Subfloor preparation is the work completed before the flooring is installed. It determines how the finished floor performs.
Any condition in the subfloor at the time of install — out of flatness, outside moisture range, contaminated, or structurally unsound — transfers into the finished floor. Classy Flooring ATL installs engineered prefinished hardwood, engineered site-finished hardwood, and solid hardwood across the Atlanta metro area, and subfloor preparation is built around the substrate being worked on: concrete slab or plywood.
What we check before installation begins
Before any preparation work starts, the subfloor is evaluated against five conditions:
- Substrate type — concrete slab or plywood
- Flatness, measured against published tolerances
- Moisture — in the substrate and in the ambient air
- Surface contamination that would interfere with adhesive bond
- Structural defects — loose panels, movement, squeaks, cracks, delamination, hollow zones
Each condition is assessed separately. Measurements are recorded before any corrective work begins.
What a ready subfloor looks like
A subfloor ready for hardwood installation meets four conditions: flat, clean, dry, and structurally sound.
Flat. Flatness is measured. The National Wood Flooring Association specifies a maximum deviation of 3/16 of an inch across a 10-foot span, or 1/8 of an inch across 6 feet, for glue-down installations. Flat is not the same as level. A room can be level and fail a flatness check.
Clean. The substrate must be free of anything that blocks adhesive bond. On concrete: curing compounds, sealers, paint overspray, drywall and joint compound residue, oil. On plywood: debris, paint, drywall mud, and surface contamination left by other trades. An adhesive applied over residue bonds to the residue.
Dry. Dry is confirmed with measurement, not inspection. On concrete, slab moisture is tested and recorded before work continues — a common failure point in Atlanta, where most homes sit on slab-on-grade construction. On plywood, both the moisture content in the panel and the relative humidity in the air are measured.
Structurally sound. On plywood: no loose panels, no movement underfoot, no squeaks, no fasteners above the panel surface. A squeaky subfloor stays squeaky after the floor is installed. On concrete: no active cracks, no delamination, no hollow zones.
The steps we take
The process differs by substrate. Measurements are recorded before any work begins, and a moisture barrier is part of the installation scope on every project.
On plywood subfloors
- Moisture readings. Moisture content in the plywood and relative humidity in the air are measured and recorded.
- Debris removal. Loose material is cleared and the surface is vacuumed.
- Structural correction. Loose panels are re-fastened. Protruding fasteners are reset below the panel surface. Squeaks are traced and corrected.
- Seam sanding. All plywood seams are sanded flat so the panels meet as a continuous plane.
- Surface buffing. The entire plywood surface is buffed to remove residual paint, drywall mud, and surface contamination.
- Moisture barrier. One coat of moisture barrier is rolled across the prepared surface.
- Installation. The flooring is installed using a combined nail-down and glue-down method.
On concrete slabs
- Moisture readings. Slab moisture is measured and recorded.
- Grinding. The slab is ground to open the pores of the concrete so the moisture barrier bonds into the substrate rather than sitting on top of it.
- Moisture barrier. Two coats of moisture barrier are applied to the prepared slab.
- Flatness correction. Where the slab is out of tolerance, self-leveling compound is applied over the moisture barrier to bring the surface into plane.
- Bonding primer. Where self-leveling compound has been applied, a bonding primer is applied over it to prepare the surface for adhesive bond. This step is not required on slabs that were already within flatness tolerance.
- Installation. The flooring is installed glue-down.
Before any flooring is opened, the prepared subfloor is verified once more — flatness, cleanliness, moisture, and structural soundness. Installation either proceeds or waits.
How the process applies across the floors we install
The same evaluation standards apply across the hardwood products we install. The attachment method and what happens after installation depend on the product.
Engineered prefinished hardwood. The flooring arrives finished, sealed, and micro-beveled from the factory. Once the subfloor is prepared, the flooring is installed — nail-down combined with glue-down over plywood, glue-down over concrete. There is no on-site sanding or finishing step after installation, which is why subfloor preparation is less forgiving for this product. Any substrate condition remaining at install becomes a permanent condition of the finished floor.
Engineered site-finished hardwood. Subfloor preparation and attachment are the same as for engineered prefinished. The flooring arrives unfinished and is sanded, stained if specified, and finished on site after installation. Site finishing produces a continuous, seamless surface — but it does not correct substrate issues. A recent example of this approach is our engineered white oak installation at Château Élan, documented from substrate preparation through site finishing.
Solid hardwood. Solid hardwood is installed over plywood subfloors, using the same preparation process as engineered hardwood on plywood, and the same combined nail-down and glue-down method. The flooring is then sanded and finished on site.
For Atlanta homes built on slab-on-grade construction, engineered hardwood is typically the more practical choice because its layered construction is more dimensionally stable with seasonal changes in temperature and humidity. Solid hardwood is also possible over concrete when the installation calls for it. Either product requires the same subfloor preparation process — grinding, moisture barrier, flatness correction, and a bonding primer where self-leveler has been used — and either will fail over time without it. Preparation is not optional for either product. The difference is in long-term dimensional behavior, not in how much preparation the substrate needs. For homeowners deciding between product types, our guide on prefinished versus site-finished engineered hardwood covers the trade-offs in more detail.
What happens when this step is skipped or rushed
Subfloor preparation is the part of a project a homeowner rarely sees. The consequences are visible.
Hollow spots. A board bridging a low point has no adhesive contact underneath. The area sounds hollow underfoot. The joint carries stress over time.
Squeaks. A squeak is mechanical — a loose panel, a moving fastener, two surfaces rubbing. Installing over it does not remove it.
Telegraphing. Seams, ridges, raised fasteners, and unfilled dips print through the finished floor. On prefinished material, this is permanent.
Adhesive failure. An adhesive that cannot develop full bond releases at the edges, creates hollow zones, or allows boards to shift. Correction requires removing the floor.
Cupping, peaking, or separation. A floor installed outside manufacturer moisture requirements moves as it reaches equilibrium with the home. The finished floor shows the movement.
Voided warranty. Hardwood manufacturers publish subfloor requirements in their installation instructions. A floor installed outside those requirements has no manufacturer recourse.
How Classy Flooring ATL handles this
On every hardwood installation we specify, a moisture barrier is part of the scope — one coat rolled over plywood, two coats applied over the prepared slab on concrete. Our Monroe herringbone installation documents this process on a concrete slab, from grinding through moisture barrier application to finished installation. On projects where an architect or manufacturer specifies an alternative system that includes moisture control in the adhesive, we install to that specification.
Measurements are recorded before any work begins. Where subfloor conditions fall outside the flooring or adhesive manufacturer’s requirements, the installation is postponed until the substrate is corrected.
Subfloor and moisture questions
How do I know if I have a moisture problem before installing hardwood?
Moisture is not always visible. A dry-looking slab can still hold moisture internally, and a plywood subfloor can read high on a meter without any surface signs. Efflorescence on concrete, dark stains on plywood, a musty smell, or cupping on an existing floor often indicate moisture is already a problem. What confirms it is measurement, not inspection. Every hardwood installation we perform begins with moisture readings documented before any other work starts.
What is a moisture barrier and do I need one?
A moisture barrier is a product applied to the subfloor before the flooring is installed. It controls how moisture moves between the substrate and the hardwood above. On every hardwood installation we specify, a moisture barrier is part of the scope — one coat rolled over plywood, two coats applied to prepared concrete. In Atlanta, where most homes are built on slab-on-grade construction, a moisture barrier is standard for installations over concrete.
What is self-leveling compound and when is it used?
Self-leveling compound is a cementitious material poured onto a subfloor to correct flatness. It spreads to a level plane and cures hard. On concrete slabs, we apply it after the moisture barrier has been installed, in areas where the slab is out of flatness tolerance. On plywood subfloors, flatness is corrected by sanding peaked seams and filling low spots with a compatible compound; full self-leveling is rarely needed on plywood.
Can hardwood floors be installed directly over a concrete slab in Atlanta?
Engineered hardwood, yes — and it is typically the right choice for Atlanta homes built on slab-on-grade construction. The slab is ground, a moisture barrier is applied, flatness is corrected where needed, and the engineered hardwood is glued down. Solid hardwood is not installed directly over concrete. Where solid hardwood is specifically required on a slab home, a plywood overlay must be installed first, which raises the floor height and adds cost. In most cases, engineered hardwood is the more practical choice for slab construction.
What happens if subfloor moisture is not addressed before installation?
Moisture that was not controlled before install continues to affect the floor afterward. The most common results are cupping (the edges of the boards lift higher than the centers), peaking (the edges press against each other and rise at the seam), and gapping (the boards separate at the seams in drier conditions). In severe cases, the adhesive bond releases and boards shift. Most of these failures fall outside the flooring manufacturer’s warranty because the installation did not meet the published moisture requirements.
Can hardwood be installed over tile or vinyl without removing it?
In most cases, no — not at a premium standard. Tile and vinyl are not the flat, bondable subfloor a hardwood installation requires. Tile introduces grout lines that will telegraph through the finished floor, and most vinyl surfaces are designed to resist adhesion. Removing the existing floor back to the structural subfloor allows the substrate to be evaluated, corrected, and prepared the same way as any other installation. We remove existing flooring as part of the installation scope when it is present.
For more answers on hardwood installation, refinishing, and material selection, see our full Atlanta hardwood flooring FAQ.
For hardwood installation in Atlanta and the Metro Atlanta area, contact Classy Flooring ATL for an on-site evaluation.