Hardwood Flooring February March 2018

AT THE SITE

Tech Troubleshooting: Installing Wood Subfloors Over Concrete (Continued)

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

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the baseboards and trimwork; 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 NWFAWood 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.

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