Wood Underlayments for Resilient Flooring

Proper Selection and Installation of Underlayment Material Paves the Way for Durable and Attractive Vinyl Floors

by Paul Fisette- © 2000

Installing floor underlayment is routine for most builders: order a load of particleboard or plywood and fasten it to the subfloor. But careful selection and fussy workmanship become more critical when the finish surface is vinyl tiles or sheet goods. Resilient flooring is thin and subsurface irregularities are readily transmitted through thin vinyl. Nail-pops, swollen wood fibers and deformed joints are the villains of the resilient underworld.

Subfloor versus Underlayment

The terms “subfloor” and “underlayment” are often used interchangeably, but there is a world of difference between the two. A subfloor is a layer intended to provide structural support. Underlayment, on the other hand, is installed over a subfloor to create a smooth, durable surface upon which finish flooring is installed.

Resilient floor coverings demand a lot from underlayments. These underlayments must be hard, smooth, dimensionally stable and stiff. Hardboard, plywood and at least one OSB product offer smooth, hard surfaces that are considered safe for thin resilient flooring. Other popular choices are particleboard and American Plywood Association’s (APA) Sturd-I-Floor, a hybrid system that combines subflooring and underlayment functions in a single panel product.

Hardboard

Hardboard underlayment provides a thin, hard, smooth surface that is popular among remodelers. The Resilient Floor Covering Institute (RFCI), a Rockville, MD, association representing seven of the largest resilient flooring manufacturers, recommends hardboard and appropriately graded plywood as the only two acceptable underlayments for resilient flooring. Not just any hardboard will do, however. Only class 4, 0.215-inch, service-grade hardboard is recommended by RFCI. And this product is hard to get.

Few lumberyards carry this material. And even fewer manufacturers make it any longer, probably because many resilient flooring manufacturers do not allow any hardboard under their fully-adhered floors. Even with RFCI approval, manufacturers cite hardboard’s inadequate uniformity, poor dimensional stability and variable surface porosity as reasons to avoid its use under fully-adhered systems. However, some manufacturers will warranty perimeter-bonded flooring laid over hardboard.

Since resilient flooring is thin, any subsurface irregularities like swollen underlayment joints or nail-pops are “telegraphed” through the flooring, creating a bumpy finished surface. Product standards allow 1/4-inch service-grade hardboard to swell as much as 30%(.075 inch) when subjected to a 24-hour soak test. Compare this to 12% (.063 inch) for 1/2-inch particleboard and 2%-4%(>.021 inch) for 1/2-inch plywood and you’ll find reason for concern. Avoid using hardboard underlayment in bathrooms and kitchens where high humidity and occasional wetting is expected.

Hardboard is very densely packed wood fiber. When you drive a nail into it detached fibers are either forced beneath the panel, causing a bump, or the fibers mushroom out of the top near the nail head. Pre-drill nail holes and use thin-shanked nails for best results. When installing hardboard sheets, follow the nailing schedule shown in Table 1 and the placement specifications shown in Figure 1.

Particleboard

Particleboard is smooth, knot-free, and hard. It has no core voids and has great impact resistance. Sounds like a winner! But the RFCI doesn’t recommend its use for fully adhered sheet vinyl or tile floors.

Thickness edge-swelling is the number one complaint when it comes to particleboard installed under resilient flooring. Particleboard soaks up moisture at its edges first, creating ridges in the finish flooring. If vinyl tiles are used, they create a finish floor with many seams. These seams can expose particleboard underlayment to wetting and swelling when the floor is washed. Here, wood fibers will swell and tiles will lift around their edges. Particleboard is not a strong candidate for underlayment in moist locations like basements and bathrooms.

Rich Margosian, general manager with National Particleboard Association (NPA) claims, “The biggest problems are usually related to installation.” Examples leading to failure include laying particleboard underlayment:

  • before the structure is weathertight

  • over unvented crawlspace

  • over crawlspace without a groundcover

  • improperly stored on site (store flat & keep dry)

  • before plaster and concrete have cured dry

If particleboard is used, a glue-nail fastening system will produce the best results. White carpenter’s glue, not subfloor adhesive, is recommended by NPA. Spread the glue onto the subfloor with a paint roller and then nail down the panels.

OSB

While there are over a dozen APA-approved oriented strand board (OSB) subfloor and sheathing products, there are no APA-approved OSB underlayment products. Only one manufacturer, Weyerhauser, seems to be seeking APA approval for their 1/4-inch OSB underlayment, Structurwood. The lack of APA approval for Structurwood appears to be a procedural technicality based on the fact that APA just hasn’t developed a standard for non-plywood underlayment yet. APA and Weyerhauser promise a standard is in the works. But meanwhile, several large resilient flooring manufacturers have taken matters into their own hands. Some companies have tested and approved Structurwood for use under fully-adhered and perimeter-bonded floors. Weyerhauser backs its product with a one-year warranty.

Surface smoothness can be a problem with OSB underlayment because strands lying next to each other in the panel’s matrix may shrink and swell differently. The irregular surface will telegraph through thin resilient flooring. Weyerhauser claims to have solved this riddle with a proprietary stabilizing and conditioning process.

APA Plywood

Tried-and-true is appealing in an environment where everyone wants to blame the other guy for problems that might arise. Plywood gets a clean bill-of-health from everyone. All resilient flooring manufacturers approve the use of appropriately-graded APA plywood under all types of resilient flooring, provided it is installed correctly

>Approved plywood underlayments for resilient floor coverings have the following characteristics noted in their grade standards or stamp markings

  • “underlayment” or “plugged crossbands”

  • exposure classification

    • Exposure 1 – limited exposure to moisture

    • Exterior – repeated exposure to moisture

  • fully sanded face (not PTS, plugged and touch-sanded)

Lauan Plywood

Quarter-inch lauan plywood is a popular choice as an underlayment among remodelers. It has a solid track record and is readily available. All resilient floor manufacturers allow lauan plywood under at least some resilient floor applications and some manufacturers recommend it as an underlayment for all applications. Check with your manufacturer before using lauan plywood.

There are two types of lauan: Type 1, with an exterior glue and Type 2 with a water-resistant glue. Only Type 1, which is more resistant to delamination, should be installed under resilient flooring. The “type” is stamped on the very edge of the panel, usually on the 4-foot edge. Three face-grades are available – BB, CC and OVL(overlay). BB is the highest grade and should be used whenever possible.

APA’s installation guidelines for 1/4-inch plywood should be followed when installing lauan plywood, but installers BEWARE: Lauan is made under a wall-panel specification of the International Hardwood Products Association (IHPA). No manufacturing specifications support its use or gauge its performance as an underlayment.

Don’t Fill Nail Holes

Whatever underlayment you use, all gouges, gaps, chips and sunken edges must be filled with a patching compound and sanded flat. Most resilient floor manufacturers specify a portland cement-based compound mixed with latex. Other less effective fillers like calcium sulfate, plaster-of-Paris or gypsum mixed with a latex binder are readily available too. With the exception of hardboard, all the joints between panels must be filled and leveled. But don’t fill the nail holes. If a nail works loose, it can force the patching compound up and form a small bump in the finish floor. Instead, drive the nail heads just below the surface or the plywood and leave the holes unfilled.

Specifying and installing underlayment for resilient floor covering seems like a game of dodge-the-bullet. Everyone wants to pass the responsibility for a failed floor onto someone else. The safest course to follow is one paved by common sense. Know what you are asking your material to do and understand how your material will perform. And be sure to follow the manufacturer’s recommendations.

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Last updated: May 5, 2009