May 7, 2019|

The Basics of OSB Construction

  • Plywood, OSB and Engineered Lumber | via ECHOtape

Chances are that housing development going up around the corner has OSB (oriented strand board) as exterior sheathing.  Indeed, when you count walls, roofs, and sub-floors, OSB claims about 70% of the wood sheathing market in North America over plywood.

But why? Both products have benefits and drawbacks.  The most important thing is to understand and respect their limitations to avoid a wall or structure failures. Here’s what you need to know about building with OSB.


What is OSB?

Oriented strand board was created in the late 1970s as inventors extended the use of waferboard. It differs from other wood-scrap products because its long strips of wood are placed strategically rather than randomly. That’s where it gets its name, since the strands are “oriented” as a whole [Source: OSB Guide].

The process of making OSB involves cutting the logs into strands that are then dried, organized an­d treated with wax and binders. To form panels, these strands are grouped into big sheets and pressurized at a high temperature.

Manufacturers of OSB engineer their product to match a performance-rated scale. Manufacturers want to make sure their product is strong, ­multifunctional, uniform and workable. It comes in various sizes, usually ranging from a quarter-inch (6 mm) to three-quarters of an inch (18.5 mm), though customers may put in special size requests.

If OSB sounds a lot like plywood, you wouldn’t be entirely wrong.

APA – The Engineered Wood Association says that there’s no real difference between the two panels: OSB’s and plywood’s structural characteristics are equivalent, and they can be used interchangeably.  Both are rated Exposure 1 for temporary vulnerability to the weather; they have equivalent nail withdrawal resistance; and they’re installed using the same methods and construction details. 

However, outside of OSB’s lower price point, there are differences that builders should be aware of.


Pros and Cons of OSB Construction

OSB has more going for it than just cost. Green builders appreciate that it can be made sustainably from small, fast-growing trees, many of which come from tree farms rather than forests. It can also be bought in 9-foot sheets, which means you can sheathe a wall from the top plate to the bottom of the floor joists with single, vertical sheets, leaving no horizontal seams. OSB panels can be manufactured in lengths up to 16 feet (or sometimes even higher), while plywood is generally limited to 8 to 10 feet.

OSB boasts a more consistent density. While a sheet of plywood might be 5 to 7 plies thick, a sheet of OSB is made from as many as 50 strand layers packed and compressed into the same thickness. There’s no equivalent of the weak spots that can be left in plywood when knotholes in adjacent plies overlap.

On the downside, the material is a bit heavier than plywood—two pounds or more per sheet depending on its thickness and intended use—but this difference has no effect on the panels’ performance. It just takes a bit more muscle to handle on the job site.

The biggest difference between the two panels is how they react when exposed to large amounts of moisture over extended time periods. With the exception of projects in very arid regions like the Southwest, sheathing and flooring panels are routinely covered with rain, snow, and ice during construction delays. It’s here that plywood has the edge.

When plywood gets wet, it tends to swell consistently across the sheet, and then returns to its to normal dimensions as it dries out. It dries out relatively quickly, and the swelling is usually not enough to affect floor or roof finishes.

OSB takes longer to get wet than plywood but also takes longer to dry out. When used as a roof sheathing, this tendency to hold moisture means it can degrade faster than plywood when exposed to chronic leaks.

When OSB does get wet it also tends to swell along the edges, and those edges stay swollen even after the material has dried out. Swollen edges have been known to telegraph visible ridges called “ghost lines” through asphalt roof shingles.

Manufacturers insist that OSB’s moisture problems have been corrected, thanks to advancements in the engineered lumber marketplace.


On The Job: Choosing OSB or Plywood

Here’s something interesting:  Outside of North America, OSB is not commonly used in construction. In 2005, the combined production of OSB in Europe and Latin America was just 3.5 billion square feet – less than seven times as much as was produced in North America that year.  We also know that buildings in Europe vastly exceed the airtightness and efficiency of building in North America.  That’s not OSB’s fault, per se, but when half of the world’s top builders don’t use the product, we pay attention.

Building Science Educator Joe Lstiburek has this to say: “In the same microclimate, plywood works significantly better than OSB. Plywood dries much faster, while OSB needs to be more protected than plywood. OSB can’t get as wet and it needs to be more deliberately dried.”

Water penetration also makes decay more likely in OSB than plywood. Of course, tree species plays a large role in this determination. OSB made from aspen or poplar is relatively susceptible to decay. In one of the biggest consumer class-action lawsuits ever, Louisiana-Pacific (LP), a building materials manufacturer, was forced to pay $375 million to 75,000 homeowners who complained of decaying OSB in their homes.

So, it all boils down to water.  Are you in a wet climate or a dry climate?

With the exception of projects in very arid regions like the Southwest, sheathing and flooring panels are routinely covered with rain, snow, and ice during construction delays. This encompasses the Northeast, Midwest, Mid-Atlantic and Pacific Northwest regions.  Furthermore, in the Southeast and Gulf Coast, there’s the added issue of flooding and topographical drainage issues.  When cost is not a deciding factor,  these regions should choose plywood over OSB.

Austin Builder and High-Performance Building Professional Matt Risinger did a deep dive on the issue on his YouTube Channel.  You can watch the video here:

Spoiler alert — Risinger says, “If you have the budget and the choice, plywood is the preferred sheathing to use.  OSB isn’t terrible and your house won’t fall down because it’s sheathed with OSB, but in my opinion, plywood is the better choice.”


The Case for Coated OSB and Engineered Lumber

OSB manufacturers have been working on solutions to overcome the moisture issues surrounding OSB.  The newest building materials actually combine OSB (oriented strand board) with a facer or laminate applied. Prominent among these products is Huber Woods’ ZIP system.

Builders should be aware that OSB sheathing is available in various grades and the appropriate grade should be selected in correlation with the circumstances of the job. The grade level will have an impact on the cost of the product and not all grades will perform the same. The following list of available product grades are from the manufacturers’ websites.

Georgia-Pacific: 

  • Blue Ribbon® OSB for roofs and walls with a limited lifetime warranty.
  • DryGuard® Enhanced OSB with a Limited Lifetime and 200-day no-sand guarantee.
  • DryMax® High-Performance OSB with a Limited lifetime and 500-day no-sand guarantee.
  • ForceField® Air and Water Barrier as well as Thermostat® Radiant Barrier Sheathing.

Huber Engineered Woods:  

  • Huber Blue™ OSB Sheathing is for walls and roofs and has a 25-year limited warranty.
  • AdvanTech® Sheathing for walls and roofs with a 500-day weather resistance guarantee and a limited lifetime warranty.
  • PerormMax® 500 laminated facer, Tru-Spec® for millwork, and FormRite® concrete forming board.  
  • ZipSystem™ wall with a laminated weather resistive barrier, ZipSystem™ roof panels with a laminated underlayment and ZipSystem™ R-sheathing with continuous foam insulation applied to the back side of ZipSystem™ wall sheathing.

LP: 

  • LP® TechShield® Radiant Barrier sheathing and LP® FlameBlock® Fire Rated sheathing.

Norbord: 

  • Trubord is for use on roofs and walls and has a 25-year limited warranty.
  • TallWall, Windstorm, and Quakezone have variable sizes to reduce blocking and provide a continuous load path
  • Solarbord radiant barrier

Roy OMartin: 

  • WindBrace wall sheathing panels.
  • Eclipse Radiant Barrier Roof sheathing
  • Eclipse OSB Wall System sheathing with a radiant barrier air and water barrier
  • Tuff-StrandXL and StructWall XL are longer format sheets of these product types.

Telko T-Strand:

  • T-Strand™ OSB Roof and Wall sheathing carries a 25-year limited warranty.
  • T-Strand™ Pro Roof and Wall sheathing carries a 50-year limited warranty as well as a 365 day no-sand guarantee.

Weyerhaeuser: 

  • Edge Gold™ roofing
  • Radiant Barrier Sheathing (RBS)

Best Practices for Building with Coated OSB

Although OSB manufacturers have come out with promising new formulations, most consultants would still advise that the builder ensures that the OSB is protected from moisture as much as possible. 

Ideally, gaps around penetrations are first sealed with tape on the outer side of the OSB, and then canned foam was sprayed from the inner side of the sheathing. Although this method is somewhat redundant, it provides for a more robust seal because the tape directs the foam expansion to fill the gap between the penetration and the OSB instead of beyond the exterior face of the sheathing. Additionally, the tape protects the foam from water.

To ensure the best outcome, a builder must ask themselves the following questions:

  • Where are these structures being built?
  • In which weather conditions?
  • Are the subcontractors on the job properly trained on installation and best building practices?
  • Are the product warranty details being followed?
  • Which product would best protect my investment?

Good building practices will help to prevent damage to the OSB sheathing and result in its effective use.  Proper weather-resistive barriers and waterproofing materials to protect the sheathing should be used. And here’s the best news:  A coated OSB product with taped seams covers the walls to serve as both sheathing and weather-resistant barrier.

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