Homeowners planning a hardwood installation often ask Classy Flooring ATL the same question during the estimate: “Is the subfloor level?”
It is a reasonable question. For hardwood installation, however, flatness is the measurement that matters.
Hardwood flooring responds to flatness, not level. The two are different measurements. A subfloor can be level and still fail the test that matters. And a subfloor that fails a level check can still be correct for a hardwood installation. Understanding the difference determines whether the finished floor performs the way it should. At Classy Flooring ATL, flatness is one of the first conditions verified before any hardwood installation begins.
What level means
A floor is level when it sits perpendicular to gravity. If you place a bubble level on it, the bubble rests in the center. Level is a single reference — the floor is aligned with horizontal.
Level matters for some things. It matters when installing a refrigerator, a pool table, or a washing machine. It matters when a countertop needs to sit correctly against a wall. It does not, in most cases, matter for a hardwood floor.
Older homes are rarely level. Foundations settle. Joists sag. Slabs shift. A home that has been standing for forty years almost certainly has floors that are out of level by some amount. In most cases, this does not affect how a hardwood floor performs.
What flat means
A floor is flat when its surface stays in a single plane across a given distance. No dips, no humps, no sudden rises where one panel meets the next.
Flatness is measured against a tolerance. For glue-down hardwood installations, 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, unless the flooring manufacturer specifies otherwise. On glue-down work, that is the number Classy Flooring ATL uses to determine whether a subfloor is ready.
The tolerance shifts slightly depending on installation method. Glue-down, floating, and nail-down installations with fasteners shorter than 1-1/2 inches use 3/16 inch in 10 feet or 1/8 inch in 6 feet. Nail-down installations with fasteners 1-1/2 inches or longer use a slightly wider 1/4 inch in 10 feet or 3/16 inch in 6 feet. The installation method and the flooring manufacturer’s instructions set the number.
A flat subfloor can be out of level. A subfloor can slope gently from one side of a room to the other and still be flat — as long as there are no sharp changes in elevation within the 10-foot span. Hardwood installs over that without issue.
What happens if you install hardwood on an uneven floor?
It depends on which way the floor is uneven. NWFA’s installation guidelines call for a subfloor that is flat, not necessarily level. A gradual slope across a room usually doesn’t affect how the floor performs; dips, humps and raised seams can leave boards that sound hollow, creak, shift or, on a glue-down, lose contact with the adhesive.
Hardwood is a rigid material. When a board is placed on a subfloor, it bridges whatever small irregularities the subfloor has. If those irregularities are gradual — a room that slopes slightly across 20 feet — the boards follow the slope without strain. The floor is not perfectly horizontal, but it is continuous. Nothing rocks, nothing sounds hollow, nothing carries stress at the joints.
If the irregularities are sharp — a high spot in one corner, a dip in the middle of the floor, a raised seam between panels — the board does not follow them. It bridges them. Where there is a dip, the board is unsupported underneath. Where there is a high spot, the board rocks or loses contact with the adhesive. These are the conditions that cause the problems homeowners notice after installation: hollow sounds underfoot, joints that creak, boards that shift, adhesive that fails.
These conditions often go unnoticed before installation. Their consequences show up in the finished floor, sometimes months later. On a prefinished floor, which arrives finished from the factory and is not sanded on site, substrate-caused irregularities are permanent — there is no finishing step that can correct them after install.
How a flat subfloor is confirmed
A straightedge is the primary tool. A long rigid bar — typically 6 or 10 feet — is set across the subfloor and moved across the surface. Where the subfloor rises, the straightedge pivots and reveals a gap on one side. Where the subfloor dips, the straightedge reveals a gap underneath. The size of the gap is measured against the tolerance.
Flatness is measured across the entire floor, not just a few representative points. A room that meets tolerance in one area can fail in another. On every hardwood installation Classy Flooring ATL performs across metro Atlanta and North Georgia, flatness measurements are recorded before any corrective work begins.
If the gap exceeds the tolerance, the area is corrected before installation. On concrete, Classy Flooring ATL grinds high spots and applies self-leveling compound to low spots; our post on how to level a concrete floor for hardwood covers when each is used, how thick the compound is poured and how long it takes to dry. On plywood, correction may include sanding peaked seams, re-fastening loose panels, resetting protruding fasteners, and building up low areas with plywood glued and screwed in place or with cedar shims set in adhesive, the methods NWFA’s installation guidelines give for wood subfloors.
Our guide to what kind of subfloor hardwood floors need covers panel thickness for the joist spacing, gluing, overlays, and when a floor leveler can be used on a wood floor.
What a homeowner should ask instead
Instead of “is the subfloor level,” the more useful question during an estimate is: “How do you verify the subfloor is flat enough for this installation, and what is corrected before the flooring is installed?”
The answer should reference a measurement — 3/16 of an inch in 10 feet for glue-down is the NWFA standard — and a method for correction.
For hardwood installations in Atlanta, where slab-on-grade construction is common, flatness is one of four conditions a subfloor has to meet before installation. The others are clean, dry, and structurally sound. Each is measured, and each is corrected independently where required. The full process Classy Flooring ATL follows for subfloor preparation on both concrete and plywood is documented on our service page.
Planning a hardwood installation? Contact Classy Flooring ATL.
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.