for residential and commercial construction. While both are strong and durable building materials, their advantages vary. The final choice depends on how and where you need to use the materials.
Oriented strand board is primarily used as a sheathing material for walls and roofs. It is composed of coniferous woods, which are processed and cut into strands, then compressed with resins and wax into sheets. The strands form a two-ply mat, which runs lengthwise on one layer and perpendicular on the second ply. The cross-ply technique makes OSB boards strong and durable.
Plywood is an engineered composite wood material made from thin sheets of wood veneer, or plies. One panel of plywood can have five or more layers or plies. The plies are glued together with adhesives and resins and bonded together using heat and pressure. Unlike OSB, multiple, thin plywood layers are slanted at angles. This makes plywood structurally stable, with minimal tendencies to warp, crack, shrink or twist. Like OSB, plywood is used for indoor and outdoor applications as a sheathing material for walls and floors and to structure roofs. Plywood is also used for building cabinetry and furniture.
The weight of a panel can have an impact on the strength of a panel. Plywood panels are typically lighter in weight. The average weight of a plywood board made from southern yellow pine can range from 34 to 36 pounds per cubic foot, while an OSB board made of southern yellow pine ranges from 38 to 42 pounds per cubic foot.
Strength to Withstand Moisture
OSB has a tendency to delaminate, or swell, more than plywood when it is exposed and saturated by water. The swelling is particularly evident around the edges of panels and shows when the panels dry. In testing conducted by the American Plywood Association, where plywood and OSB boards were exposed to high amounts of water, OSB was found on average to swell by 10 to 15 percent, while swelling for plywood ranged from 6 to 8 percent compared to OSB, which had swelling percentages that ranged from 10 to 15 percent.
The standard measurement for building materials is called the R-value. Plywood and OSB have similar R-values. According to the American Society of Heating, Refrigeration and Air Conditioning Engineers, which actively tests and measures the R-value of various materials, a 3/8-inch-thick sheet of OSB has an R-value of 0.45, while a 3/8-inch-thick sheet of plywood has an R-value of 0.47. This means that in terms of insulation strength, plywood and OSB have relatively similar capabilities. In fact, ½-inch-thick panels of OSB and plywood have the same R-value rating of 0.62.
The tendency for OSB to swell can present issues on using OSB for residential applications such as using it for as a subfloor material and for roofing installations. Over time, the swelling may become more pronounced when OSB is subjected to high humidity or a major rain and water-related occurrence such as a hurricane or heavy snowfall. Once OSB or plywood delaminate, they are structurally unsound and should be replaced.