Subfloor Preparation for Hardwood Floor Installation
The subfloor beneath your hardwood determines installation success more than almost any other factor. Problems hidden under existing flooring—moisture issues, uneven surfaces, structural weakness—become your problems if not addressed before installation. Professional subfloor preparation protects your investment and ensures decades of trouble-free performance.
Why Subfloor Preparation Matters
Your subfloor must provide a stable, flat, dry foundation for hardwood. Deficiencies in any of these areas cause problems that appear months after installation—squeaking from inadequate attachment, telegraphing of uneven spots through the hardwood surface, or moisture-related cupping and buckling.
Proper preparation follows National Wood Flooring Association (NWFA) standards. These industry guidelines exist because generations of installers learned (often painfully) what happens when corners get cut. Taking time upfront prevents expensive callbacks and floor failures.
Moisture Testing: The Critical First Step
Moisture issues cause more hardwood floor failures than any other factor. Both your subfloor and the hardwood itself must be tested before installation proceeds.
Concrete Subfloor Testing: The NWFA approves two testing methods for concrete. The calcium chloride test (ASTM F1869) measures vapor emissions—acceptable readings fall at or below 3 pounds per 1,000 square feet per 24 hours. The relative humidity in-situ probe test (ASTM F2170) measures moisture within the slab—acceptable readings stay at or below 75% RH. Results exceeding these thresholds require moisture mitigation before installation can proceed.
Wood Subfloor Testing: Wood subfloor moisture content should stay below 14%. More importantly, the difference between subfloor and flooring moisture content must stay within acceptable ranges. For strip flooring under 3 inches wide, this difference can be up to 4%. Wide plank flooring 3 inches and wider requires a tighter 2% maximum difference. These tight tolerances explain why acclimation matters so much.
Testing Throughout the Space: Single readings don’t suffice. Professionals test multiple locations in each room, paying special attention to areas near plumbing, exterior walls, and any spots with past water damage. One problem area can compromise an entire installation.
Flatness Requirements by Installation Type
Subfloor flatness requirements vary by installation method. Checking flatness involves moving a straightedge across the floor and measuring gaps beneath it.
| Installation Method | Flatness Tolerance |
| Nail-Down (1.5″+ fasteners) | 1/4 inch in 10 feet |
| Glue-Down | 3/16 inch in 10 feet or 1/8 inch in 6 feet |
| Floating | 3/16 inch in 10 feet or 1/8 inch in 6 feet |
Floating and glue-down installations demand tighter tolerances because the floor has no mechanical fasteners pulling it down to the subfloor. High spots create bounce; low spots allow flex. Neither creates a satisfactory result.
Preparing Concrete Subfloors
Concrete preparation for hardwood installation requires attention to age, surface condition, and moisture.
Curing Time: New concrete must cure at least 30 days before moisture testing, and many professionals recommend 60-120 days before hardwood installation. Rushing this timeline risks moisture problems that appear months later.
Surface Preparation: Concrete must be free from sealers, waxes, curing compounds, adhesive residue, and contaminants. Burnished or slick surfaces require grinding with 30-grit abrasive to create proper adhesion. High spots get ground down; low spots are filled with approved leveling compound.
Moisture Barriers: All on-grade and below-grade concrete installations require vapor barriers, even when moisture tests pass. Standard specification calls for 6-mil polyethylene film with low permeability (≤0.15 perm rating), with seams overlapped at least 4 inches and taped. Severe moisture conditions may require epoxy moisture barriers at additional cost.
Preparing Wood Subfloors
Wood subfloors (plywood or OSB) must be properly secured, appropriately thick, and in good condition.
Thickness Requirements: Minimum plywood thickness depends on joist spacing. For 16-inch on-center joists, 5/8-inch plywood suffices. Joist spacing of 16-19.2 inches requires 3/4-inch plywood. Wider joist spacing demands 7/8-inch or thicker material. Inadequate thickness leads to bounce and flex.
Fastening: All panels must be properly secured to joists—every 6 inches along bearing edges and every 12 inches on intermediate supports. Loose panels squeak. Re-fastening existing subfloors with screws often eliminates existing squeaks and prevents new ones.
Expansion Gaps: Plywood panels should have 1/8-inch gaps between them to allow expansion. If existing panels are butted tight, expect some additional movement. Properly installed subfloor panels rarely need adjustment.
Damage Assessment: Water-damaged sections must be replaced entirely—you cannot effectively dry and reuse damaged plywood. Look for discoloration, soft spots, delamination, and mold growth. Problem areas must be cut out and replaced before installation proceeds.
When Leveling is Required
Subfloors often need leveling work before they meet installation standards. Common scenarios include older homes with settlement-induced dips, additions where slabs weren’t poured perfectly level, and areas where previous flooring removal revealed underlying issues.
High Spots: Localized high spots on concrete can be ground down. On wood subfloors, high spots typically indicate a joist crown or plywood seam issue—sometimes sanding helps, other times the plywood needs replacement.
Low Spots and General Unevenness: Self-leveling compounds address low spots and overall unevenness on concrete. For wood subfloors, adding a thin plywood layer over the existing subfloor (with appropriate fastening) can create a new flat surface.
Leveling adds cost and time but prevents permanent problems. A floor that feels bouncy or shows visible waves never becomes satisfactory regardless of the hardwood quality above it.
Professional Assessment Matters
Subfloor conditions often surprise homeowners. What looks fine under carpet may reveal significant issues when exposed. Problems hidden for years become your responsibility to address before hardwood installation can succeed.
Leicester Flooring’s pre-installation assessment identifies subfloor issues before you commit to materials. Our team tests moisture levels, checks flatness, and evaluates structural conditions throughout your installation areas. When issues exist, we explain options and include remediation in your comprehensive quote.
This thorough approach prevents unpleasant surprises during installation. You’ll know exactly what preparation work is needed and what it costs before your project begins.
Schedule Your Subfloor Assessment
Before selecting your hardwood flooring, let us evaluate what’s underneath. Our free in-home consultation includes thorough subfloor assessment throughout Asheville, Hendersonville, and Western North Carolina.
Contact Leicester Flooring to schedule your consultation. Understanding subfloor conditions upfront ensures realistic budgeting and successful installation outcomes.