Back when I started framing houses, subfloor and sheathing choices were limited to solid, spaced sheathing and a still relatively new building product, plywood. The latter was gaining market share, but many old-timers resisted the thin, bendable, sometimes delaminating sheets of cross-grained veneer.
But eventually not a single floor, wall, or roof had anything but plywood over the joists, studs, and rafters, and today the same could be said for oriented strand board (OSB). Just as with plywood, some builders regarded OSB suspiciously before it became well established.
Today, a new generation of sheet materials is pushing this category toward new levels of performance, with products made from more durable raw materials and healthier resins, and panels that combine features that address moisture control, air infiltration, and energy performance in addition to their structural functions.
Before we explore these alternatives, let’s review the pros and cons of the old warhorses, plywood and OSB, as the former has made a comeback among quality-conscious builders, and concerns over deforestation and indoor air quality have muddled the question of sheathing with either plies or strands.
PLYWOOD VS. OSB
Plywood consists of an odd number of sheets of wood, glued together with the grain of each ply in a perpendicular direction, to create a structural panel with shear strength in all directions. Plywood’s cross-grain provides strength and greater holding power for screws and nails than solid-sawn wood.
Oriented strand board (OSB) uses a similar engineering principle, but instead of creating the multidirectional structure with large sheets of wood glued together, manufacturers of OSB arrange small strands of wood (2 to 3 inches in length) into a cross-grain pattern, and then bind them into a solid, structural panel using adhesives, pressure, and heat.
From an environmental perspective, the notable difference between the two panel products comes with the natural resources required to make them. The sheets of lumber used to make plywood are peeled in thin veneers off a log with a sophisticated lathe. The logs are older and larger, and from a more limited number of tree species, than those shredded for strands of OSB. Also, the plywood peeling process leaves a spindle of wood at the center, whereas manufacturers of OSB shred the entire log.
Green building certification agencies recognize the ecological advantage of engineered lumber products, principally OSB, which is used not only to make sheathing, but also joists, rafters, and substitutes for dimensional lumber.
Nevertheless, plywood has maintained its place and has seen some gains because of its greater resistance to moisture, especially around the edges, and slight advantage in nail- and screw-holding power. Many flooring and some roofing manufacturers prefer plywood under their products because of its greater stability versus OSB in humid conditions. Miami-Dade County, Fla., prohibits the use of OSB roof sheathing, given a comparatively high failure rate once wetted during historic storms.
With the exception of Miami-Dade, all national and international building codes regard plywood and OSB as equal, and use the generic phrase “wood structural panel” to clearly denote that the code recognizes these two materials on par. The leading green certification agencies, the USGBC and the NAHB, provide points for both products.
Both products carry similar performance-based certifications, primarily from APA-The Engineered Wood Association, and the U.S. Department of Commerce Voluntary standard for Wood Based Structural Panels (PS1 and PS2) that allow consumers and inspectors to know the exposure (outdoor, indoor, or marine), strength (structural capacity), span rating over framing members (adequate for 16, 24, 32 inches on-center), surface finish quality, and, more recently, compliance with air-quality standards.
But OSB has become the clear leader in construction not so much for its ecological as economic advantages: OSB is generally several dollars per sheet cheaper.