In this guide
- What Goes Under Hardwood Flooring?
- Start With the Subfloor
- What to Put Under Hardwood by Installation Method
- Nail-Down Solid Hardwood
- Glue-Down Engineered Hardwood
- Floating Engineered Hardwood
- Engineered Hardwood Over Wood Subfloor
- What to Put Under Hardwood on Concrete
- Choosing an Underlayment
- Moisture Control and Acclimation
- What Not to Put Under Hardwood
- How to Decide What Goes Under a Hardwood Floor
What goes under hardwood flooring depends on the floor type, installation method, and existing subfloor. A typical installation may include a sound, level subfloor and, when appropriate, a thin underlayment or moisture-control layer. But adding the wrong material—or adding layers the flooring manufacturer does not approve—can lead to movement, noise, moisture damage, or a voided warranty. Here is how to choose the right layers for solid or engineered hardwood over plywood, concrete, or an existing floor.
What Goes Under Hardwood Flooring?
In most homes, hardwood flooring is installed over a prepared subfloor. The subfloor is the structural, load-bearing surface beneath the finished floor; it is usually plywood or oriented strand board (OSB) over floor joists. Some installations also require a separate underlayment, adhesive, or moisture-control product.
The appropriate assembly depends on three main factors: whether the hardwood is solid or engineered, how it will be installed, and what material is underneath it. A nailed solid-wood floor over plywood does not use the same layers as a floating engineered floor over concrete.
- Subfloor: The stable base that supports the flooring.
- Underlayment: A thin layer used for a specific purpose, such as sound reduction, cushioning, or moisture control.
- Moisture barrier or retarder: A material or system that limits moisture movement where the flooring assembly requires it.
- Adhesive: A flooring-specific bonding product used for glue-down installations; some adhesives also provide moisture control when specified for that use.
These terms are not interchangeable. A foam underlayment, for example, is not automatically an appropriate vapor retarder, and a moisture barrier cannot correct a weak or uneven subfloor.
Start With the Subfloor
Before choosing underlayment, the installer should confirm that the subfloor is sound, clean, dry, and sufficiently flat for the flooring product. Loose panels, protruding fasteners, debris, and excessive dips or ridges can cause squeaks, flexing, or visible problems after installation.
Wood subfloors commonly consist of plywood or OSB. The required thickness, panel condition, joist spacing, and fastening pattern depend on the structure and the flooring manufacturer’s instructions. A subfloor that is too thin or poorly attached may need repairs or an additional plywood layer—not a soft underlayment to disguise the problem.
For concrete, the slab must be sufficiently flat and dry for the chosen flooring system. Concrete can look dry while still allowing moisture vapor to move through it. Testing should follow the flooring and adhesive manufacturers’ requirements; acceptable test methods and limits vary by product. If a slab fails, the cause should be addressed before covering it.
For guidance on wood structural panels, the APA—The Engineered Wood Association provides information about panel products and their uses. Flooring installation requirements, however, should come from the specific flooring manufacturer and applicable building codes.
What to Put Under Hardwood by Installation Method
Nail-Down Solid Hardwood
Traditional solid hardwood is commonly nailed or stapled to a suitable wood subfloor. A thin flooring paper, such as asphalt-saturated paper or an approved synthetic product, may be used between the subfloor and flooring when the flooring manufacturer calls for it. This layer can help reduce friction and limit minor moisture movement, but it does not replace a sound subfloor or fix a moisture problem.
Do not place thick foam, carpet padding, or a soft cushion beneath a nail-down floor unless the manufacturer explicitly approves it. The floor needs a firm base so fasteners can secure the boards properly. An overly soft layer may allow movement, loosen fasteners, and contribute to squeaks or gaps.
Glue-Down Engineered Hardwood
Engineered hardwood is often glued directly to a prepared wood or concrete subfloor using an adhesive approved for the flooring. Some adhesives are designed to provide both bonding and moisture control, but that performance is product-specific. The adhesive’s stated moisture limits, application instructions, and compatibility with the flooring must be verified before installation.
A separate underlayment is generally not placed beneath a direct glue-down floor unless the flooring system specifically requires one. Applying an unapproved layer can interfere with adhesion. Concrete preparation may also involve patching, grinding, or an approved primer, depending on slab condition and the adhesive system.
Floating Engineered Hardwood
A floating floor is not fastened directly to the subfloor; the boards connect to one another and rest over an approved underlayment. The underlayment may provide cushioning, reduce some impact sound, or help manage moisture. Its thickness and properties should match the flooring manufacturer’s specifications.
When installing over concrete, a vapor-retarding layer may be required beneath a floating floor. Some underlayments include an attached vapor barrier; others require a separate sheet or seam-taped system. Do not assume that every product labeled “underlayment” offers adequate moisture protection. Check the product’s stated permeance and the flooring instructions.
Engineered Hardwood Over Wood Subfloor
Engineered flooring can be nailed, glued, or floated over a wood subfloor, depending on the product. The required layer changes with the method: a nail-down system needs a fastening surface, a glue-down system needs compatible bonding surfaces, and a floating system needs the specified underlayment. The word “engineered” alone does not determine what goes underneath.
What to Put Under Hardwood on Concrete
Concrete requires particular attention because moisture can move through a slab and affect wood flooring. The correct approach depends on the flooring type and installation system. A floating engineered floor may use an approved underlayment with a vapor-retarding layer, while a glue-down system may rely on a compatible moisture-control adhesive or a separate approved mitigation system.
Solid hardwood is more commonly installed over a wood subfloor than directly over concrete. Some products and systems allow other approaches, but they must be explicitly approved. Installing solid hardwood over a slab without a specified system can expose the floor to moisture-related cupping, warping, or other damage.
Moisture checks should be completed before installation, not after symptoms appear. The National Wood Flooring Association (NWFA) offers technical resources on wood-flooring installation and moisture considerations at NWFA.org. Its guidance does not replace the product-specific installation instructions, which control the flooring warranty.
Choosing an Underlayment
Underlayment should be selected for a defined purpose and approved for the flooring assembly. Product labels may mention sound reduction, cushioning, moisture control, or compatibility with floating floors. Those claims do not mean one product performs every function equally well.
| Underlayment or layer | Common use | Key caution |
|---|---|---|
| Flooring paper or approved synthetic sheet | Sometimes used beneath nail-down hardwood over a wood subfloor | Does not correct subfloor defects or guarantee moisture protection |
| Foam or specialty floating-floor underlayment | Used beneath some floating engineered floors for cushioning or sound control | Use only the thickness and type approved by the flooring maker |
| Separate polyethylene sheet | May be specified as a vapor retarder over concrete for certain floating systems | Not appropriate for every assembly; seams and edges must follow instructions |
| Moisture-control adhesive or system | Used in some glue-down installations, especially over concrete | Must match the slab conditions and flooring manufacturer’s requirements |
| Plywood overlay | Can provide an approved wood substrate over some existing surfaces | It is a subfloor layer, not a soft underlayment; installation details matter |
Sound-control requirements in apartments and condominiums can involve building rules or association standards. A product’s laboratory sound rating does not necessarily guarantee the same result in a specific building. The complete floor-ceiling assembly, installation details, and local requirements affect performance.
Moisture Control and Acclimation
Wood changes dimension as its moisture content and surrounding humidity change. Excessive moisture from a slab, plumbing leak, wet crawlspace, or damp subfloor can damage hardwood; very dry conditions can contribute to gaps. A barrier is only one part of moisture management, and it cannot stop an active leak or correct a wet building condition.
Installers should measure moisture in both the flooring and the subfloor using methods appropriate to the materials. Wood subfloor readings and concrete moisture tests are different, and results should be compared with the flooring manufacturer’s limits. The National Wood Flooring Association recommends addressing job-site moisture and environmental conditions as part of installation planning.
Acclimation instructions vary. Some products require a period of acclimation, while others specify that acclimation is unnecessary or use different procedures. The product’s instructions should determine when materials are delivered, how they are stored, and what indoor temperature and humidity conditions are required. A fixed number of days does not apply to every floor.
What Not to Put Under Hardwood
- Carpet padding or thick foam: These materials are usually too soft for nail-down hardwood and may be unapproved beneath floating or glue-down floors.
- Multiple unapproved layers: Stacking sheets, pads, and barriers can create excessive softness or interfere with fastening and adhesion.
- Plastic sheeting beneath a wood subfloor: An improvised layer can trap moisture in the assembly. Use only a moisture-control detail designed for the specific floor and building conditions.
- Underlayment over an unstable base: It will not repair loose panels, weak framing, major unevenness, or a damp slab.
- Products that conflict with the warranty: Even a high-quality material may be unsuitable if the flooring maker does not approve it.
Underlayment also cannot eliminate every floor sound. Squeaks often come from movement between a subfloor and joists, loose fasteners, or gaps in the structure. Those causes should be diagnosed and repaired before the hardwood is installed.
How to Decide What Goes Under a Hardwood Floor
- Identify the flooring: Confirm whether it is solid or engineered hardwood and locate its installation guide.
- Identify the installation method: Determine whether the floor will be nailed, glued, or floated.
- Inspect the substrate: Establish whether the base is plywood, OSB, concrete, or an existing floor, and repair defects.
- Test moisture: Use the test methods and limits specified for the flooring, adhesive, and underlayment system.
- Choose only approved layers: Select a barrier, underlayment, adhesive, or overlay only when the installation instructions call for it.
- Check local requirements: Confirm building-code, sound-control, and multifamily-building rules where applicable.
For a straightforward wood-subfloor installation, the answer may be a properly prepared subfloor and a thin approved layer—or no separate underlayment at all. Over concrete, the flooring system may need a carefully selected vapor-retarding product or moisture-control adhesive. The flooring manufacturer’s installation instructions, combined with verified subfloor conditions, are the most reliable guide to what belongs under hardwood flooring.