Installing Wood Subfloors Over Concrete

In many situations, the installation method calls for an installation of a wood subfloor over a concrete subfloor. Different methods may be necessary depending on the subfloor system and/or the flooring being installed. Always follow the flooring manufacturer’s recommendations for adequate and recommended subfloor system requirements below their flooring.

The wood subfloor panels used in these scenarios should conform to most current U.S. Voluntary Product Standard PS1, Construction and Industrial Plywood and/or most current US Voluntary PS 2, Canadian standards CSA 0151 or CSA 0121, and/or Canadian performance standard CAN 0325 (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. Alternative wood-based structural panels are permitted if recognized in a current evaluation report from an approved testing source as satisfying the requirements for subflooring in the governing building code. Do not use pressure treated plywood unless it has been kiln dried after treatment (KDAT), and moisture content coincides with the in-use environment of the facility in which the flooring is being installed.

A moisture control membrane is recommended any time a wood subfloor is installed over concrete. In on-grade and below-grade installations, moisture control systems are always recommended, and required when calcium chloride readings are greater than three pounds, or when relative humidity readings are greater than 75 percent. These general rules hold true for all three of the following subfloor installation methods.

Glue-Down
When direct gluing a wood subfloor over concrete, follow the adhesive manufacturer’s recommendation for moisture requirements and limitations, type of adhesive, application method, trowel notch type, and spread rates. When the adhesive requires, add a moisture control system before applying the adhesive. Use a minimum 3/4” (23/32, 18.3 mm) subfloor panel cut to smaller 2’ x 8’ or 4’ x 4’ sections. Kerf the back of the panels 1/2 the thickness of the material (3/8”) on a 12” x 12” grid. Lay sections in a staggered joint pattern in the adhesive, with 1/8” spacing between sheets, and 3/4” minimum expansion space at walls and all vertical obstructions. Flatness tolerances should be to within 3/16” in 6’ or 1/4” in 10’ for nail down over the wood subfloor. Do not use flooring fasteners longer than 1 3/4” to be certain not to puncture the moisture control membrane.

Floating
For floated wood subfloors installed on-grade and below-grade, be certain moisture control is taken into account. This may be as simple as a 6 mil polyethylene plastic covering 100 percent of the concrete and brought up all walls, then trimmed after the floor has been installed. Use a minimum of two layers 3/8” (10 mm) CD Exposure 1 Plywood subfloor panels (CDX) in 4’ x 8’ sheets. Place the first layer with edges parallel to the longest starting wall. Lay the second layer so that no seams line up with the below layer. All of the plywood panels should be placed with 1/8” gaps between abutting panels and a 3/4” minimum expansion space at all vertical obstructions and wall lines. Staple/screw and glue (with urethane adhesive) the second layer to the first layer on a 12” interior grid pattern (6” on the perimeter). Do not penetrate the vapor retarder.

Alternative materials can include minimum 3/4” (23/32”, 18.3 mm) CD Exposure 1 Plywood sheathing in 4’ x 8’ sheets. For this material, cut sheets to 16” x 8’ or smaller panels, and kerf the back 3/8” deep a minimum of every 12” across the width. Orient planks perpendicular or diagonally to the direction of the flooring, staggering planks every 2’. Space planks 1/8” between ends with 3/4” minimum expansion space at all vertical obstructions.

Nail-Down
For nail-down wood subfloors installed on-grade and below-grade, add a moisture control system below the subfloor material. Use a minimum 3/4” (23/32, 18.3 mm) 4’ x 8’ subfloor panels. Fasteners may be powder-driven pins, pneumatic driven nails, screws, deformed pins, or other fasteners suitable for concrete application. Stagger panel joints allowing approximately 1/8” expansion space around all panels to prevent edge peaking due to compression caused by panel swell. Allow 3/4” minimum expansion space at all vertical obstructions. Panels should be mechanically fastened. Areas with higher humidity may require additional fasteners. When fastening the subfloor into the slab, be mindful that each penetration also creates a hole in the moisture control membrane, which could become a source for moisture entry into the wood floor system.

Screeds (also known as sleepers) are strips of 2x material laid on-end in a pattern that allows for nailing the wood floor directly to them. The nailing schedule for the flooring being installed becomes a factor to the spacing of these screeds. Solid 3/4”, 25/32”, and 33/32” T&G strip flooring may be installed directly to screeds. 3/4” engineered flooring also may be installed directly to screeds. Solid or engineered wood flooring less than 3/4” (23/32”) thick and thin-classification strip flooring (including 1/2”) cannot be installed directly to screeds. For these flooring types, the screed system must be overlaid with proper subflooring material. Installation over screed systems includes properly spaced and oriented screeds laid perpendicular to the direction of the flooring and across two or more spans.

Regardless of the installation method, it is necessary to take into account the additional costs involved with these underlayment systems. The customer should be fully aware of the system being used, the additional costs incurred, and the time necessary to complete the entire project. The elevation gain that coincides with these installation methods also may affect much more than just the baseboards and trim work; you must also take into account the inward swinging doors, appliances, plumbing fixtures, and anything else that may come into contact with the floor.

More detailed information about wood subflooring installed over concrete is available in the NWFA Wood Flooring Installation Guidelines and Methods publication, or at member.nwfa.org/default.asp?page=InstallGuidelines.

Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at brett.miller@nwfa.org.