Technical Troubleshooting: Starting from the Bottom – Evaluating Wood Substrates

Evaluating wood substrates is one of the most critical aspects of successful wood flooring installations. This is because the substrate is the foundation upon which the wood will be placed, and any deficiencies with the substrate could impact the wood. It is, therefore, important that you understand how the conditions of the wood substrates can affect the performance of the wood floors, and how to address those conditions to maximize the end results.

Joists and Trusses

A joist is a horizontal support that reinforces a flooring system. It runs between foundations, walls, or beams and is typically laid out in repetitive patterns. Joists can be made of wood, engineered wood, steel, or even concrete.

Traditional lumber joists are sized according to the species and grade of wood, spacing and span of the joists, and the design load. I-Joists have a higher strength-to-weight ratio than lumber joists and are used for longer spans. They use top and bottom flanges that are typically solid lumber, structural composite lumber, or laminated veneer lumber (LVL).

Floor trusses are usually made up of lumber on top and bottom chords, with an open web configuration, and also use metal plates. The lumber in the floor truss flanges is usually oriented flat-wise. These trusses are often used for longer spans than lumber joists.



Identification of Proper Subfloor Panels Based on the Joist Spans

When installing wood flooring, follow the information in the chart above (from NWFA Installation Guidelines) in order to understand which subflooring material is required to proceed with wood floor installation.

Subfloor Panels
Subfloor panels should follow the most current standards: U.S. Voluntary Product Standard PS1 Construction and Industrial Plywood, and/or U.S. Voluntary Product Standard PS2, or Canadian Standards CSA 0151 or CSA 0121, and/or Canadian Performance Standard O325 (Reference R503.2 of the 2015 IRC; 2305.1.5 of the 2015 IBC). Wood structural panels suitable for use as subflooring material should be plywood or oriented strand board (OSB) with a bond classification of Exposure 1 or Exterior.

When using OSB subfloors, keep these things in mind:

  • OSB thickness varies and will be determined by joist type, span, and anticipated load.
  • OSB is composed of more than 95 percent wood.
  • Most OSB is manufactured with bonding resins and waxes compressed with 1,100 pounds per square inch pressure and heat.
  • Some OSB is edge-sealed to repel water. Without this, OSB is prone to swell on the edges.
  • In general, OSB has lower moisture tolerances.
  • OSB is typically more cost effective.
  • OSB requires 1/8” spacing between all panels, on all four sides.
  • OSB shrinks/swells nearly twice as much in width as it does in its length.

When using plywood subfloors, the following should be considered:

  • Plywood thickness varies and will be determined by joist type, span, and anticipated load.
  • Plywood has good moisture resistance and quicker acclimation times than OSB.
  • Plywood is typically more expensive than OSB.
  • Plywood requires 1/8” spacing between all panels, on all four sides.

Double Layer Subfloors
When a second layer of subfloor material is necessary to meet the minimum subfloor panel requirement:

  • The second layer must be minimum 15/32”.
  • The top layer should be offset by 1/2 panels in each direction to the existing subflooring, with no seams falling in the same space.
  • The top layer also may be installed on a diagonal.
  • The panels must have 1/8” spacing between sheets and must have 3/4” perimeter expansion at all vertical obstructions.
  • Before installing, allow both panels to acclimate to the area in which they will be installed.
  • The second layer of panels should be installed with fasteners (ring or screw shanked nails or equivalent) only to the subfloor panel and not the floor joists, and at 6” on center along all panel edges and an 8” on center grid through the field of the panel.

Installation Over Existing Floors
When installing over solid board subflooring or existing wood floors, consider:

  • If safe, sand off the old finish and/or any high spots, to properly flatten the substrate.
  • Prep the existing floor to be clean, dry, sound and flat.
  • Repair, re-nail or replace loose flooring in the existing floor.
  • Ensure that the moisture content of the existing floor and the new wood flooring are compatible.
  • Solid board subflooring is 3/4” x 5 1/2” (1” x 6” nominal), Group 1 dense softwoods. These subfloors should be installed at a 45-degree angle to the joists.
  • Wood flooring should always run perpendicular to the floor joists or on the opposite diagonal to the subflooring.
  • Some engineered flooring, parquet, and thin classification solid flooring may not be recommended over these subfloors without additional underlayment.
  • Installing over an existing wood floor is also possible, but different scenarios call for different methods. Follow the flooring manufacturer recommendations when available.
  • Existing glue-down floor: glue the new wood floor directly to the existing wood floor, running perpendicular or diagonal to the direction of the floor. If the thickness of the floor will allow, staple or nail the new floor to the existing floor. When installing new wood flooring parallel to an existing solid nail-down wood floor, add a minimum of 15/32” plywood underlayment over the existing floor to increase stability.
  • When installing new wood flooring at a 45-degree to 90-degree angle to the existing solid nail-down wood floor, additional underlayment may not be required.

When installing over existing floors, including ceramic tile, terrazzo, marble, sheet vinyl, or cork tile, keep these things in mind:

  • Follow flooring and/or adhesive manufacturer instructions first and foremost.
  • New flooring installed over existing flooring will raise the floor elevation. This can pose issues with cabinets, appliances, exterior or interior doors, and stairs.
  • Be sure to check with local and federal Environmental Protection Agency (EPA) or Occupational Safety and Health Administration (OSHA) regulations for asbestos concerns. Violating these kinds of regulations can cost you tens of thousands of dollars in fines and expose you, your employees, and customers to the harmful effects of asbestos.
  • Glue-down parquet applications requiring Poly Vinyl Acetate (PVA) adhesives are not recommended over existing sheet vinyl, vinyl, or cork tile unless underlayment is put down first. Be sure to follow underlayment manufacturer recommendations if you choose to do this.
  • Other types of adhesives may require primer or vinyl blocker when installing over sheet vinyl, vinyl or cork tile flooring. Always follow adhesive manufacturer recommendations in these cases.
  • Nail-down applications over existing sheet vinyl or vinyl are acceptable when moisture from below is not a concern, when fastener penetration is not significantly diminished, and when the subfloor meets all other requirements. Fasteners must penetrate into the subfloor by at least 5/8”.
  • When considering a glue down installation over ceramic tile, terrazzo, or marble check with the adhesive manufacturer for specific instructions. These floor coverings must be sound, secure and in-tact. They also must be flat to within tolerance (1/8” in 6’ or 3/16” in 10’). And finally, they must be properly prepared to receive adhesive, which may include scarification.

Installation Checklist
Before installing wood floors over a wood subfloor, a few items must be checked.

The NWFA provides a Jobsite Checklist that can aid in checking the wood subfloor before each job. The newly updated Jobsite Checklist is available for download by visiting For more information, contact the NWFA at 800.422.4556.

Brett Miller is VP of Certification and Education at the National Wood Flooring Association in St. Louis. He can be reached at

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