Nailing down a quality hardwood floor with OSB

by Elaina Adams | April 1, 2012 11:22 am

[1]

By Chris Degnan
Hardwood floors are popular for their striking grain patterns, rich colours, and natural beauty. Their eye-catching esthetic is not solely dependent on these surface characteristics, however, as what goes under a floor is often as important as the finish material itself.

As the support structure for the flooring, a sound subfloor provides stability underfoot and helps enable a high-quality finish. Excess moisture in the substrate can damage overlying hardwood flooring, potentially creating cupping, cracking, or other defects.

To ensure a first-rate, visually appealing hardwood floor, building professionals are increasingly using oriented strandboard (OSB) subfloors. Engineered to optimize value and performance, OSB panels are typically more consistent than plywood, knot- and void-free, and lower in cost. OSB panels provide a flat, solid base for floorcoverings, and help yield even, stable, and long-lasting floor finishes.

OSB attributes
OSB is a structural panel with uniform strength and stiffness. Manufacturers produce the material by applying durable adhesives to thin wood strands and aligning them into layers of alternating strand orientation for strength. The stacked layers (typically four deep) form large mats, which manufacturers bond together under high heat and pressure.

[2]

While some early OSB formulations were prone to edge swell and delamination—causing some building professionals to favour plywood—manufacturing advances have improved the moisture resistance. Now, some products have 50-year limited lifetime warranties against delamination. Today, nearly all OSB panels are produced with ‘exposure 1 bond’ adhesives. As with plywood, OSB panels with this bond classification are unsuitable for permanent exposure to weather, but resist the effects of moisture during construction delays. Most manufacturers also treat OSB panel edges with sealants to further protect against moisture intrusion. Additionally, panels are available with self-draining grooves on their edges to eliminate standing water during construction.

All major North American building codes, including the National Building Code of Canada (NBC), recognize OSB for structural applications. Such panels meet stringent performance criteria in Canadian Standards Association (CSA) O325, Construction Sheathing, and the U.S. Department of Commerce (DOC) PS2, Performance Standard for Wood-based Structural Panels.

Creating a high-quality floor system
With proper specification and installation, OSB works with other structural elements to produce a consistent and stable base for hardwood floor finishes. To aid in this process, OSB manufacturers provide a range of technical information on product capabilities and performance, along with installation tips for OSB subfloors.

This article summarizes some of the general factors for using OSB subfloors under hardwood flooring. To keep from voiding warranties (as with any subfloor material), it is important to check with the hardwood flooring manufacturer for specific requirements.

[3]

Installation
Contractors can saw, drill, sand, and fasten OSB panels easily with standard tools, though carbide-tipped blades tend to wear best.

Most manufacturers recommend leaving a 3-mm (¹/8-in.) gap between panels to allow for normal linear expansion. Some panels have tongue-and-groove (T&G) edges to provide edge support and a self-gapping fit. For such panels, it is necessary to ensure the correct side is installed down so the profiled edges fit together. Manufacturers typically stamp the panels to show which side goes down.

To help stiffen the entire floor assembly, an approved adhesive is applied on the joists before placing and fastening panels. Using adhesive in panel grooves also helps lessen floor movement.

While a typical OSB floor panel thickness is 18 mm (²³/32 in.) in residential applications, using thicker 22- or 29-mm (⁷/8- or 1 ¹/8-in.) panels can boost floor performance—reducing vibration and providing a more rigid base for hardwood floors. Thicker panels usually cost more, but it is possible to achieve a net savings for the overall floor assembly, as fewer joists might be needed. Specifically, thicker panels can allow an increase in joist on-centre (oc) spacing from 406 mm (16 in.) to either 488 or 610 mm (19.2 or 24 in.), depending on the building type, joist size, occupant expectations for floor performance, and local code requirements.

Moisture management 
A critical part of the installation process is to pay careful attention to the moisture content of the subfloor and hardwood flooring—problems can arise later in the finished floor if the moisture differential between the OSB and hardwood is too great. For example, OSB that has absorbed moisture on the jobsite contracts as it dries, causing loosening around nails and staples that attach the hardwood flooring. This can result in pops when occupants walk across the finished floor. If there is a high moisture differential, with the OSB panels having a higher MC than the hardwood, moisture will migrate into the hardwood, potentially causing the flooring to cup or buckle.

[4]

To help ensure balanced moisture content (MC) levels between the subfloor and hardwood flooring, it is important to first confirm the OSB is adequately dry. While a subfloor may appear dry on the surface, moisture content within the panels may be too high. Although it can only accurately be determined with a moisture meter, checkpoints to help identify whether the subfloor might require additional drying time include:

• length of time since construction;
• most recent exposure to rain and/or standing water;
• presence of factory-cut drainage grooves in OSB (this reduces the drying period by preventing standing water during construction); and
• whether HVAC equipment is installed and running.

When testing wood subfloor moisture content, one should use policies set forth by the U.S. National Wood Flooring Association (NWFA). One NWFA recommended policy is to use a moisture meter on at least 20 locations on the OSB subfloor per 93 m² (1000 sf), and average the results. While a MC of 10 per cent or less is the accepted industry standard, one must still ensure the panels’ MC aligns with the hardwood flooring manufacturer’s recommendations. Further, even if the MC average is within an acceptable range, high readings in one or more areas indicate trouble zones that need additional attention. Areas prone to moisture include portions of the subfloor near exterior and plumbing walls, as well as panels over crawlspaces.

Hardwood acclimation
Proper moisture management requires acclimating the hardwood flooring to the interior environment, which aids in preventing gaps between boards, heaving, or buckling. Hardwood flooring typically arrives at the jobsite with a MC of six to 10 per cent. Since moisture tolerance varies by wood species, floor type (i.e. solid strip versus engineered), and strip width, it is important to check the hardwood flooring manufacturer’s specifications.

[5]

To protect the hardwood flooring from absorbing moisture before installation, it is crucial to keep materials covered before and during delivery, and then wait to deliver the flooring until after the building is enclosed and the temperature and humidity conditions match occupancy levels. It is also important to unwrap the flooring and allow it time to adjust to the interior heated/air-conditioned environment per the manufacturer-recommended acclimation period.

As with testing the OSB subfloor, it is also important to use a good-quality moisture meter to confirm moisture levels in the hardwood flooring before installing it. NWFA recommends sampling the moisture in 40 individual boards for every 93 m² of flooring.

NWFA also recommends installing hardwoods only when MC is within four per cent of that of the subfloor for solid strip flooring less than 76 mm (3 in.) wide. The recommendation for wide-plank flooring (i.e. wider than 76 mm) is to ensure moisture levels are within two per cent of the substrate’s moisture content before installing the flooring. Recommended moisture differential values for engineered hardwood flooring might vary from these figures, as these types of products incorporate a mix of wood species or components. (The manufacturer should confirm this.)

Subfloor preparation
After the OSB and hardwood flooring are adequately dry, and the hardwood flooring has been acclimated, it is important to evaluate the subfloor’s surface condition before flooring installation begins. Careful inspection can help determine a floor’s short- and long-term performance. Conditions to evaluate include:

• subfloor panels’ proper attachment to the underlying joists—in other words, whether there are any missing or misplaced fasteners (e.g. ‘shiner’ nails or screws);
• panels with edge swell or delamination;
• presence of contaminates or debris that might interfere with flooring adhesives; and
• surface irregularities, such as popped nails.

In addition to visually inspecting the substrate above and below, it is important to walk the floor in all areas and multiple directions. This can help detect loose panels, excessive movement, and squeaks that can be fixed before the floorcovering is installed.

Additional subfloor preparation includes:

• sanding the surface to manufacturers specifications, as needed (some premium OSB floor panels are factory-sanded to help eliminate this requirement);
• removing contaminants, such as excess adhesives, oil, and grease; and
• sweeping, vacuuming, and cleaning.

It is also critical to ready the substrate for the specific flooring selected. For instance, some hardwood flooring products require covering the subfloor with a quality-grade 6.8-kg (15-lb) asphalt felt/building paper lapped 50 to 100 mm (2 to 4 in.) along the edge seams to minimize dust, reduce moisture movement, and prevent squeaks in dry seasons.

[6]

Beyond the subfloor
Oriented strandboard can accommodate glue-down, nail/staple, or floating hardwood installations. This makes it possible to install a wide array of hardwood species and flooring types, solid strip or engineered. Since hardwood flooring performance varies by wood species, flooring type, and manufacturer, it is important to consider several factors when choosing products.

Location
A floor’s location within a building can determine the amount of foot traffic it must withstand. Hardwood floors in a restaurant dining room, for instance, are exposed to frequent movement of waiters, customers, and chairs, as well as dropped dinnerware. In such applications, harder and denser woods are typically preferred given their better wear resistance. Woods that surpass red oak on the Janka Ball Hardness Test (i.e. 8894 kPa [1290 psi])—the industry benchmark for wear resilience—are more durable. Examples of hardwood species that exceed this threshold include ash, sapele, ipe, mahogany, and eucalyptus.

Another option for highly trafficked commercial applications is engineered hardwood flooring. Designed to provide greater stability, particularly where moisture or heat might pose problems, engineered hardwood materials resist cupping, swelling, and splitting.

Although it is reasonable to expect dense or engineered hardwood flooring options to withstand scratches and dents longer than softer hardwoods, it is important to note all hardwoods are susceptible to wear over the long term. Applying a protective wood finish, such as phenolic or polyurethane varnishes, can enhance the
flooring’s durability.

Surface appearance
A hardwood’s visual characteristics—including grain pattern, colour, growth marks, and plank size—help support interior design schemes. Dark tones are a primary choice within contemporary styles, as confirmed by a 2010 USDA Forest Service study, “Enhancing Utilization of Hardwoods in the Eastern U.S.,” which found consumers prefer dark-toned eucalyptus and cherry hardwoods. As esthetic preferences can change over time, it is important to select woods with good sanding and re-staining properties.

[7]Wood species such as eucalyptus and others that can absorb various stains—from oil- to water-based finishes—can be re-stained for a new look or returned to their original colour. While engineered hardwood flooring has less flexibility for refinishing than solid strip flooring, sanding and refinishing engineered products is possible with those that have a wear layer (typically 3.175 mm [¹/8 in.] thick).

Maintenance
Periodic cleaning of installed hardwood flooring per manufacturer recommendations is essential to maintaining long-term beauty. Generally, a hardwood’s finish—either surface or penetrating—determines maintenance levels. Hardwood floors with penetrating treatments typically only require a weekly sweeping or dusting, and occasional application of a no-wax floor cleaner. To repair deep scratches or heavy wear, one should refer to the manufacturer’s guidelines or consult with a qualified floor care professional.

It is also important to note hardwoods often contain certain acids in their cellular structure that begin to amber when exposed to sunlight. Known as ‘patina,’ this colour change is not a defect and varies by wood species. The wood will reach its own natural warmth and patina level and stop ambering after a certain period.

Conclusion
The increasing number of buildings that use oriented strandboard panels for an enduring platform evidences its success as a subfloor material. It is strong, consistent, and dependable, and can help enhance subfloor performance and the long-term beauty of an overlying hardwood floor. As with other subfloor materials, appropriate moisture management is essential for a top-notch finish.

Chris Degnan is the oriented strandboard (OSB) product manager for Weyerhaeuser, which supplies structural frame and finish materials, including Edge Gold OSB floor panels and Lyptus eucalyptus solid and engineered hardwoods. He has 25 years of experience in the building products industry. Degnan can be contacted via e-mail at chris.degnan@weyerhaeuser.com.

Source URL: https://www.constructioncanada.net/nailing-down-a-quality-hardwood-floor-with-osb/

---