Synthetic Roof Underlayment vs Felt: Choosing the Best Underlayment

The choice between synthetic roof underlayment and felt affects roof longevity, installation speed, cost, and warranty outcomes. This article compares performance, installation, code implications, and lifecycle value so homeowners and contractors can make an informed decision.

Feature	Synthetic Underlayment	Felt Underlayment
Water Resistance	Superior (true water-shedding)	Moderate (absorbs moisture, slower drying)
Tear Strength	High resistance to ripping	Lower; vulnerable when wet
UV Exposure	Often rated for temporary UV exposure	Poor; degrades quickly
Slip Resistance	Varies by product; many have textured surfaces	Typically has good traction initially
Cost	Higher material price; lower labor cost	Lower material cost; may increase labor/repair cost

Overview Of Synthetic Underlayment And Felt

Synthetic roof underlayment and asphalt-saturated felt are the two primary underlayments used under shingles and other roofing materials in the U.S. Synthetic underlayments are polymer-based sheets engineered for strength and water resistance, while felt is a traditional bitumen-saturated paper or fiberglass mat. Both serve as secondary water barriers and a temporary roof covering during installation.

Materials And Types

Synthetic underlayments are made from woven or non-woven polypropylene, polyethylene, or multi-layer laminates with a polymer coating. They are available in lightweight, heavy-duty, and high-slip variants.

Felt underlayments generally come as 15-pound or 30-pound asphalt-saturated rolls, with 30-pound offering thicker construction. Fiberglass-based felts offer improved dimensional stability over organic felts historically used.

Performance: Water Resistance And Leak Protection

Synthetic underlayment provides superior water-shedding capabilities because it does not absorb moisture and often features sealed seams when taped. It acts as an effective secondary barrier against wind-driven rain and ice dams.

Felt relies on the asphalt saturation to repel water but can absorb moisture, swell, and lose adhesion. When wet, felt’s protective properties diminish, increasing leak risk during prolonged exposure.

Durability, Tear Resistance, And UV Stability

Synthetics typically exhibit higher tear strength and puncture resistance due to polymer reinforcement. They resist tearing during installation and withstand job-site foot traffic better than felt.

Felt tears more easily, especially when wet, and has much lower UV tolerance. Felt left exposed can degrade within days to weeks depending on sunlight and weather.

Slip Resistance And Job-Site Safety

Synthetic products vary in surface texture; manufacturers produce versions with coarse, slip-resistant top layers for safer walking. Selecting a textured synthetic can improve installer safety on steep slopes.

Traditional felt generally provides decent traction when dry but becomes slippery when wet. Felt is also prone to shifting, which can create hazards on the roof deck.

Installation Speed And Labor Considerations

Synthetic underlayment is lighter and comes in wider rolls, enabling faster unrolling and fewer seams. Contractors often find installations quicker and cleaner, reducing labor hours and disposal volume.

Felt is heavier and produces more waste during removal and replacement. Handling wet or damaged felt can slow a job and increase labor costs.

Cost Analysis And Long-Term Value

Material cost for synthetic underlayment is higher than felt on a per-square basis, sometimes 2–4 times the price depending on grade. However, synthetic can reduce total project cost through faster installs and fewer callbacks.

Felt’s lower upfront cost may appeal for tight budgets, but potential hidden costs include replacement, leak repairs, and shorter effective useful life. Evaluating lifecycle cost often favors synthetic for long-term value.

Building Codes, Insurance, And Manufacturer Warranties

Local building codes increasingly reference underlayment performance rather than material type; many codes now allow synthetic if it meets specific standards such as ASTM D4869 alternatives or ICC-ES reports. Contractors must verify code compliance for the project location.

Some shingle manufacturers require specific underlayments to maintain shingle warranties; using a non-approved product can void portions of the roof warranty. Always check shingle warranty guidelines before selecting underlayment.

Environmental Impact And Disposal

Synthetic underlayments are petroleum-based and less biodegradable, but many are recyclable in specialized streams. Reduced replacement frequency and lower job-site waste can offset environmental drawbacks.

Felt includes asphalt and paper or fiberglass; organic felts historically had higher landfill impact, while fiberglass felts are more dimensionally stable. Felt disposal is straightforward but may occur more frequently due to shorter lifespan.

Selecting The Right Underlayment: Practical Tips

Assess climate, roof slope, and expected exposure during installation. For climates with heavy rain, ice dams, or prolonged UV exposure, synthetic underlayment is usually the better option.

Match underlayment to roof covering: Certain metal, tile, and slate systems may have specific underlayment requirements. Consult manufacturer installation guides to ensure compatibility.

Choose textured, high-slip synthetic on steep slopes and consider higher-temperature rated underlayments for low-slope or dark roof systems. Prioritize products with independent test reports and clear UV exposure ratings.

Maintenance, Inspection, And Troubleshooting

Regular roof inspections can identify underlayment failures early. Look for signs of buckling, shredded edges, or water staining in attics after storms.

If leaks occur, determine whether flashing or underlayment is responsible; underlayment replacement typically requires removing roofing material. Synthetic underlayments often allow faster temporary repairs during emergencies.

Cost Comparison Table

Category	Synthetic Underlayment	Felt Underlayment
Material Cost	Higher	Lower
Installation Time	Faster	Slower
Expected Lifespan	Longer	Shorter
Maintenance Needs	Lower	Higher
Warranty Compatibility	Varies; often compatible	Widely accepted

Common Myths And Misconceptions

Myth: Felt Is Always Better For Breathability. Felt does not significantly improve roof breathability; ventilation depends on venting design, not underlayment type. Proper attic ventilation remains the key to moisture control.

Myth: Synthetic Causes Shingles To Slip. Modern synthetic underlayments come with textured surfaces to preserve shingle traction; proper nailing pattern and installer technique matter more. Selecting a high-friction synthetic mitigates slip concerns.

Resources And Standards To Consult

Reference materials include ASTM standards, ICC-ES evaluation reports, manufacturer technical data sheets, and local building codes. Using products with third-party approvals and clear installation instructions reduces risk and ensures compliance.

Contractors and homeowners may consult the National Roofing Contractors Association (NRCA) and state licensing boards for best practices and qualified installers. Hire licensed professionals who follow manufacturer instructions to preserve warranties.

Practical Recommendation Summary

For most U.S. climates and long-term roofing projects, synthetic underlayment offers a stronger performance profile in water resistance, durability, and installation efficiency. Felt may remain acceptable for limited budgets or traditional restoration jobs but carries higher long-term risk.

When choosing, prioritize compatibility with roof covering and warranty requirements, consider slip-resistant textures on slopes, and verify local code acceptance. Investing in a quality synthetic underlayment often yields lower lifecycle costs and fewer repairs.

Further Reading And Manufacturer Links

Consult technical data sheets from leading manufacturers and independent test reports to compare UV exposure limits, tear strength, and slip ratings. Manufacturers typically publish installation guides that clarify compatibility and warranty conditions.

For additional technical guidance, review NRCA publications and local building code resources. Engaging a certified roofing contractor to perform a site assessment provides the best path to selecting the right underlayment for a specific property.