Subfloor Preparation for Hardwood Installation in Atlanta

Classy Flooring ATL is a hardwood flooring company serving metro Atlanta and North Georgia, and subfloor preparation for hardwood floor installation is one of the things we specialize in. We prepare concrete slabs and plywood subfloors by measuring moisture and flatness, then correcting whatever falls outside the flooring and adhesive manufacturers’ requirements 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.

Subfloor correction is priced according to the condition of the subfloor, $1.00 to $4.00 per square foot; self-leveling compound, where the floor needs it, is $110 per bag, material and labor included. WAKOL PU 280 is $1.00 to $2.00 per square foot: one coat on plywood, two coats on concrete. For projects of 1,000 sq ft and up, installation starts at $3.50 per square foot for solid hardwood and $4.00 per square foot for engineered and prefinished hardwood; smaller projects are priced by the job.

This page covers how much unevenness a hardwood floor tolerates, what goes between the subfloor and the hardwood, and how to tell when a subfloor needs to be replaced. The answers come from our projects: white oak herringbone on the 24th floor of a Buckhead high-rise, solid white oak on the 21st floor of a Buckhead high-rise, an engineered floor replacement at Atlantic Station, engineered herringbone in Monroe, engineered white oak at Château Élan, prefinished engineered white oak in Canton and solid red oak in Alpharetta.

How much uneven floor is acceptable?

A hardwood floor needs a flat subfloor, not necessarily a level one. The NWFA Installation Guidelines call for a subfloor flat within 3/16 inch in 10 feet or 1/8 inch in 6 feet for glue-down floors, floating floors and nail-down floors with fasteners shorter than 1½ inches, and within 1/4 inch in 10 feet or 3/16 inch in 6 feet for nail-down floors with fasteners 1½ inches and longer. Both apply unless the flooring manufacturer specifies otherwise, in which case the manufacturer’s number governs.

We measure flatness room by room before preparation starts and correct the subfloor to the tolerance for the installation method. Our post on flat versus level subfloors explains the difference, and herringbone subfloor flatness requirements covers pattern floors.

On concrete slabs

We grind high areas down and fill low areas with patching or self-leveling compound compatible with the adhesive, after the slab’s moisture has been tested. Grinding also takes off sealers, curing compounds and old adhesive residue, leaving the slab clean and absorbent for the moisture barrier or adhesive applied to it. WAKOL’s data sheet puts D 3045 primer on the cured PU 280 moisture barrier before the leveling compound, and the flooring is installed once the compound has cured: 24 hours for WAKOL Z 631, with a mat test first on pours deeper than 3/8 inch.

On the 24th floor of a Buckhead high-rise, the slab was brought into tolerance by grinding alone before the solid white oak herringbone was glued down. At Atlantic Station, a planetary concrete grinder with HEPA vacuum extraction prepared the slab, with hand scraping at the edges and around the in-floor outlets. At Château Élan, the slab took two coats of WAKOL PU 280, D 3045 primer and WAKOL self-leveling compound before the engineered white oak was glued down. In a Savannah penthouse, we checked the lower-level slab with a straightedge against the glue-down tolerance, and the general contractor’s crew leveled it to specification before we installed the European ash herringbone.

On plywood subfloors

We sand the plywood seams flat and buff the whole surface to take off paint, drywall mud and residue left by other trades. For low spots in wood panels, NWFA’s methods are overlays, plywood build-up set in elastomeric adhesive and screwed down, and cedar shims set in adhesive; self-leveling compound is not normally approved over plywood and can’t be nailed through. For a glue-down floor, WAKOL’s data sheet rates its Z 631 self-leveling compound for plywood at least 3/4 inch thick, securely screwed and free of deflection, over joists 16 inches on center; we apply it over one coat of WAKOL PU 280 and WAKOL D 3045 primer. Our guide to what kind of subfloor hardwood floors need covers when a floor leveler can be used on a wood floor.

In Canton, we edge-sanded the plywood seams and removed high points and dried drywall compound with a rotary buffer before installing the RW Floors Kells prefinished engineered white oak.

Does anything go between a subfloor and a hardwood floor?

In most installations, yes: a moisture retarder or barrier, the adhesive, or both, and in some high-rise condominiums an acoustic underlayment. Which of these is used depends on the subfloor, the installation method and the moisture readings we take first: relative humidity probes in concrete, and a moisture meter on plywood and on the flooring itself.

On plywood subfloors

NWFA’s guidelines consider a vapor retarder unnecessary over plywood above a conditioned space unless the flooring manufacturer calls for one; over an unconditioned space, a Class II retarder may be used. We roll one coat of WAKOL PU 280 over the plywood. WAKOL’s data sheet describes one coat of PU 280 on wood subfloors up to 20% moisture content as a primer and moisture retarder, and WAKOL specifies a single coat on wood.

In Alpharetta, solid red oak strip was nailed over plywood primed with one coat of PU 280. In Canton, the plywood read 6.0% and the prefinished engineered white oak 5.5% before the nail-assisted glue-down.

On concrete slabs

NWFA recommends a Class I vapor retarder over slabs on and below grade, and when a slab tests above 80% relative humidity, calls for waiting or installing a Class I retarder per the adhesive manufacturer. Under a glue-down, NWFA has the adhesive manufacturer set the moisture limits and the moisture control, and the retarder has to be compatible with the adhesive. After the moisture readings, our standard system on a slab starts with grinding the concrete. We then apply two coats of WAKOL PU 280, which WAKOL rates to 98% relative humidity; above that, WAKOL evaluates the slab first. We glue the floor with WAKOL MS 260, which is applied directly over the cured PU 280. MS 260 on its own is rated to 80% relative humidity.

At Monroe, the slab read 65% relative humidity; it was ground and given two coats of PU 280, and a third coat was applied after WAKOL’s 72-hour installation window had passed. At Château Élan, the slab read 55% to 65% relative humidity and took two coats of PU 280 before the engineered white oak was glued down with MS 260. Our post on hardwood over a concrete slab in Atlanta covers slab installations in more detail.

In high-rise condominiums

Where a building sets a minimum sound rating (IIC), an acoustic underlayment goes between the slab and the hardwood, and the underlayment manufacturer’s requirements govern the slab. On the 21st floor of a Buckhead high-rise, the slab read 55% relative humidity, and 4-inch solid white oak was glued over Roberts Natural Cork with WAKOL MS 260 to meet the building’s IIC 65 requirement. A 21st-floor slab is above grade, and NWFA allows solid hardwood on or above grade. At The Atlantic in Atlantic Station, the building requires IIC 54: the ECO-Ultra Quiet underlayment, rated by its manufacturer at IIC 73 under engineered hardwood, was set in WAKOL MS 230 at both layers to meet that requirement, and slab moisture was held within its manufacturer’s limit of 4 lbs per 1,000 sq ft per 24 hours.

How do I know if my subfloor needs to be replaced?

A subfloor needs replacing when the panels themselves are damaged: plywood or OSB that water has damaged, delaminated or swollen. NWFA’s guidelines call for replacing water-damaged plywood or OSB when the damage is evident, with the new panels installed at an acceptable moisture content. OSB loses density and fastener holding power once it swells.

Before installation, we check the subfloor for movement, noise, water damage and delamination, the conditions NWFA lists for a sound subfloor. From above, they show as bounce or movement underfoot, squeaks, water stains, plies separating at the panel edges and swollen OSB edges.

NWFA sets a minimum panel thickness by joist spacing: 19/32-inch plywood or 23/32-inch OSB over joists spaced up to 16 inches apart. A subfloor thinner than the minimum gets a second layer, a structural fix or replacement. Joists that sag or are out of plane need a framing repair, which may require an engineer and permits, and that work comes before the flooring.

Panels that move or squeak are re-fastened. In Alpharetta, the plywood was screwed down where needed before the solid red oak was nailed over it. Our post on squeaky subfloors under new hardwood floors covers squeaks in more detail.

Client reviews

Three clients wrote about the subfloor work in their Google reviews.

★★★★★ 5-star review — Source: Google Business Profile

“Claudiu is a true craftsman. He is very organized and makes sure to go above and beyond for his customers. We recently completed projects in Canton and Woodstock, and I couldn’t be happier with the craftsmanship and professionalism of his installation team. These were not simple installations—we required extensive subfloor preparation, a custom herringbone hardwood pattern, and intricate solid brass inlay work. The attention to detail throughout every phase of the project was exceptional.The installers took great pride in their work, communicated well, kept the jobsite clean, and delivered a finished product that exceeded expectations. The herringbone layout is flawless, and the brass inlay adds a stunning custom touch that was executed perfectly. If you’re looking for a team that can handle high-end, detailed flooring projects with precision and professionalism, I highly recommend them.”

— Caleb Measures, Canton and Woodstock GA

See the Canton project

See the Woodstock project

★★★★★ 5-star review — Source: Google Business Profile

“We installed engineered prefinished hardwood in our home in Alpharetta. We chose Classy Flooring ATL because we did some research and understood that subfloor preparation it’s really important for the longevity of this floor. Claudiu have extended knowledge about it. It was a pleasure working with the team, good communication, daily updates.
Highly recommend Classy Flooring ATL.”

— Nagender Khokhar, Alpharetta GA

★★★★★ 5-star review — Source: Google Business Profile

“We hired Classy Flooring ATL to replace the carpet in our upstairs bedrooms in Buford with pre-finished engineered hardwood, and I’m so glad we did. We already had hardwood in the main living areas, and I wanted the bedrooms to feel more connected to the rest of the house, but I did not want the inconvenience of doing unfinished floors and having sanding and finishing done inside the home. Claudiu explained the options clearly and helped us choose a dark 5-inch floor that ended up being the right fit for us.

He communicated very well throughout the project, answered our questions patiently, and made the whole process feel easy and manageable. They also took the time to prepare the plywood subfloor properly before installation, which gave me confidence that the job was being done the right way.

I also appreciated that Claudiu was honest from the beginning that the new floor would not match the existing hardwood perfectly. That said, the overall transition turned out very well, and once everything was in place, it felt right with the rest of the house.

The project moved quickly, the team was respectful and pleasant to have in our home, and the bedrooms look so much better now. It was a pleasure working with them, and I would definitely recommend Classy Flooring ATL.”

— Maria Neamtu, Buford GA

Frequently asked questions

Should I level my floor before installing hardwood floor?

It needs to be made flat, though not necessarily level, to the tolerance for the installation method. On concrete, we grind high areas and fill low areas with a compatible patch or self-leveling compound after moisture testing; on plywood, we sand the seams and build up low spots with wood. At Château Élan, the slab took WAKOL Z 631 self-leveling compound before the engineered white oak was glued down.

Do I need underlayment for solid hardwood flooring?

Underlayment goes under solid hardwood when the subfloor or the building calls for it: a moisture retarder or barrier for moisture, or an acoustic underlayment where a building sets a minimum sound rating. Solid hardwood itself is nailed or glued to the prepared subfloor. In Alpharetta, solid red oak strip was nailed over plywood primed with one coat of WAKOL PU 280. On the 21st floor of a Buckhead high-rise, 4-inch solid white oak was glued over Roberts Natural Cork to meet the building’s IIC 65 requirement.

How much does it cost to have your floor leveled?

Leveling is priced in two parts: subfloor correction according to the condition of the subfloor, $1.00 to $4.00 per square foot, and self-leveling compound, where the floor needs it, at $110 per bag, material and labor included. Where WAKOL PU 280 is applied, it adds $1.00 to $2.00 per square foot, with one coat on plywood and two on concrete. The price follows what the subfloor needs: on the 24th floor of a Buckhead high-rise, grinding alone brought the slab into tolerance, while at Château Élan the slab also took self-leveling compound.