In this guide
- The Basic Layers Under Laminate Flooring
- What Is Laminate Flooring Underlayment?
- Underlayment and attached-pad laminate
- When a Moisture Barrier Is Needed
- What Goes Under Laminate on Different Subfloors?
- Concrete
- Plywood and OSB
- Existing vinyl, tile, or other hard flooring
- Carpet
- How to Choose the Right Underlayment
- Subfloor Preparation Matters as Much as Underlayment
- Installation Mistakes to Avoid
- Where to Check Requirements
Meta description: Learn what goes under laminate flooring, including underlayment, vapor barriers, and subfloor requirements for concrete and wood floors.
What goes under laminate flooring depends on the existing floor, the laminate product, and the manufacturer’s installation instructions. In many installations, the layers include a clean, level subfloor and a suitable underlayment; concrete may also require a moisture barrier. Some laminate planks have underlayment attached, which changes what should be added below them. Choosing the right layers helps limit moisture damage, excess movement, and noise while supporting a stable installation.
The Basic Layers Under Laminate Flooring
Laminate flooring is generally a floating floor: its planks lock together and rest on the surface below rather than being nailed or glued to the subfloor. A typical installation has two or three components beneath the planks: the subfloor, an underlayment when required, and sometimes a moisture barrier.
- Subfloor: The structural surface that supports the flooring, commonly plywood, oriented strand board (OSB), or concrete.
- Underlayment: A thin layer designed to provide specified performance benefits, such as sound reduction or cushioning.
- Moisture barrier: A layer that limits water vapor moving from a concrete slab into the flooring. It may be a separate polyethylene film or part of an approved underlayment.
These functions are not interchangeable. Underlayment may or may not include a vapor barrier, and a vapor barrier does not fix an uneven or damaged subfloor. The flooring manufacturer’s instructions determine which materials are compatible and whether separate layers are needed.
What Is Laminate Flooring Underlayment?
Laminate flooring underlayment is a thin sheet or roll installed between the subfloor and the laminate, unless the product already has an attached pad and its instructions say otherwise. Common materials include foam, cork, and fiber-based products. Underlayment can help reduce minor sound transmission, provide a small degree of cushioning, and support the manufacturer’s specified installation system.
Underlayment is not a substitute for a solid, flat subfloor. It should not be used to hide dips, ridges, loose boards, or major surface defects. Too much softness or the wrong thickness can allow planks to flex at the joints, potentially contributing to noise or damage. Use the type and thickness specified for the floor rather than assuming thicker is better.
Underlayment and attached-pad laminate
Some laminate planks have a pad bonded to their underside. These products are often described as having attached underlayment or an attached pad. Do not automatically add another cushion layer. A second layer may make the floor too soft or interfere with the locking system, and many manufacturers prohibit it. Check the product’s installation guide to determine whether an additional moisture barrier is required over concrete.
When a Moisture Barrier Is Needed
Moisture is a key concern because many laminate products have a fiberboard core that can swell if exposed to excess water. Concrete slabs can release water vapor even when their surface looks dry. For that reason, laminate instructions commonly call for a moisture barrier over concrete, including some slabs above grade. Requirements vary by product and site conditions, so the manufacturer’s guidance and any required moisture testing take priority.
A moisture barrier is often a polyethylene film or an underlayment designed and labeled to provide vapor protection. If a separate film is specified, follow the instructions for thickness, seam overlap, and taping. If the underlayment includes an approved vapor barrier, a second layer may be unnecessary or prohibited. Confirm that the product is rated for the installation rather than relying on a general product description.
Wood subfloors generally do not get covered with polyethylene film unless the flooring manufacturer specifically requires it. Trapping moisture between layers can create problems. Before installation, check for leaks, dampness, and signs of moisture damage. A moisture barrier does not make a wet slab or a leaking room suitable for flooring.
What Goes Under Laminate on Different Subfloors?
| Existing surface | Typical approach | Important checks |
|---|---|---|
| Concrete slab | Approved underlayment and, when required, a vapor barrier | Test moisture as directed; confirm the slab is clean, sound, and sufficiently flat |
| Plywood or OSB | Approved underlayment if required by the laminate | Secure loose areas, replace damaged panels, and correct unevenness |
| Existing vinyl or linoleum | May be acceptable if the flooring instructions permit it | Check that it is firmly attached, flat, dry, and in good condition |
| Carpet | Usually remove it and its pad | Carpet is generally too soft and unstable for laminate installation |
Concrete
For laminate over concrete, verify whether the product requires a vapor barrier, a particular underlayment, or both. Some underlayments combine cushioning and vapor protection; others do not. Concrete must also meet the flooring maker’s requirements for flatness and moisture. A slab that is too uneven can stress the plank joints, while excess moisture can damage the floor.
Follow the manufacturer’s specified moisture-test method and limits. Testing may involve a relative humidity test, a calcium chloride test, or another method recognized in the product instructions. A visual inspection alone cannot establish that a slab meets moisture limits. If a test result exceeds the product’s limit, resolve the moisture issue with an appropriate system before installation.
Plywood and OSB
Over a wood subfloor, the laminate usually rests on a suitable underlayment if the product calls for one. The subfloor should be structurally sound, dry, and flat within the limits stated in the installation guide. Fasten down loose panels or boards, address squeaks and movement, and replace areas with rot, swelling, or other damage.
Do not install polyethylene over a wood subfloor as a routine step. Wood can need to dry, and adding a non-breathable layer in the wrong place may trap moisture. The exact assembly should follow the laminate manufacturer’s written instructions and applicable building requirements.
Existing vinyl, tile, or other hard flooring
Laminate can sometimes be installed over an existing hard surface if the manufacturer allows it and the surface is stable, clean, dry, and flat. Loose tiles, cracked areas, deep grout lines, or raised edges may need repair or leveling. The installer should also check that the added floor height will not interfere with doors, transitions, appliances, or nearby flooring.
Existing flooring does not eliminate the need to follow the laminate’s underlayment and moisture-barrier instructions. For example, an approved hard surface over concrete does not necessarily remove the slab’s moisture risk. Do not cover unknown or damaged materials without checking local guidance and taking appropriate precautions.
Carpet
Carpet and carpet padding are generally unsuitable beneath a floating laminate floor because they compress and move underfoot. That movement can stress the joints and make the floor feel unstable. In most cases, both the carpet and pad should be removed so the installer can inspect and prepare the actual subfloor. A thin, firm carpet-like surface is not automatically acceptable; the product instructions control.
How to Choose the Right Underlayment
Start with the laminate manufacturer’s installation guide, not with a generic underlayment recommendation. The guide should identify whether the product needs a separate layer, what types are approved, and whether the attached pad changes the requirements. It may also state rules for concrete, wood subfloors, radiant heating, sound control, and moisture testing.
When comparing underlayments, consider these specifications:
- Compatibility: The product should be approved for the particular laminate and subfloor.
- Moisture protection: Check whether it includes a vapor barrier and whether that barrier meets the flooring instructions.
- Thickness and density: Follow the flooring maker’s limits; an overly thick or soft layer may affect the locking joints.
- Sound performance: Look for relevant ratings when sound reduction is important, while recognizing that results depend on the entire floor assembly.
- Radiant heat suitability: Confirm that both the laminate and underlayment are approved for the heating system and follow temperature limits.
Product labels and performance ratings can help with comparisons, but they do not override installation requirements. If the guide is unclear, contact the flooring manufacturer before buying materials.
Subfloor Preparation Matters as Much as Underlayment
A successful laminate installation starts with a subfloor that is clean, dry, firm, and flat within the manufacturer’s stated tolerance. Remove dust and debris, and make sure there are no protruding nails, loose fasteners, or material that could interfere with the floating floor. Repair damaged sections before covering them.
Check flatness with a straightedge or another method recommended by the manufacturer. High spots may need to be sanded or ground, while low areas may require an approved leveling compound. The allowable variation and measurement distance differ among products, so there is no single tolerance that applies to every laminate floor. Underlayment should not be expected to correct major unevenness.
Also consider the floor’s movement and expansion requirements. Laminate commonly needs a perimeter expansion gap and may require additional gaps across large installations or at certain transitions. These are not layers beneath the floor, but they are essential parts of a floating-floor installation. Follow the product guide for spacing, doorways, and transition moldings.
Installation Mistakes to Avoid
- Adding a second pad to laminate with an attached pad: This may create too much cushioning and can void product requirements.
- Assuming every underlayment blocks moisture: Some products provide sound or cushioning benefits without a vapor barrier.
- Using underlayment to hide a poor subfloor: Major dips, humps, loose boards, and damage need correction first.
- Skipping moisture testing on concrete: A slab can appear dry while still exceeding the flooring’s moisture limit.
- Leaving carpet underneath: Soft carpet and padding can make a floating floor unstable.
- Ignoring room conditions: Flooring may need to acclimate or be installed within specified temperature and humidity ranges. Follow the manufacturer’s directions.
- Using an unapproved moisture treatment: Coatings and sealers can affect adhesion, moisture performance, or warranty coverage.
Where to Check Requirements
The most reliable source for a specific installation is the laminate manufacturer’s current installation guide and warranty. These documents explain approved underlayment, vapor-barrier requirements, subfloor limits, moisture testing, and installation conditions. Guidance from the National Wood Flooring Association can provide broader flooring context, while concrete moisture-testing practices may be addressed in standards from ASTM International. Neither general guidance nor a retailer’s recommendation should replace the product-specific instructions.
For a practical decision, identify the subfloor first, determine whether the laminate has an attached pad, and then check the guide for underlayment and moisture-barrier requirements. If the floor is over concrete, moisture testing and vapor protection deserve particular attention. If the subfloor is damaged, damp, or substantially uneven, correct that condition before covering it. The right layers are the ones approved for the specific floor and the actual conditions beneath it.