Plywood Vs. OSB

O

sb looks like, and is, a lot of wood chips glued together. This view has a familiar tone. Plywood suffered the same criticism not too long ago. Delamination of ancient plywood sheathings gave plywood that a bad name. Most “old-timers” chased by strong board sheathing before the day they hung up their aprons. Not many builders share that opinion today.

Background

Portland Manufacturing Company produced the initial structural plywood out of western forests in 1905. This plywood, such as all structural plywood created before the mid 1930’s, was secured using non-waterproof blood and soybean adhesive. Delaminations were routine until waterproof synthetic resins were created during world war II. The technical repair for delamination was inspired by the 1950’s housing boom. Now, southern pine plywood accounts for approximately half of all structural plywood offered.

MacMillan Bloedel opened the first viable waferboard center at Hudson Bay, Saskatchewan in 1963. Aspenite, the very first generation waferboard (known as chipboard by most builders), has been made in the abundant source of aspen located in the area (charge julio). Technology involving the random alignment of wood-fiber at waferboard shortly gave way to the growth of superior oriented strandboard. Elmendorf Manufacturing Company made the first osb in Clairmont, NH just 14 decades ago.

 

Technical Merits

Model building codes normally use the term “wood structural panel” to characterize using plywood and osb. Codes recognize these two materials as precisely the same. Similarly, APA that the Engineered Wood Association, the agency responsible for approving more than 75 percent of those structural panels used in residential building, treat osb and plywood as equals in their published performance guidelines. And wood scientists concur that the structural performance of osb and plywood are equivalent.

Osb and plywood share exactly the same exposure durability classifications: Interior, Exposure 1 (95 percent of structural panels), Exposure two and Exterior. They share the identical set of performance criteria and span evaluations. Both substances are installed on roofs, walls and floors using one set of recommendations. Installation requirements prescribing using H-clips on roofs, blocking on flooring and tolerance of single-layer floor systems are indistinguishable. But, 3/4-inch Sturd-I-Floor plywood weighs 70 pounds, 10 pounds less than its osb counterpart. Even the storage recommendations are exactly the same: keep panels from the floor and protected from the weather.

Professor Poo Chow, a researcher at the University of Illinois, studied the withdrawal and head pull-through operation of nails and staples in plywood, waferboard, and osb. Chow found that in both dry and 6-cycle outdated tests: osb and waferboard performed equivalent to or better than CD-grade plywood. The results of another independent research conducted by Raymond LaTona in the Weyerhauser Technology Center in Tacoma also revealed that withdrawal strengths in osb and plywood are exactly the same. However, while both products can perform the exact same structurally, they’re different materials.

To begin with, the makeup of each material differs. Plywood is made from thin sheets of veneer which are cross-laminated and glued together using a hot-press. Picture the raw log as a pencil being sharpened in a big pencil sharpener. The wood veneer is peeled in the log as it is spun. Resulting veneers possess pure tangential grain orientation because the clipping follows the growth rings of the log. Through the thickness of the panel, the grain of each layer is positioned in a vertical direction to the adjoining layer. This strategy leaves plywood secure and less likely to shrink, swell, cup or warp.

Osb is different. “Strand plies” are positioned as alternating layers which run perpendicular to each other. This structure mimics plywood. Waferboard, a less-stiff cousin of osb, is a homogeneous, random makeup. Osb is designed to get stiffness and strength equivalent to plywood.

Performance is similar in a lot of ways, but there are gaps in the service provided by osb and plywood. All wood products extend when they become wet. When osb is exposed to wet conditions, it expands faster around the outside of the panel than it will in the middle. Swollen advantages of osb panels can telegraph through thin sheeting such as asphalt roof shingles.

The period ghost roof or lines ridging was coined to describe the results of osb edge swelling under thin shingles. The Structural Board Association (SBA), a trade association that represents osb producers in North America, has issued a technical bulletin outlining a strategy to prevent this phenomenon. SBA correctly suggests that dry storage, proper setup, adequate roof ventilation and use of a warm-side vapor barrier will assist in preventing roof ridging.

Irreversible border swelling has become the biggest knock osb. Producers have done a good job of addressing this issue at the production facility and during transport by coating panel edges. However, the reality is that builders don’t limit osb utilize to full-sized sheets. The advantages of cut sheets are seldom if ever treated within the specialty. Houses under construction get rained on.

It takes longer for water to soak osb and conversely, once water gets into osb it is extremely slow to leave. The more that water remains within osb the more likely it’s to rust. Wood species has a significant effect. If osb is made of aspen or poplar, it will get a big fat zero with regard to natural decay resistance. Many of the western forests used to manufacture plywood have moderate decay resistance.

In the past, we’ve heard that walls in many Southeastern houses covered with the Exterior Finish and Insulation System (EIFS) were rotting. Rigid foam insulation has been implemented over osb and coated with a stucco-like covering. When the exterior foam boards were removed, wet, rotted, dimmed osb was exposed. Osb was slammed in the press. The issue is in factn’t OSB’s fault. All cases I am familiar with were caused by improper installation of flashing or protective sheeting.

Louisiana-Pacific’s osb inner-seal siding made the information. The claims were that osb siding has been rotting on the walls of many homes in the South and Pacific Northwest. Both are extremely moist climates. LP said the issues were due to improper installation. But builders and consultants engaged in this case believe the substance does not operate in permanently subjected applications. In my knowledge, there hasn’t been a issue of scale connected with plywood siding. Osb, in its present state of development, is more sensitive to moist conditions. Plywood, though not immune, is somewhat pliable. Plywood really gets soaked considerably quicker compared to osb, but it is not prone to edge swelling and it dries out much more quickly.Irreversible border swelling has been the biggest knock on osb. Producers have done a fantastic job of addressing this issue at the manufacturing facility and during transportation by coating panel edges. But the truth is that builders don’t limit osb use two full-sized sheets. The advantages of cut sheets are seldom if ever treated in the specialty. Houses under construction get rained on. And if you utilize osb in a place of very high humidity, like over an improperly vented attic or over a badly assembled crawlspace, you are asking for trouble.

Osb reacts more slowly to changes in relative humidity and exposure to liquid water. The longer that water stays within osb the more probable it is to rust. Wood species has a substantial impact. If osb is created from aspen or poplar, it gets a big fat zero regarding natural corrosion resistance. A number of the western forests utilized to manufacture plywood at least have moderate decay resistance.

On the plus side, osb is a consistent product. It is really an engineered material. You never have a soft spot in the panel because 2-knot holes stink. You don’t have to worry about knotholes at the edge of a board where you’re nailing. Delaminations are virtually nonexistent.

Osb is perhaps 50 strands thick, so its attributes are averaged out over many more “layers” than plywood. Osb is always stiff. Plywood has a broader range of variability. During the manufacturing process, plywood veneers are randomly chosen and piled into panels. You may get 4 veneers of earlywood piled above 1 veneer of latewood. Who knows? Osb, normally, is 7% less stiff because it stays closer to its goal spec. Smaller trees may be used to earn osb. Wood fiber is used more effectively in osb.

Osb is more powerful than plywood in shear. Shear values, through its thickness, are approximately 2 times larger than plywood.This is one of the reasons osb is used for webs of wooden I-joists. But, nail-holding ability controls functionality in shear wall software. So both products perform equally well as shear-wall components.

Approved Use

It’s human nature to be scared of a new item. A builder’s reputation often hangs on the ability of new technologies to deliver on its promise. Homeowners anticipate contractors to select materials and systems which perform well. Builders want assurance from manufacturers that new products will work. Producers are not always right. But right or wrong, a manufacturer’s support is frequently where the rubber meets the road.

Subfloors and underlayments function as structural systems and as a foundation for flooring solutions. Osb and plywood are equals structurally, but floor producers make various recommendations regarding their use as a substrate.

The National Oak Flooring Association (NOFA) at Memphis recommends both 5/8-inch and thicker plywood, 3/4-inch osb or 1-x6-inch compact, group1 softwood boards installed in a diagonal beneath hardwood floors. In his analysis, Loferski mimicked what occurs on a real building website. He constructed several full-sized flooring out of planks, plywood, and osb and weathered them for 5 months prior to installing hardwood floors. Finished floor systems have been cycled within a living room to simulate the changes which happen in summer and wintertime.

The research demonstrated that strong boards set up in a diagonal were far and away the best system. But two important observations were made throughout the analysis: Some of those plywood delaminated during the weathering experiment and fresh patches had to be spliced into the subfloor system. Also, researchers learned that the best floor of was that the controller specimen, which was shielded from any weathering. This speaks volumes to the value of shielding materials during transport, storage and early phases of construction.

If you’re planning to use osb for a subfloor OR underlayment for your next tile flooring, you might wish to believe again. Joe Tarver, Executive Director of the National Tile Contractors Association, Jackson, MS says, ” Osb isn’t a suitable substrate to get ceramic tile, period!” NTCA lists osb, together with pressboard and luan plywood, as “not suitable” in its reference guide. It’s to do with depth swell. They feel that if osb becomes wet, it transfers stress and causes the tile to fail. Subfloors and underlayments function as structural platforms and as a foundation for flooring products. Osb and plywood are equals structurally, but floor manufacturers make various recommendations regarding their use as a substrate.

The National Oak Flooring Association (NOFA) at Memphis recommends both 5/8-inch and thicker plywood, 3/4-inch osb or 1-x6-inch compact, group1 softwood boards installed in a diagonal under hardwood floors. TIn his study, Loferski simulated what happens on a real building website. He built several full-sized flooring from planks, plywood, and osb and weathered them for 5 months before installing hardwood floors. Finished floor systems have been cycled in a living room to simulate the changes which occur in summer and wintertime.

RFCI installation specifications advocate plywood as an underlayment material. Osb is acceptable as a subfloor material. Manufacturers haven’t seen a deluge of failures because of the usage of osb under resilient flooring. But they have received complaints of advantage swelling that has telegraphed through their flooring products. Manufactures feel more comfortable guaranteeing their products when they are installed on plywood.

All producers 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 haven’t been any problems reported by the area. Nail-holding and racking resistance would be the same.” Chung has noticed some concern with the usage of osb among builders but is quick to add, “There is no good reason for any concern. Both products serve equally well as a nail base.”

Roof sheathing is a mixed bag. However, ARMA, NRCA and representatives from at least 2 roofing producers, Cellotex and TAMKO, prefer plywood roof decks. Warranties on shingles are extended to both substrates but manufactures feel more comfortable with plywood. Mark Graham, NRCA’s associate manager of technical services states, “We hear a lot of complaints related to dimensional stability. Along with a disproportionate number are related to osb. So we are a little bit careful.” Grahm also admits that APA, a company that he definitely respects, is standing firmly behind the osb product.

Florida’s Dade county is the only building code district in the nation that prohibits using osb for a roof deck. Damage to roofs during hurricane Andrew was originally blamed on OSB’s poor nail-holding power. Dade’s banning of osb spawned several research initiatives to explore the suitability of osb because of structural sheathing. Research conducted by APA, Chow, LaTona, and others have conclusively verified osb seaworthy. Many experts believe the ban makes no sense. Dade’s standing is perceived by many industry insiders to be a political maneuver to satisfy public concern.

Market Rap

Osb has earned its reputation as a low-cost substitute for plywood. In fact, recent price quotes from Denver, Boston, and Atlanta put 7/16-inch osb $3.00 to as much as $5.00 per sheet lower than 1/2-inch CDX plywood. This price spread means that a builder can save $700.00 on a 2,500 square foot house if osb is substituted for plywood sheathing on floors, walls, and roofs. A substantial savings to be sure. The trend among builders is to switch to osb. APA’s market data indicates that more than half the structural panels used in residential construction in 1995 were OSB. But the price is not the whole story.

A bumper crop of news stories highlighting contractor ripoffs has left consumers reeling. Reports indicate that some homeowners worry about builders “cheaping out” when they use OSB. Customers become suspicious that builders are trying to put something over on them: charging for an expensive product like plywood and substituting it with something cheap, like OSB. When it comes to structural integrity, the cost is less of an issue among consumers than structural performance.

“It looks like a bunch of junk wrapped collectively.” , is how one homeowner described osb to me personally. Are not there any more trees?” Public perception is that we are becoming stuck with scraps. The uninitiated do not appreciate the high-level of science and technology used to create engineered wood products. They believe that “glued-together” isn’t as great as “nailed-together”.

Customers don’t want technical explanations about several things. For instance: Many do not wish to know how blown-in fiberglass insulation performs differently than batt insulation. They typically do not wish to learn what weight roof shingle you’re using, or even what thickness floor joist you’ve specified. But, customers are nervous about engineered wood since all they see are small pieces of wood stuck together. Osb is so visually striking that customers need a technical explanation about this substance in their contractors.

In summary here at A Cottage Collection, we stick to the tried time-tested quality of plywood for our outdoor structure to insured the longevity of your outdoor structure. Weather in the Cincinnati area can be quiet wet and we maintain great warranties on our sheds with quality materials. Take a look at our amazing selection of outdoor sheds and structures.

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