What kind of subfloor do I need for hardwood floors?

For a hardwood floor over a wood frame, the subfloor is structural-rated plywood or OSB, or solid boards in some older homes. It needs to be thick enough for the spacing of the joists below it, fastened tight to them, and dry, flat and clean when the flooring is installed. Hardwood is also installed over concrete slabs, which are prepared differently; our post on hardwood over a concrete slab in Atlanta covers them. At Classy Flooring ATL, we check each of these conditions before installing hardwood in homes across metro Atlanta and North Georgia, against the flooring manufacturer’s instructions and the NWFA Installation Guidelines.

Classy Flooring ATL is led by Claudiu Toma, an NWFA Certified Installer, NWFA Certified Sand & Finisher and NWFA Certified Sales Advisor, who completed NWFA Precision Patterns: Parquet Flooring and NWFA Precision Patterns: Medallions. Every project is carried out by our in-house crew.

Dry means the subfloor reads within range on a moisture meter. The house needs to be closed in, with drywall, paint and other wet work finished and the heating and air running for at least five days before the flooring is delivered, so the readings match the conditions the floor will live in. NWFA’s guidelines call for at least 20 moisture readings over the first 1,000 square feet of a wood subfloor and 4 more for every additional 100 square feet, with at least 3 in each room and extra attention along exterior walls and near plumbing. During the humid season, the subfloor shouldn’t read above 13% moisture content. For solid hardwood, NWFA’s general rule of thumb is a subfloor no more than 4 points wetter than strip flooring under 3 inches wide, and no more than 2 points wetter than plank 3 inches and wider. For engineered hardwood, the flooring manufacturer’s instructions set the limit. We record our readings on the subfloor and on the flooring before installation.

How flat the subfloor needs to be for each installation method, what belongs between the subfloor and the hardwood, and when a subfloor has to be replaced are covered on our subfloor preparation service page.

How thick does the subfloor need to be for hardwood?

It depends on how far apart the joists are. NWFA’s Installation Guidelines set a minimum subfloor panel for each joist spacing, measured center to center: over joists 16 inches apart or less, that is 19/32-inch plywood or 23/32-inch OSB, and the panel gets thicker as the spacing widens. We check the subfloor against these minimums before an installation. Where a subfloor falls short, NWFA’s options are a second layer of panels, a structural fix by a qualified professional or a new subfloor; the second layer is covered below.

Joist spacing, center to center	Minimum plywood	Minimum OSB
16 inches or less	19/32 inch	23/32 inch
Over 16 to 19.2 inches	23/32 inch	23/32 inch
Over 19.2 to 24 inches	7/8 inch	7/8 inch
Over 24 to 32 inches	1-1/8 inches	1-1/8 inches

The edge of the subfloor usually shows at a floor vent opening, and from a basement or crawlspace the joists and the grade stamp on the panels can be seen. NWFA calls for plywood made to the PS 1 standard or OSB made to PS 2, with an Exposure 1 or Exterior bond classification, which the grade stamp normally shows. The panels are installed with the long side across the joists and continuous over at least two joist spaces, with the panel ends landing on a joist and tongue-and-groove edges wherever an edge isn’t supported.

NWFA’s guidelines call for a gap of 1/16 to 1/8 inch around every plywood or OSB panel. The gap lets the panels expand with moisture without pressing against each other at the seams.

Plywood, OSB and solid boards are all acceptable subfloors under NWFA’s guidelines when they meet its requirements; particleboard isn’t acceptable under a nailed floor. A board subfloor is 3/4-inch boards no wider than 6 inches, set at 45 degrees to the joists with each end on a joist. Which is better depends on the floor that will be installed over it. Sound boards work under 3/4-inch solid hardwood nailed in the right direction: perpendicular to the boards or to the joists when the boards are diagonal, and on the diagonal, or over a 19/32-inch plywood overlay, when the boards run straight across the joists. For engineered flooring under 3/4 inch, thinner solid flooring and parquet patterns such as herringbone, NWFA calls for an overlay or a new subfloor over boards, and a glue-down over boards gets a 19/32-inch plywood overlay unless the flooring and adhesive manufacturers direct otherwise. For those floors, plywood or OSB in good condition is the more direct base.

Should a subfloor be glued?

Yes. NWFA’s Installation Guidelines have subfloor panels glued and fastened to the joists, and where the joists are more than 16 inches apart, gluing as well as fastening is required. The adhesive keeps each panel from shifting against the joist below it, and movement at that joint is a common source of squeaks and loosened fasteners.

Nails and screws both meet NWFA’s guidelines. For panels up to 23/32 inch, NWFA lists ring-shank or screw-shank nails, common nails or screws made for subfloors, spaced every 6 inches along the panel edges and every 12 inches along the joists in between. The rings and threads on deformed-shank nails grip the wood fibers, so they hold better than smooth nails. On joists more than 16 inches apart, NWFA’s own text and drawings give different edge spacing, so the panel manufacturer’s fastening schedule is the one to follow.

On new construction, the subfloor is usually installed by the framing crew, and we check it before the flooring is installed. On an existing subfloor, panels that move or squeak are re-fastened with screws, which pull the panel down tight to the joist. In Alpharetta, we screwed the plywood down where it needed it before nailing the solid red oak strip over it. Our post on squeaky subfloors under new hardwood floors covers squeaks in more detail.

Can you put a subfloor over an existing subfloor?

Yes, when the existing subfloor is sound, dry and fastened tight, and NWFA’s Installation Guidelines describe how. A second layer is one of NWFA’s answers when the existing panels are thinner than its minimum for the joist spacing, when a board subfloor sits under engineered flooring thinner than 3/4 inch, a parquet pattern such as herringbone or a glue-down, and when a nailed floor will run parallel to the joists. Water-damaged or delaminated panels are replaced rather than covered; our subfloor preparation service page covers when. A second layer also raises the floor, which changes the transitions to adjoining floors and the clearance under doors.

Over a plywood or OSB subfloor, NWFA’s second layer is at least 15/32 inch thick, installed perpendicular or diagonal to the joists, with its seams offset from the panels below: edges by at least 4 inches and ends by at least one joist space. The panels get 1/16 to 1/8 inch between them and 3/4 inch at walls, and they’re fastened every 6 inches along the edges and every 12 inches across the field with ring-shank or screw-shank nails or screws that go into the subfloor only, not into the joists, over elastomeric adhesive applied as the panel manufacturer directs.

Over a board subfloor, when taking the boards out isn’t an option, NWFA’s second layer is at least 19/32 inch, and adhesive between the layers is often necessary. So 1/2-inch plywood over a plank subfloor falls short of NWFA’s guidelines: 1/2-inch sheathing is usually 15/32 inch thick, enough over plywood or OSB but under the 19/32 inch NWFA calls for over boards.

An overlay can also be installed over a sound subfloor for flatness rather than strength. On the upper level of a Savannah penthouse, we installed a 1/4-inch plywood overlay over the existing plywood to give the glue-down European ash herringbone a flat, sound surface. That overlay was for flatness under the adhesive, not a structural second layer under NWFA’s guidelines.

Can you use a floor leveler on a wood floor?

Not on bare plywood, and not under a nailed floor. Under NWFA’s Installation Guidelines, self-leveling compound isn’t normally approved over plywood or OSB, and a nailed floor can’t be fastened through it. The flooring and adhesive manufacturers’ instructions come first, though, and some leveling systems are made for wood subfloors under a glue-down floor. WAKOL’s data sheet approves its Z 631 leveling compound over plywood at least 3/4 inch thick, screwed down and free of deflection, on joists 16 inches on center, poured at least 1/8 inch thick.

Where a plywood floor that meets those requirements needs leveling under a glue-down, we follow WAKOL’s system: one coat of WAKOL PU 280 rolled over the plywood, WAKOL D 3045 primer on the cured PU 280 where the compound will be poured, then WAKOL Z 631. PU 280 leaves a non-absorbent surface, which is why the D 3045 primer comes before the leveler. The flooring is installed once the compound has cured for 24 hours, with a mat test first on pours deeper than 3/8 inch.

Instead of a leveler, NWFA’s guidelines flatten a wood subfloor with sanding and wood. High spots, including swollen panel edges at the seams, are sanded flat with an edger or a buffer and coarse grit. Low spots are built up with an overlay, with plywood set in elastomeric adhesive and screwed down, or with tapered cedar shims, from nothing to 3/8 inch over 15 inches, set in adhesive. In Canton, we edge-sanded the plywood seams and took down high points with a rotary buffer before the prefinished engineered white oak was installed.

Can I level my floor myself?

You can check how flat it is yourself. NWFA’s guidelines measure each room with a laser, a string line or a straightedge, and a 6-foot or 10-foot straightedge moved across the floor shows the high and low spots. What counts is flatness, not level, and the tolerance depends on how the floor will be installed; our post on flat versus level subfloors explains how to check and gives NWFA’s numbers for each method.

The correction is where the tools and the order of the work matter. On plywood, high seams are sanded down with an edger or a buffer, and low spots are built up with plywood, adhesive and screws or with shims, as covered above. On a slab, the moisture is tested first, and the moisture barrier, primer and leveling compound are applied in the order the manufacturer’s data sheet sets: without the primer the data sheet calls for, the leveler may not bond to the surface beneath it, and a floor glued over it is only as secure as that bond. When the floor dips because the joists sag or are out of plane, the fix is a framing repair that may need an engineer and permits, and it comes before any flooring work.

The least expensive correction is usually the smallest one that brings the floor within tolerance: sanding or grinding the high spots and filling only the low areas, instead of pouring compound across a whole room. A self-leveler’s cost follows its depth: WAKOL Z 631 covers about 50 square feet per bag at 1/8 inch, its minimum pour, so a room poured 1/4 inch deep takes about twice the bags. We measure each room and price the correction by what the subfloor needs; our prices for subfloor correction and leveling compound are on our subfloor preparation service page.