Engineered hardwood is versatile but requires proper subfloor preparation and underlayment for each base type to ensure longevity, performance, and warranty compliance. Here's a breakdown for each compatible subfloor:
1. Concrete Subfloors
Compatibility:
✅ Excellent for basements, slabs, or ground-level installations.
⚠️ Critical Risk: Moisture vapor emission from concrete can warp planks or cause adhesive failure.
Preparation:
Moisture Testing: Mandatory. Use ASTM F2170 (in-situ RH probes) or ASTM F1869 (calcium chloride test). Maximum allowable vapor emission:
≤ 3 lbs/1,000 sq ft/24 hrs (calcium chloride)
≤ 75% RH (probe test).
Flatness: ≤ 3/16" over 10 ft or ≤ 1/8" over 6 ft. Grind high spots; fill low spots with leveling compound.
Curing: Concrete must cure ≥ 60 days before installation.
Underlayment Requirements:
Vapor Barrier: Use ≥ 6 mil polyethene sheeting (lapped 6–8", taped seams) if moisture is borderline.
Combination Underlayment: Opt for a vapor-retarding + cushioning underlay (e.g., cork/rubber with integrated membrane).
Glue-Down Method: Moisture-curing urethane adhesives (e.g., Bostik's Best) directly over concrete with no underlayment.
2. Plywood/OSB Subfloors (Wood Framing)
Compatibility:
✅ Ideal for above-ground floors (joist systems).
Preparation:
Thickness & Integrity: Minimum ⅝" plywood/OSB (19/32") for 16" joist spacing. Replace swollen, delaminated, or uneven panels.
Flatness: ≤ ⅛" over 6 ft or 3/16" over 10 ft. Sand ridges; fill gaps with leveling compound.
Fastening: Screw down every 6" along seams and 8" in the field to eliminate squeaks.
Cleanliness: Remove dust, debris, and protruding fasteners.
Underlayment Requirements:
Basic Underlayment: Use 1.5–3mm foam, cork, or felt for noise reduction and minor smoothing.
Vapor Barrier: Only needed if installed over a damp space (e.g., crawl space). Use kraft paper or foil-laminated underlay.
Direct Glue-Down: Possible with approved wood adhesives (no underlayment).
3. Existing Flooring (Overlay)
Compatibility:
⚠️ Conditional – Depends on the existing material:
✅ Suitable: Ceramic tile, vinyl tile/sheet (non-cushioned), laminate, or stable existing wood.
❌ Unsuitable: Carpet, cushion-backed vinyl, cork, or any soft/compressible flooring.
Preparation:
Stability: Existing floor must be rigid, fully adhered, and level (meets flatness standards above).
Height Transition: Ensure the new floor won't interfere with doors/appliances.
Surface Prep:
Clean thoroughly; remove wax/grease.
Sand glossy surfaces for adhesive grip (if gluing).
Use cementitious leveler over tile grout lines if > ⅛" deep.
Underlayment Requirements:
Thin Underlayment: Use ≤ 2mm foam or cork to avoid excessive height gain.
Decoupling Mat: Essential over tile/stone (e.g., Schlüter®-DITRA) to prevent grout line telegraphing.
Vapor Barrier: Add if overlaying concrete.
Key Considerations for All Subfloors
Acclimation: Engineered planks need 48–72 hrs onsite (spaced in the room) at 60–80°F and 30–50% RH.
Expansion Gaps: Leave ½" gaps at walls/fixed objects.
Manufacturer Guidelines: Always prioritize product-specific requirements (e.g., some mandate attached underlayment pads).
Radiant Heat: Requires low thermal resistance (≤ 0.5 R-value) underlayment and manufacturer approval.
By meticulously preparing the subfloor and selecting the correct underlayment, you ensure a stable, silent, and durable engineered hardwood floor. When in doubt, consult a professional installer or the flooring manufacturer.
