Better Floor Performance Starts With a Solid Subfloor System

By Mark Halverson

Beautiful hardwood floors are an investment expected to last a lifetime. To ensure the best long-term performance and appearance, careful attention must be given to proper subfloor installation and mitigating moisture risks. Proper installation starts with specifying and installing the right subfloor materials.

A variety of floor framing and wood structural panel products can be used in floor construction. To select the appropriate products and floor design, it’s important to consider the predicted loads, the structural requirements, and compatibility with the finished floor requirements.

Quality through and through
Hardwood flooring is most commonly installed over a single layer of tongue-and-groove combined subfloor-underlayment or sheathing-grade panels. APA Performance Rated Panels are manufactured to meet performance criteria for specific designated construction applications, as defined in the Voluntary Product Standard PS 2-10, Performance Standard for Wood-Based Structural-Use Panels. The standard applies to plywood and oriented strand board (OSB) panels. Plywood may also be manufactured to the prescriptive provisions of Voluntary Product Standard PS 1-09 for Structural Plywood. The standards are developed and maintained cooperatively by the wood structural panel industry, user groups, and the U.S. Department of Commerce, with APA – The Engineered Wood Association serving as sponsor.

In Canada, APA-trademarked OSB is manufactured in conformance with CSA-0325. The maximum joist support spacing and corresponding span rating is identified in Part 9 of the National Building Code of Canada (NBCC). APA-trademarked plywood is certified to CSA-0151, Canadian Softwood Plywood; CSA-0121, Douglas-Fir Plywood; or CSA-0153, Aspen/Poplar Plywood. Part 9 of the NBCC specifies minimum panel thicknesses for these products at common joist spacing.

The certification trademark that appears on the product is the manufacturer’s assurance that the product conforms to the standard indicated on the mark (Figure 1). Several independent agencies provide quality assurance services to U.S. and Canadian manufacturers of which APA – The Engineered Wood Association, TECO Corporation, and Timber Products Inspection are three major providers.

Use the trademark to specify the right panel for the job
Performance rated panels, such as APA Rated Sturd-I-Floor and APA Rated Sheathing, are easy to use and specify for floor applications because the maximum recommended spacing of floor joists – or span rating – is included in each trademark stamp. Panels are manufactured with span ratings of 16”, 20”, 24”, 32”, and 48”. These assume use of the panel continuous over two or more spans with the long panel dimension or strength axis across supports.

APA panel subfloor spans for 3/4” hardwood strip flooring are limited to maximum spacing of floor framing listed in Table 1. For improved stiffness, and to help eliminate floor squeaks when hardwood flooring is installed, spans reduced from the maximum are recommended by the National Wood Flooring Association (NWFA). The NWFA also recommends the use of minimum 19/32 performance category wood structural panels as a subfloor material when joists are spaced at 16” on center. For wider spacing, thicker panels are recommended.

The glued floor system
To minimize the potential for floor squeaks, APA recommends that the subfloor panels should be glued and nailed to the floor framing using recommended fasteners and construction adhesives conforming to ASTM D3498 or APA Specification AFG-01. For increased stiffness, tongue-and-groove or blocked panel edges also may be glued.

Glue-nailed subfloor panels up to 3/4” thick should be fastened with 6d ring- or screw-shank nails or 8d common nails spaced 6” on center along panel edges and 12” on center along intermediate supports. For subfloor panels thicker than 3/4”, and for nailed-only subfloor installation, see APA’s Engineered Wood Construction Guide (Form E30) for fastener size and spacing.

Note: The NWFA recommends gluing subfloor sheathing when framing is spaced more than 16”.

Preparation of subfloors for hardwood flooring installation
Both the subfloor and hardwood flooring are sensitive to moisture. Most hardwood flooring is delivered dry (less than 10 percent moisture content). Expansion and buckling of the hardwood flooring can occur if exposed to excessive moisture, or opening of gaps between flooring strips can occur upon subsequent drying, and floor squeaks may result. This reaction to moisture can occur regardless of the panel type used for the subfloor.

If the subfloor has become wet during construction, it should be allowed to dry prior to the installation of hardwood flooring. A hand-held moisture meter can be used to check the condition of the subfloor, which should be within a range consistent with recommendations of the hardwood flooring manufacturer. (Note: Moisture content measurement results will vary depending on the panel and meter type. In order to ensure accurate results, it is important that the moisture meter be calibrated before use. Calibration should be conducted in accordance with the moisture meter manufacturer’s recommendations.)

For homes with crawlspace floors, the builder should ensure that the crawlspace is well-drained and dry when hardwood flooring is installed. Also, minimum 6-mil polyethylene sheeting should be installed as a vapor retarder on the ground in the crawlspace prior to installation of the hardwood flooring.

Inspect the subfloor for flatness between joists. When necessary, the builder should install extra blocking and refasten the subfloor to flatten uneven areas. Check the subfloor for squeaks or loose panels and refasten as necessary before installing the hardwood flooring.

The installer should inspect the subfloor for smoothness along joints between panels. Any ridges at panel edges should be sanded smooth prior to installation of the hardwood flooring using a heavy-duty floor sander with a moderately coarse grit sandpaper.

Hardwood flooring installation
Follow the recommendations of the NWFA for handling, storing, acclimating, and installing hardwood flooring. Installation of the flooring material onto the subfloor requires significant consideration of the job site, including ambient temperature and relative humidity at the time of installation. Where possible, hardwood flooring strips should be oriented perpendicular to floor framing for maximum floor stiffness. APA, in cooperation with the NWFA, created the following checklist, for floor installers to ensure the best long-term performance of the floor system.

Floor installer’s checklist: Seven steps to minimize the risk of moisture

  1. Be sure to have a vapor barrier membrane in place on the crawlspace ground and ventilate/condition the crawlspace or basement prior to wood floor installation.
  2. Consider methods to remove excessive moisture from the framing/floor sheathing and the rest of the interior of the building prior to installing the flooring (such as using an auxiliary dehumidifier in the short term).
  3. Follow the recommended fastener schedule or consider augmenting the attachment by spacing the fasteners at closer intervals. For example, if the recommendations for fastening are 8” to 10” apart, keep fasteners no further than 8” apart.
  4. Make sure that the length and diameter of the flooring fasteners meet or exceed flooring industry or the manufacturer’s recommendations. Flooring manufacturers typically recommend a minimum length of fastener. The greater the length of the fasteners’ embedment in the wood subfloor or the larger the shank diameter of the fasteners, the greater the fastener withdrawal capacity.
  5. The recommendation for fastening of flooring boards is often within 1” to 3” of end joints. Keeping fasteners within 1” to 2” of end joints will better stabilize movement.
  6. Consider using an approved flooring adhesive in conjunction with the recommended fastener schedule for flooring being installed with a liquid vapor-retarding system to control moisture migration.
  7. In some situations, it may be necessary to build in expansion rows within the flooring system. This may include built-in gaps within the floor, t-molding transitions, cork, or other methods.

Useful Resources:
The following resources are available to assist in the planning, preparation, and proper installation of hardwood flooring over wood panel subflooring. These and other resources can be found on APA’s website at

  • APA’s Engineered Wood Construction Guide (Form E30)
  • APA’s Engineered Wood Floor Systems Construction Guide (Canada; Form R300)
  • APA’s Technical Note: APA Performance Rated Panel Subfloors under Hardwood Flooring (Form R280)
  • APA’s Technical Note: Prevent Callbacks in Wood Flooring Installation (Form T350)’

Mark Halverson has served as a manager for APA since 1994, starting with management of industrial markets and then moving on to management of APA’s field staff. In this capacity, Mark oversees a team of engineers, architects, and technical experts who provide engineered wood and systems information and recommendations to construction and design professionals. He can be reached at

2 thoughts

  1. Can 3/4” hardwood be laid parallel to joist? We have a long wall, which would seem that the hardwood would run parallel to that wall for aesthetics . But on inspection of the joist, they too run parallel. Please advise.

  2. Hello David – Your wood floor can run parallel to the floor joists as long as the installer takes follows one of these two options:
    1- Add a layer of 15/32″ plywood (minimum) to the existing subfloor.
    2- Add additional structural support between the joist/truss system in accordance with the joist/truss manufacturer recommendations. I’d recommend hiring a builder or a structural engineer before modifying or adding anything to the substrate system.

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