PRODUCT PROS AND CONS: ORIENTED STRAND BOARD VS. PLYWOOD
Which panel is right for your homes?
If you’ve been around long enough, you’ve seen plywood go from being the top structural panel in the industry to becoming an also-ran that's now second to oriented strand board (OSB) in usage among home builders.
To get a sense how widespread OSB usage has become, you can do your own unscientific research. Drive through a subdivision under construction. House after house will be sheathed in the panels made with wood chips.
OSB hit the market in the late 1970s and was almost immediately a hit. According to the Structural Board Association, “in 1980, North American OSB panel production was 751 million square feet (3/8" basis),” says the Website of the Structural Board Association, which dissolved in 2008. “By 1990, this figure was 7.6 billion square feet. In 2005, this figure had grown to 25.0 billion square feet."
The product immediately found an audience with contractors and builders, despite the fact that many thought it looked like chipboard or particleboard, says Mary Jo Nyblad, sales and marketing manager for Boise Cascade’s plywood and particleboard division. “All it takes is for one guy to use it, and more followed,” she says. Boise, which used to make both types of panels, sold its OSB division to Ainsworth Lumber Co. in 2004.
Mainly due to its look, consumers and home buyers had the perception that OSB was cheap and an inferior product, but the panels found success among multifamily builders, Nyblad says. “It was readily accepted because there was no home buyer per se,” she continues. “It was a good fit.” The fact that the panel was economical also played a large role in its acceptance.
At first, plywood and OSB competed hammer and nail for market share. In the 1980s, Nyblad says, OSB started appearing on sidewalls and roofs, but plywood was still largely used for floors. Plywood tried hard to fend off OSB, but it could not compete well against OSB's lower cost. As a result, by the late 1990s, the panels had achieved parity in terms of usage, with OSB continuing to gain.
“In 2000, for the first time, OSB production marginally exceeded plywood production,” according to the Structural Board Association. “By 2006, OSB production grew to nearly 60% of the North American panel market share.” Manufacturers of both panel types agree OSB’s share of the production home market might even be closer to 75%.
To be sure, there is no difference between OSB and plywood as a structural panel, says APA - The Engineered Wood Association. The Tacoma, Wash.-based group is a nonprofit trade association that represents U.S. and Canadian manufacturers of plywood and OSB as well as other structural engineered wood products.
“Both products meet the local and national building codes,” says APA market research director Craig Adair. They are equally interchangeable for walls and roof sheathing, and for flooring underlayment, he adds. Moreover, both panels install fast and easily. Specified correctly, they perform as intended. Manufacturers from both sides claim their products offer better nail holding ability, but Adair says both meet the same requirements.
Still, after Hurricane Andrew tore through South Florida in 1992, leaving well over 250,000 people homeless and almost $30 billion in damage, the Miami-Dade County Board of County Commissioners banned OSB for use as roof sheathing.
Even though OSB dominates the construction market, some contractors and builders have problems with the performance of the product and have remained plywood loyalists. Nyblad says, for example, that there are regional preferences for plywood such as the Northeast, Northern California, Southwest, and Seattle in the rainy season. (In the dry season, these regions use OSB.) Plywood is particularly popular among DIYers and custom builders as well, she adds.
“We typically use plywood for all of our roof sheathing and subfloors,” says Steve Ronchelli, a senior project manager with custom builder Jim Murphy & Associates in Santa Rosa, Calif. Ronchelli says it’s partly due to the performance, but he allows that “it might just be a regional thing.” Northern California is close to the forest of the Pacific Northwest, so the lumber comes from nearby, he adds.
Brian St. Germain, an OSB technical expert at LP Building Products in Nashville, Tenn., says “the main stronghold for plywood is with traditional builders who have stuck with the [product].”
The main main complaint of OSB—its intolerance to moisture—has been largely fixed, manufacturers say and is no longer an issue. “We can get equivalent moisture resistance with different resins and waxes that create water repellency,” St. Germain says.
Each panel has its advantages and drawbacks, which makes it difficult for builders to choose the right product for a particular application. The decision only becomes more time-consuming given manufacturers's claims and counter-claims about a myriad of issues, including some stances that are supported by third-party sources and users and some that are not.
But enough information exists to glean a fairly accurate picture of what each panel--plywood and OSB--can and cannot do. To evaluate the options for your proejct, here's a handy guide that outlines the pros and cons of traditional plywood and OSB in various applications
OSB Pros
Versatile. In order to adapt to certain applications, OSB manufacturers can essentially just tweak the recipe for their product. Chris Degnan, marketing director of strand technologies at iLevel by Weyerhaeuser, says OSB “can be made in different formulations to suit various climates and budgets. This is especially important for floor panels, since OSB resists buckling and helps provide a flat and stable floor.”
Available in large sizes. Plywood can be produced in lengths from 8 feet to 10 feet, but that’s pretty much it. OSB, on the hand, says APA’s Adair, can be produced in panels up to 8 feet wide and in lengths of up to 16 feet. Some manufacturers say even 24 feet is possible. This is mainly due to production. An OSB plant can easily adjust to make longer boards, while plywood producers are limited by tree sizes.
Consistent. Anyone who has worked with OSB knows that the panels are dense and solid throughout the product. This is due to the manufacturing process. Consistent-sized thin wood strands are mixed with wax and adhesive and compressed with up to 1,100 pounds per square inch. Approximately 50 layers of strands make one sheet of OSB, according to HUD’s Partnership for Advancing Technology in Housing (PATH). “There are no soft spots such as those that can occur in plywood,” PATH says on its Website.
Good shear strength. “OSB is stronger than plywood in shear,” says a report by the Building and Construction Technology program at University of Massachusetts--Amherst. “Shear values, through its thickness, are about two times greater than plywood. This is one of the reasons OSB is used for webs of wooden I-joists.”
Resource-efficient. OSB is generally considered a resource-efficient product because of how it’s made. “OSB panels can be manufactured from a wide range of fast-growing species and from relatively small trees,” APA writes on its Website. The process uses the maximum amount of wood fiber from each tree, the group adds. The trees are often farm-raised, PATH adds, which reduces the demand for old growth timber.
Affordable. You don’t have to be a genius to figure this out. A simple trip to the average lumberyard or home supply store is all it takes to realize that OSB is almost always cheaper than plywood. “It can be $3 to $5 a panel less expensive than plywood,” PATH writes. “For a typical 2,400 square foot home, OSB will save about $700 if used as the subfloor, sheathing, and roof decking instead of plywood."
OSB Cons
Heavy. Depend on your point of view, OSB’s relative density could be seen as a bad thing.According to APA’s Website, a 23/32 plywood panel weighs 2.2 pounds per square foot compared to 2.4 pounds for an OSB panel of the same thickness. Atlanta-based Georgia Pacific, which manufactures both types of panels, says on its site that “although weights vary by species, on average, the weight of Southern yellow pine plywood is lighter than Southern yellow pine OSB.” The company continues: “Plywood is approximately 15% to 19% lighter than OSB. While the additional weight of OSB does not mean increased strength, it just means that it is heavier to handle on the job. In addition, OSB's higher weight means higher thermal conductivity (thus slightly less R value) than plywood.”
Lower moisture tolerance. Early on in its existence, OSB had a problem with moisture. (It should also be noted that plywood had similar performance problems with moisture early on its existence). “OSB used to swell a great deal or it could have taken market share faster,” Boise’s Mary Jo Nyblad says. “But manufacturers have improved the performance of the product.” Though third-party sources agree that the product’s moisture tolerances have greatly improved, many say it still does not perform as well as plywood. Georgia Pacific says that when plywood is exposed to moisture it swells evenly throughout the panel and returns to its nominal thickness as the wood dries. “OSB will remain swollen to some degree after it dries because the panel will still have the higher ‘compaction ratio’ that was present as of the date of manufacture,” the company says.
Prone to swelling edges and telegraphing. Even though manufacturers now use better adhesives and resins in OSB, the products’ edges are still susceptible to swelling once they are cut in the field. “The major disadvantage of OSB is that if it gets exposed to significant amounts of water or moisture, the edges expand by up to 15% ... especially if they are cut edges. This swell will then telegraph onto the shingles or some flooring," according to PATH.
Lower perceived value. Despite the fact that plywood and OSB meet the same performance requirements, the average consumer assumes OSB is an inferior product based on its look. “Many consumers are concerned when they see oriented strand board (OSB) being installed in their home,” according to PATH. “Because it costs less and looks different than plywood, they feel like they are getting a lower quality product, or that their contractor is trying to pull something on them. You need not worry; in many applications OSB has a comparable quality to plywood.”
Plywood Pros
Stellar reputation. Builders know and love plywood, and they trust its proven performance, whether they use this product or not. It’s the panel of choice for most DIY projects and can be found in a variety of applications throughout the home. To the average home buyer, it looks like real wood. It’s available in a wide variety of appearances, ranging from construction-grade products that are used for sheathing and underlayment to smooth panels for furniture and finish-grade work.
Strong and stiff. Plywood’s main attribute is that it’s stiff, strong, and durable. Citing an APA technical document, Georgia Pacific says “plywood panel bending stiffness is 10% greater than OSB at equal joist spacing.” This, the company says, results in stiffer floors that are less likely to be soft and springy.
Good moisture resistance and fast drying times. Clarence Young, vice president and general manager of Georgia Pacific’s structural panels business, says that even though OSB’s water absorption has improved, plywood overall is highly moisture tolerant and dries faster than OSB when it does get wet. Georgia Pacific now offers DryPly subfloor that has been treated to be even more moisture resistant. “It offers up to 40% less water absorption [during the normal construction cycle than uncoated plywood],” Young says.
Excellent as an underlayment. Though APA says the products are equal structurally, different flooring manufacturers recommend plywood under their products. “If you are installing a tile floor, the National Tile Contractors Association recommends that you not use OSB as a subfloor or underlayment because of the potential that swollen edges will weaken the tiles,” PATH writes.
The Building and Construction Technology program at University of Massachusetts Amherst says the Resilient Floor Covering Institute also recommends plywood over OSB under vinyl sheet-flooring. “[The Resilient Floor Covering Institute] installation specifications recommend plywood as an underlayment material,” the group says on its Website. ”OSB is acceptable as a subfloor material. Manufacturers have not seen a deluge of failures due to the use of OSB under resilient flooring. However, they have received complaints of edge swelling that has telegraphed through their flooring products. Manufacturers feel more comfortable guaranteeing their products when they are installed over plywood.”
Plywood Cons
More expensive than OSB. Though pricing for both OSB and plywood can sometimes be very similar depending on the market or region of the country, plywood generally costs more. Differential pricing hasn’t changed much over the years, Young says, but the difference itsef is noticeable. For example, at the time of this writing, Georgia Pacific says retail pricing in Atlanta is $5.22 sheet of 7/16-inch OSB and $10.97 per sheet for 3-ply plywood.
Panel voids. To make plywood, manufacturers shave thin sheets of veneer from a log. Once these sheets have been dried, the manufacturer applies adhesives and arranges the sheets alternating 90 degrees to each other and applies heat and pressure. The cross lamination of the sheets give the boards their strength, stiffness, and helps with expansion and contraction. But sometimes sheets have weak spots, which results in core voids in the panels. This can lead to reduced nail holding effectiveness. A quick glance on the edges of plywood, and the voids are often noticeable.
Limited sizes. Most builders get along fine with 4-by-8-foot plywood sheets, but that's also because they have to. Sometimes a large sheet could make life easier, but Young says more than 90% of plywood plants produce this basic size.
SOURCE : builderonline.com
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