Two decisions made before ordering decide how a hardwood floor performs: whether the subfloor is ready to receive it, and which product the site can support. This guide covers both, with the checks a reviewer can repeat.
What does the subfloor need before hardwood goes down?
The subfloor must be structurally sound, flat within the tolerance for the installation method, close to the flooring in moisture, and made of a panel the fastening or adhesive method accepts. Check all four before the product choice is final.
The National Wood Flooring Association states that the subfloor is the foundation of the wood floor, and that the finished floor is only as good as the subfloor under it.
Which subfloor panels accept hardwood?
Wood structural panels such as plywood and oriented strand board (OSB) are the normal base for nail-down and glue-down hardwood. Walk the subfloor first: it must be sound, with no movement, objectionable noise, water damage, delamination, or damaged areas. Fix any of those before installation.
Particleboard has limits. The NWFA guidelines require a minimum 40-pound density stamped underlayment grade where it is used, state that particleboard is not an acceptable underlayment for nailing down any wood floor, and note that glue-down over particleboard is normally not recommended. If particleboard sits under a planned nail-down floor, replace it or add a second subfloor layer.
Refasten loose panels, and drive protruding fasteners deeper or remove them. Squeaks often trace to panel movement at the fasteners.
How flat must the subfloor be?
The tolerance depends on the fastening method. For mechanical fasteners 1-1/2 in and longer, the subfloor must be flat to within 3/16 in over 6 ft or 1/4 in over 10 ft (about 5 mm over 1.8 m or 6 mm over 3 m). For glue-down floors, floating floors, and fasteners shorter than 1-1/2 in, the tolerance tightens to 1/8 in over 6 ft or 3/16 in over 10 ft (about 3 mm over 1.8 m or 5 mm over 3 m). The flooring manufacturer's instructions control when they state a different number.
Check flatness across each room with a laser level, a taut string-line, or a straight-edge, measuring the gaps between the reference and the subfloor at several points. The floor does not need to be level, but it must be flat.
Sand high spots, including swollen panel seams. Build up low spots with overlays such as plywood rather than thick patch material, because heavy build-up shortens fastener penetration and raises the risk of squeaks. Where a low area is structural, a qualified builder or engineer should assess it before the subfloor is altered.
How do you check subfloor moisture?
Test both the subfloor and the flooring with a moisture meter. Set the meter's species correction for the subfloor material being tested. For solid strip flooring (boards under 3 in wide), the average subfloor reading should sit no more than 4 percentage points above the flooring's moisture content. For solid plank (3 in and wider), the gap must stay within 2 percentage points. Take at least 40 flooring-board readings for the first 1,000 sq ft, with 4 more per additional 100 sq ft, and average them.
In hot and humid climates or seasons, keep wood subflooring at or below 13% moisture content. Flooring made to NOFMA certification is manufactured at 6 to 9% moisture content, which sets the target range the subfloor should approach.
For concrete slabs, test the slab and put moisture control in place before installation. On the moisture-meter scale the NWFA guidelines reference, a reading over 2.5% calls for a Class I vapor retarder, and a reading over 4% may not be acceptable for wood flooring at all. Test each slab separately.
If the wood floor goes down before the subfloor dries adequately, the subfloor's later shrinkage opens gaps at flooring joints and weakens fastener holding, while moisture moving up into the flooring expands the boards.
How do you choose between solid and engineered hardwood?
Solid hardwood is one piece of wood from top to bottom. It can be sanded and refinished many times during its service life, and it installs on-grade or above-grade. It should not go below-grade unless the flooring manufacturer specifically recommends it. Below-grade means any part of the subfloor sits 3 in or more below ground level.
Engineered hardwood bonds several wood veneer or lumber core layers with adhesive under a real-wood wear layer (the lamina or lamella), which determines the visible species. Finished thickness runs 3/8 in to 3/4 in. Its layered construction makes it generally more dimensionally stable than solid, and it can install above-, on-, or below-grade.
| Site condition | Solid | Engineered |
|---|---|---|
| Below-grade room (subfloor 3 in or more below ground) | No, unless the manufacturer recommends it | Yes, per the product spec |
| On-grade or above-grade with a wood subfloor | Yes | Yes |
| Concrete slab | Needs an approved wood subfloor system over the slab | Yes, glue-down or floating per the product spec |
| Wide humidity swings or radiant heat | Higher movement risk; check the manufacturer | More stable; confirm radiant-heat approval for the wear species |
| Future refinishing priority | Sands and refinishes many times | Depends on wear-layer thickness; read the product spec |
Refinishing potential on engineered floors follows the wear-layer thickness, which varies by product. Compare that number on the spec sheet rather than relying on the word "engineered" alone.
What do species, width, and finish change?
Species sets appearance and dent resistance. Janka hardness ratings indicate approximately how each species resists indentation, so check the rating for the species on the spec sheet instead of judging hardness by color or name. Being a hardwood does not automatically mean dense, and some softwoods outrank some hardwoods.
Width sets the visual scale and the movement behavior. The NWFA categories are strip (under 3 in), plank (3 to 5 in), and wide plank (over 5 in). Wider boards show fewer seams and move more per board with humidity changes, which makes the flatness and moisture checks matter more.
Finish affects maintenance and timing. Factory-finished boards arrive with the finish already applied, so the floor is usable once installed. Unfinished flooring is sanded and finished on site, which adds schedule and dust control to the job. Choose the finish with the install sequence in mind.
What should the record contain before ordering?
Keep two records separate from the order math: the subfloor record (panel type and condition, flatness check with the tolerance used, moisture readings with the meter setting, slab test results where they apply) and the product record (solid or engineered, species, width, wear-layer thickness, finish, grade level). Hand both to the reviewer before quantities are calculated.
Send the checked records to the Flooring Calculator with the room measurements from the hardwood measurement guide, and keep the allowance and carton math as a separate documented step.
Common checks that get skipped
- Testing the subfloor in several areas instead of one spot.
- Matching the flatness tolerance to the fastening method.
- Reading the wear-layer thickness on engineered product specs.
- Confirming grade-level and slab limits before choosing solid.
- Recording the moisture meter's species correction setting.
- Checking that the manufacturer's instructions do not override a guideline number used above.
Sources and scope
- National Wood Flooring Association, Installation Guidelines, 2025 edition, accessed 2026-10-20: solid and engineered definitions, refinishing, grade-level limits, wear layer, panel and fastener rules, flatness tolerances by method, moisture testing and concrete thresholds, Janka concept, NOFMA moisture context, and manufacturer-instruction precedence.
- NIST Handbook 44, 2026 Appendix C, accessed 2026-10-20: inch-to-millimetre relationships used for the metric tolerance equivalents.
Scope: the checks and thresholds above describe subfloor readiness and product choice. The flooring manufacturer's instructions supersede the guidelines where they differ, and the governing drawings, specifications, and qualified installer control the accepted installation.
Review responsibility: Saleem Sial owns the published calculation, source, and editorial checks. Waseem Sial, External Reviewer and Engineer, remains listed for ongoing external review; no completed external-review date is claimed.
Use the hardwood measurement guide to record the room before ordering, or return to the Flooring Quantity Planning hub for the published flooring resources. The calculation methodology explains units and rounding, and the corrections route accepts source or arithmetic issues.
