The right underlayment depends on what it will sit beneath. Roofing underlayment protects the roof deck beneath shingles or other coverings, while flooring underlayment can cushion a floor, reduce sound, or help control moisture. The best choice is the one approved for the specific surface, product, and installation conditions—not simply the thickest or most expensive option. This guide explains common underlayment materials, where they work, and what to check before buying.
What Underlayment Does
Underlayment is a layer installed between a structural surface and its finish. Its job varies by application. On a roof, it provides a secondary water-shedding layer between the roof deck and roofing material. Under flooring, it may smooth minor imperfections, reduce impact noise, provide cushioning, or act as a vapor retarder.
Underlayment does not replace a sound roof deck, a properly prepared subfloor, or correct installation. It also cannot compensate for leaks, significant floor unevenness, or moisture problems that need to be fixed at their source. In some systems, adding an unapproved layer can interfere with adhesion, trap moisture, or void a product warranty.
What to Use for Roofing Underlayment
Roofing underlayment is installed over the roof deck and beneath shingles, metal panels, or other roof coverings. The appropriate material depends on the roofing product, roof slope, local building code, climate, and manufacturer instructions. Common choices include asphalt-saturated felt, synthetic underlayment, and self-adhered membrane.
Asphalt-Saturated Felt
Often called roofing felt or tar paper, this traditional material is made from a reinforcing base saturated with asphalt. It is commonly sold in different weights, such as No. 15 and No. 30. Felt can provide a secondary water barrier, but it may wrinkle, tear, or degrade if left exposed to weather for too long. Handling and fastening it correctly matters, and the roof covering should be installed within the exposure period specified on the product label.
Felt may be a suitable option under asphalt shingles when local requirements and the shingle manufacturer allow it. However, product specifications differ, and the terms “No. 15” and “No. 30” do not by themselves establish suitability for every roof. Check the package for the product’s current standard, approved uses, and installation instructions.
Synthetic Roofing Underlayment
Synthetic underlayment is generally made from woven or nonwoven polymers. Compared with felt, many products are lighter, resist tearing, and are easier to handle on a roof. Some are designed for longer temporary exposure, but allowable exposure varies widely. A synthetic sheet is not necessarily waterproof, and its surface can become slippery, especially when wet or covered with dust.
Confirm that the product is rated for the roof covering and slope. Follow its instructions for overlaps, fastening, and exposure. Do not assume a synthetic underlayment can be used beneath any metal roof or installed as a permanent exposed roof surface unless the manufacturer specifically approves that use.
Self-Adhered Membrane
Self-adhered membranes have an adhesive backing and are often used in areas vulnerable to water intrusion, such as eaves, valleys, roof-to-wall transitions, and around penetrations. Ice-barrier requirements vary by location and building code; some jurisdictions require protection in specified parts of the roof. The International Residential Code includes provisions for ice barriers in applicable climates, but local adoption and amendments determine what applies to a particular home.
These membranes can bond tightly to the roof deck, so surface preparation and temperature limits are important. They may also affect how a roof assembly dries. Use only products compatible with the deck and roof covering, and follow the roofing manufacturer’s instructions and local code.
Quick Roofing Comparison
| Material | Typical role | Important check |
|---|---|---|
| Asphalt-saturated felt | Secondary layer beneath approved roof coverings | Weight, fastening method, weather exposure, and code |
| Synthetic sheet | Lightweight water-shedding layer beneath approved coverings | Roofing compatibility, slope rating, slip risk, and exposure limit |
| Self-adhered membrane | Sealing vulnerable roof areas or meeting applicable ice-barrier requirements | Adhesion conditions, placement, compatibility, and local code |
What to Use Under Laminate Flooring
Laminate flooring commonly needs a thin, stable underlayment, but some products have an attached pad. If the laminate includes an attached underlayment, adding another layer is usually not recommended unless the flooring manufacturer explicitly permits it. Extra cushioning can allow too much movement at the joints and may contribute to damage or affect warranty coverage.
For laminate installed over a concrete slab, the system may require a vapor retarder or a combined underlayment and vapor-control product. For wood subfloors, the requirements may differ. The product’s installation guide should specify whether a separate vapor barrier is needed, which seams must be taped, and whether the underlayment is approved for that floor.
Underlayment can reduce certain types of sound, but sound ratings are measured in different ways and depend on the full floor-ceiling assembly. A product’s rating alone does not guarantee a particular result in a home. In apartments or condominiums, building rules may set minimum sound-control requirements.
What to Use Under Hardwood Flooring
The right underlayment for hardwood depends on whether the floor is solid or engineered, whether it is nailed, glued, or floated, and whether the subfloor is wood or concrete. Some nail-down installations use a specific paper or felt layer, while floating engineered floors may require an approved foam or other underlayment. Glue-down floors generally need a compatible adhesive and substrate preparation; a loose underlayment may not be appropriate.
Moisture is a key concern. Wood flooring can expand or contract as moisture conditions change. Before installation, the flooring and subfloor should meet the manufacturer’s moisture requirements, measured with the appropriate method. A sheet of plastic or other vapor retarder should not be added by default: the correct moisture-control approach depends on the flooring system, subfloor, and site conditions.
What to Use Under Vinyl Plank Flooring
Many luxury vinyl plank and tile products are designed to install directly over a clean, flat, approved subfloor. Some have an attached backing; others require a separate underlayment only when the manufacturer permits it. Too much give beneath a thin vinyl floor can stress joints, cause movement, or make imperfections more noticeable.
For concrete, check the flooring instructions for moisture limits and any required moisture-mitigation system. A basic foam underlayment is not automatically a substitute for a specified moisture barrier. For wood subfloors, the surface may need to be smooth and structurally sound, with seams or fasteners prepared as directed. “Waterproof” flooring describes the product’s resistance to water; it does not mean the subfloor is protected from moisture or that water can safely remain trapped beneath the floor.
What to Use Under Tile
Ceramic and porcelain tile need a stiff, stable base. Tile underlayment is not primarily a cushion; it helps create a suitable surface and may provide waterproofing or crack isolation when installed as part of an approved system. Common options include cement backer board, uncoupling membranes, and liquid or sheet waterproofing membranes.
- Cement backer board: Provides a tile-friendly surface but is not inherently waterproof. Seams and surrounding areas need treatment if the installation requires waterproofing.
- Uncoupling membrane: Helps separate tile from some movement in the substrate, subject to the product’s limitations. It is not automatically a structural fix or a substitute for a sound subfloor.
- Waterproofing membrane: Designed to manage water when installed with compatible seams, corners, drains, and transitions. Details are critical, especially in showers and other wet areas.
Tile over wood framing also requires adequate stiffness. A tile underlayment cannot make an undersized or excessively flexible floor structurally suitable. Follow the tile and underlayment manufacturers’ substrate requirements, and consult applicable tile-installation guidance, such as the Tile Council of North America, for system-specific recommendations.
How to Choose the Right Underlayment
- Identify the finish material. Roofing shingles, metal panels, laminate, hardwood, vinyl, and tile each have different requirements.
- Check the installation method. Nail-down, glue-down, floating, and mechanically fastened systems may use different underlayments—or none at all.
- Assess the substrate. Determine whether it is a roof deck, concrete slab, plywood, or another material, and address damage, contamination, or unevenness first.
- Evaluate moisture conditions. Consider roof exposure, slab moisture, plumbing risks, and whether the assembly needs a vapor retarder or waterproofing layer.
- Read the product instructions. Check approved uses, thickness, seams, fasteners, exposure limits, and warranty requirements.
- Verify local requirements. Building codes and condominium rules may add requirements for roofing, ice barriers, or sound control.
For code questions, the International Code Council publishes model codes, but the locally adopted code is the one that governs a project. For roofing product selection and installation, the National Roofing Contractors Association provides industry resources. Always compare those resources with the project’s local requirements and the manufacturer’s current instructions.
Common Underlayment Mistakes
- Choosing by thickness alone: A thicker pad is not necessarily better and may be prohibited beneath some floors.
- Doubling up layers: Adding underlayment over an attached pad can create excessive movement or affect performance.
- Treating all moisture barriers alike: A roof water-shedding layer, a flooring vapor retarder, and a waterproofing membrane serve different purposes.
- Ignoring exposure limits: Roofing underlayment may not be designed to remain exposed indefinitely.
- Installing over an unprepared surface: Underlayment cannot correct structural weakness, major unevenness, or active water intrusion.
- Overlooking compatibility: Adhesives, membranes, flooring, and roof coverings must be approved to work together.
When Professional Advice Is Helpful
Professional guidance is especially useful when a roof has a low slope, complex valleys, or a history of leaks; when a concrete slab has uncertain moisture conditions; or when tile will be installed over a questionable wood floor. A qualified contractor can assess the substrate, identify applicable code requirements, and confirm that the proposed layers form a compatible system. Product instructions should still be reviewed before installation.
In practical terms, the answer to “what to use for underlayment” starts with the surface above it and the substrate below it. Choose a product approved for that exact assembly, follow its installation details, and solve moisture or structural problems before covering them.