Everyone in the construction industry is familiar with the old adage “Measure twice and cut once.” This common-sense approach reminds us to avoid mistakes and do the job right the first time. This mentality should extend to every part of the project, and that includes below the floor.

One of the most critical steps in successful wood flooring installations is the subfloor. The type and condition of the subfloor will impact the installation process and determine the success or failure of the flooring long-term. Wood floors can be installed over either wood or concrete subfloors, and the requirements for each are different.

According to the National Wood Flooring Association’s Installation Guidelines, wood subfloor panels should conform to US Voluntary Product Standard PS1-07, Construction and Industrial Plywood; and/or US Voluntary Product Standard PS2-04; and/or Canadian performance standard CAN/CSA 0325.0-92 Construction Sheathing.

Solid-board subfloors

Solid-board subflooring should be 3/4” x 5 1/2” (1” x 6” nominal) Group 1 dense softwoods, No. 2 Common and kiln-dried to less than 15% moisture content. It should consist of boards no wider than 6”, installed on a 45-degree angle, with all board ends full-bearing on the joists and fastened with minimum 8d rosin-coated or ring-shanked nails, or an equivalent.

Some types of wood flooring should not be installed directly over solid-board subflooring. Thin-classification solid-strip flooring must have a 3/8”or better plywood underlayment installed over solid board subflooring. Parquet flooring cannot be installed directly to solid-board subfloors and requires 3/8” or better underlayment panels, nailed on a 6” minimum grid pattern using ring-shanked nails or staples.


CDX and OSB subfloors

CD Exposure 1 plywood (also known as CDX) and OSB (oriented strand board) Exposure 1 subfloor panels are also appropriate subflooring materials, but the proper thickness of the material must be carefully considered depending on the truss/joist system used. For the purposes of the article, I’ll be using CDX to mean CD Exposure 1 plywood and OSB to mean OSB Exposure 1 unless otherwise noted.

For on truss/joist spacing of 16” (406mm) o/c (on-center) or less, the industry standard requires 5/8” minimum (19/32”, 15.1mm) CDX subfloor panels or 23/32” OSB subfloor panels, in 4’ x 8’ sheets. On truss/joist spacing of more than 16” up to 19.2” (488mm) o/c, the standard requires 3/4” minimum (23/32”, 18.3mm) T&G (tongue & groove) CDX subfloor panels, in 4’ x 8’ sheets, glued and mechanically fastened. Additionally, 3/4” minimum OSB subfloor panels in 4’ x 8’ sheets, glued and mechanically fastened, can be used.

Truss/joist systems spaced more than 19.2” (488mm) o/c up to a maximum of 24” (610mm) require 7/8” minimum T&G CDX subfloor panels or nominal 1” OSB subfloor panels, both in 4’ x 8’ sheets, glued and mechanically fastened. Alternatively, two layers of subflooring can be used.

For double-layer subfloors, the first layer should consist of nominal 3/4” (23/32”, 18.3mm) CDX subfloor panels or nominal 3/4” OSB subfloor panels, in 4’ x 8’ sheets. The second layer should consist of nominal 1/2” (15/32”, 11.9mm) CDX subfloor panels in 4’ x 8’ sheets. The 1/2” plywood should be offset by 1/2” panels in each direction to the existing subflooring, or the panels may be laid on a diagonal or perpendicular with 1/8” spacing between sheets. Nail on a 12” minimum grid pattern, using ring-shanked nails or staples.


Concrete subfloors

For concrete subfloors, make sure the slab is flat to the wood flooring manufacturer’s specification. Typically, manufacturers will specify a flatness tolerance of 1/8” in a 6’ radius and/or 3/16” in a 10’ radius. Many high spots can be removed by grinding, depressions can be filled with approved patching compounds, and slabs also can be flattened using a self-leveling concrete product.

Concrete subfloors also must be dry. NWFA guidelines specify using relative-humidity testing (ASTM F2170), calcium chloride testing (ASTM F1869) or calcium carbide (CM) testing (ASTM D4944 and MilSpec CRD-C154-77) to identify the moisture content of the slab.

Concrete slabs with a calcium chloride reading of more than 3 require use of a vapor retarder with a perm rating of 1 or less.  It is strongly recommended to use an impermeable vapor retarder with a perm rating of .13 or less, such as 6 mil polyethylene film. If a slab tests too high in vapor emission to glue a floor down, consider using a vapor-retarder type product, installing a vapor retarder and a plywood subfloor or using an alternative installation method.

Make sure the slab is a minimum 3000 psi. To test, draw a nail across the top. If it leaves an indentation, it is probably lightweight concrete and should be avoided. Slabs also must be free of non-compatible sealers, waxes, oil, paint, drywall compound, etc. Check for the presence of sealers by applying several drops of water to the slab. If the water beads up, sealers or oils may be present.


Other considerations

Always test for moisture in wood subfloors as well. For solid strip flooring (less than 3” wide), there should be no more than a 4% moisture content difference between properly acclimated wood flooring and the subflooring materials.  For wide-width solid flooring (3” or wider), the difference should be no more than 2%.

Wood subfloors must be flat, clean, dry, structurally sound, free of squeaks and free of protruding fasteners. For installations using mechanical fasteners of 1 1/2” and longer, the subfloor should be flat to within 1/4” in 10 feet or 3/16” in a 6’ radius.

For glue-down installations and installations using mechanical fasteners of less than 1 1/2”, the subfloor should be flat to within 3/16” in 10 feet or 1/8” in a 6’ radius. If peaks or valleys in the subfloor exceed the tolerances specified above, sand down the high spots and fill the low spots with a leveling compound approved for use under wood flooring.

Once these conditions have been met, you can install wood flooring with confidence.  The NWFA offers detailed guidelines about subfloor requirements at www.nwfa.org. OSB subfloors also will be covered in education sessions and demonstrations during the NWFA’s Wood Flooring Expo to be held April 2-5 in Dallas. For more information, visit www.nwfaexpo.org.