Choosing Between Oriented Strandboard and Plywood

Manufacturers of oriented strandboard and plywood claim both products work well. But using panels made of wood chips makes some builders nervous. Like it or not, osb will define the future of the structural sheathing market.

By Paul Fisette - © 2005

The issue for most builders who choose between plywood and osb is durability. Osb looks like, and is, a bunch of wood chips glued together. Detractors of osb are quick to say: “osb falls apart”. This opinion has a familiar tone. Plywood suffered the same criticism not too long ago. Delamination of early plywood sheathings gave plywood a bad name. Many “old-timers” swore by solid board sheathing until the day they hung up their aprons. Not many builders share that view today.

Technical Merits

All 3 model building codes use the phrase “wood structural panel” to describe the use of plywood and osb. Codes recognize these two materials as the same. Likewise, APA the Engineered Wood Association, the agency responsible for approving more than 75% of the structural panels used in residential construction, treat osb and plywood as equals in their published performance guidelines. And wood scientists agree that the structural performance of osb and plywood are equivalent.

Osb and plywood share the same exposure durability classifications: Interior, Exposure 1 (95% of all structural panels), Exposure 2 and Exterior. They share the same set of performance standards and span ratings. Both materials are installed on roofs, walls and floors using one set of recommendations. Installation requirements prescribing the use of H-clips on roofs, blocking on floors and allowance of single-layer floor systems are identical. The weights of osb and plywood are similar: 7/16-inch osb and 1/2-inch plywood weigh in at 46 and 48 pounds. However, 3/4-inch Sturd-I-Floor plywood weighs 70 pounds, 10 pounds less than its osb counterpart. Even the storage recommendations are the same: keep panels off of the ground and protected from weather.

Professor Poo Chow, a researcher at the University of Illinois, studied the withdrawal and head pull-through performance of nails and staples in plywood, waferboard and osb. Chow found that in both dry and 6-cycle aged tests: osb and waferboard performed equal to or better than CD-grade plywood. The results of another independent study conducted by Raymond LaTona at the Weyerhauser Technology Center in Tacoma also showed that withdrawal strengths in osb and plywood are the same. But, while the two products may perform the same structurally, they are undeniably different materials.

To begin with, the composition of each material is different. Plywood is made from thin sheets of veneer that are cross-laminated and glued together with a hot-press. Imagine the raw log as a pencil being sharpened in a big pencil sharpener. The wood veneer is literally peeled from the log as it is spun. Resulting veneers have pure tangential grain orientation, since the slicing follows the growth rings of the log. Throughout the thickness of the panel, the grain of each layer is positioned in a perpendicular direction to the adjacent layer. There is always an odd number of layers in plywood panels so that the panel is balanced around its central axis. This strategy makes plywood stable and less likely to shrink, swell, cup or warp.

Logs are ground into thin wood strands to produce oriented strandboard. Dried strands are mixed with wax and adhesive, formed into thick mats, and then hot-pressed into panels. Don’t mistake osb for chipboard or waferboard. Osb is different. The strands in osb are aligned. “Strand plies” are positioned as alternating layers that run perpendicular to each other. This structure mimics plywood. Waferboard, a weaker and less-stiff cousin of osb, is a homogeneous, random composition. Osb is engineered to have strength and stiffness equivalent to plywood.

Performance is similar in many ways, but there are differences

The study showed that solid boards installed at a diagonal were far and away the best system. Statistically, 5/8-inch plywood and 3/4-inch osb worked the same. But two significant observations were made during the study: Some of the plywood delaminated during the weathering experiment and new patches had to be spliced into the subfloor system. Also, researchers learned that the best floor of all was the control specimen, which had been protected from any weathering. This speaks volumes for the importance of protecting materials during transport, storage and early stages of construction.

Wall sheathing: No news is good news. All manufacturers of siding products I contacted agree that osb and plywood are equals. Kevin Chung, Engineer with Western Wood Products Association in Seattle assures us, “There have been no problems reported from the field. Nail-holding and racking resistance are the same.” Chung has noticed some concern about the use of osb among builders, but is quick to add, “There is no reason for any concern. Both products serve equally well as a nailbase.”