Synthetic vs. Felt Roof Underlayment: Key Differences

When it comes to protecting your home or commercial property, choosing the right roofing materials is more than just picking shingles. One of the most important—and often overlooked—decisions is selecting the right underlayment. In this article, we’ll break down the key differences between synthetic vs felt roof underlayment, helping you decide which option offers the best performance, durability, and long-term value for your needs.

Whether you’re managing a major renovation or replacing a storm-damaged roof, this guide will walk you through a clear, side-by-side comparison to support your decision-making.

Here’s what we’ll cover:

• What Is Synthetic Roof Underlayment?
• What Is Felt Roof Underlayment?
• Side-by-Side Comparison: Synthetic vs. Felt
• Which One Is Better for Your Region or Property?
• How to Choose the Right Underlayment for Your Project
• Conclusion

Let’s start by looking at what synthetic underlayment is and why it’s become so popular in modern roofing projects.

What Is Synthetic Roof Underlayment?

Synthetic roof underlayment is a modern, engineered alternative to traditional felt, designed to sit between your roof decking and the shingles. Made from materials like polypropylene or polyethylene, synthetic underlayment offers enhanced performance characteristics that are especially useful in demanding climates and high-exposure environments.

Unlike felt, which is saturated with asphalt, synthetic underlayment is manufactured with woven or spun plastic fibers. This construction gives it a more uniform structure, making it lighter, stronger, and more resistant to tearing—especially during installation or in windy conditions.

For a deeper look at various options available, check out this comprehensive guide to roof underlayment types.

Common Materials in Synthetic Underlayment

Most synthetic underlayments are made from:

• Polypropylene: A thermoplastic polymer known for its durability, water resistance, and flexibility.
  
• Polyethylene: Another form of plastic that’s lightweight and resistant to UV rays and moisture.
  
• Fiberglass-reinforced layers (in premium products): Add even more strength without adding much weight.
  

These components work together to create a material that’s both tough and easy to work with, which is why it’s become increasingly popular among contractors and property managers alike.

Advantages of Synthetic Underlayment

• Lightweight and easy to handle: Installers can carry and roll out larger sheets, which speeds up the job and reduces labor time.
  
• Superior tear resistance: Especially valuable on steep slopes or windy job sites.
  
• Longer UV exposure rating: Some synthetic products can be left exposed for weeks without breaking down, offering flexibility during projects or in unpredictable weather.
  
• Improved water resistance: Many synthetics act as effective moisture barriers, helping prevent leaks during the installation phase or in severe storms.
  
• Slip-resistant surfaces: Many versions include added texturing for safer walkability during installation.
  

Potential Drawbacks to Consider

• Higher initial cost: Synthetic underlayment tends to be more expensive than felt on a per-roll basis.
  
• Slippery when wet (in some types): Not all products include textured surfaces, which can create safety concerns if the material becomes damp.
  
• Not always compatible with all roofing systems: Though rare, some roofing manufacturers may have preferences or requirements that affect underlayment choices for warranties.
  

Takeaway: Synthetic roofing underlayment offers a modern, high-performance solution that delivers excellent durability, water resistance, and installation efficiency—making it a strong contender for both residential and commercial properties, especially in regions with harsh weather conditions.

What Is Felt Roof Underlayment?

Felt roof underlayment, also known as asphalt-saturated felt or tar paper, is one of the oldest and most traditional materials used beneath shingles. It’s made by saturating a base material—typically fiberglass or organic paper—with asphalt to create a water-resistant layer between the roof deck and the exterior roofing material.

This type of underlayment has been a go-to choice in residential roofing for decades. Many contractors are highly familiar with it, and in some regions, it’s still considered a standard option for lower-cost or budget-conscious projects.

How Felt Underlayment Is Made

Felt is generally available in two grades:

• No. 15 (Type I): A thinner, lighter option often used for steeper-sloped roofs where water runoff is faster.
  
• No. 30 (Type II): Thicker and more durable, typically used in areas where greater protection is needed.
  

The material’s asphalt saturation helps it resist some moisture, but unlike modern synthetics, it doesn’t provide a true waterproof barrier.

Advantages of Felt Underlayment

• Cost-effective: One of the most affordable underlayment options, making it attractive for tight budgets or short-term needs.
  
• Widely available: Found at nearly every roofing supply store, and compatible with most roofing systems.
  
• Trusted by tradition: Many roofing contractors have worked with felt for years and are comfortable installing it quickly and efficiently.
  

Key Drawbacks of Felt Underlayment

• Heavier and bulkier: Rolls of felt are heavier than synthetics, making them more cumbersome to move and install—especially on large or steep roofs.
  
• Tears more easily: It’s more prone to ripping, especially if walked on during high winds or if exposed to the elements before shingle installation.
  
• Limited moisture resistance: Felt absorbs water rather than repelling it, which can lead to warping or degradation if left exposed.
  
• Shorter UV exposure window: If installation is delayed and the felt remains exposed, it can break down quickly under sunlight.
  

Takeaway: Felt underlayment remains a viable option for budget-conscious projects or where traditional materials are preferred, but its performance limitations—especially in wet or storm-prone climates—make it less ideal for long-term durability or complex roofing needs.

Side-by-Side Comparison: Synthetic vs. Felt Roof Underlayment

When choosing between synthetic and felt roof underlayment, the decision often comes down to performance across key categories—cost, durability, weather resistance, installation, code compliance, and maintenance. 

Below is a detailed comparison to help you weigh the trade-offs clearly and confidently.

Cost Comparison

Category	Synthetic Underlayment	Felt Underlayment
Material Cost (per roll)	Higher ($100–$200+)	Lower ($20–$40)
Coverage (per roll)	Larger (1,000+ sq ft)	Smaller (~400 sq ft)
Labor Cost	Often lower (easier to install)	Moderate (more time-intensive)
Long-Term ROI	High (longer lifespan, fewer replacements)	Lower (may need earlier replacement)

While felt has a lower upfront cost, synthetic underlayment may prove more cost-effective over time due to reduced labor needs, fewer repairs, and a longer service life.

Durability & Lifespan

• Synthetic underlayment typically lasts 20–30 years, especially under asphalt shingles or metal roofs. It resists tearing, cracking, and UV exposure, which helps extend roof longevity—particularly in warm or storm-prone climates.
  
• Felt underlayment generally lasts 5–15 years, depending on thickness (No. 15 vs. No. 30) and weather exposure. It’s more vulnerable to aging, rot, and deterioration over time.
  

Bottom line: For roofs that need to last decades with minimal maintenance, synthetic is usually the better investment.

Moisture & Weather Resistance

• Synthetic materials offer excellent water repellency and resist wind-driven rain, making them a reliable barrier during extreme weather events.
  
• Felt can absorb moisture, especially if exposed before shingles are installed, increasing the risk of wrinkling or roof deck damage.
  

In hurricane-prone or rainy regions like Florida, synthetic underlayment holds up better against both wind uplift and prolonged damp conditions.

Installation Ease & Safety

• Synthetic underlayment is lightweight and comes in wider rolls, allowing roofers to cover more area with fewer seams. Many versions also include anti-slip coatings for better traction during installation.
  
• Felt is heavier and prone to tearing, especially when nailed or walked on. It may become slick when wet, increasing the risk of falls.
  

Faster, safer installs with fewer interruptions give synthetic a clear edge in this category.

Code Compliance & Fire Ratings

• Many synthetic underlayments are Class A fire-rated and meet modern building codes in most states. However, always check manufacturer specs and local code requirements to ensure compliance. To learn more, see this guide to types of fire-rated underlayment.
  
• Felt is also code-compliant and has long been accepted under standard roofing systems, though fire ratings can vary by product and thickness.
  

If you’re re-roofing in a jurisdiction with stricter fire or wind-resistance codes, synthetic may provide more flexibility.

Maintenance Needs & Long-Term Value

• Synthetic underlayments require minimal attention once installed and are less prone to warping, curling, or disintegrating over time.
  
• Felt may deteriorate under moisture or UV exposure, especially if installation is delayed or shingles become damaged, exposing the underlayment.
  

Synthetic offers greater long-term value for property owners looking for a “set it and forget it” solution—particularly on roofs with difficult access or where disruption is costly.

Takeaway: While felt may offer short-term savings, synthetic underlayment consistently outperforms in durability, weather resistance, and total cost of ownership—making it the preferred choice for most modern roofing projects.

Which One Is Better for Your Region or Property?

There’s no one-size-fits-all answer when it comes to choosing between synthetic and felt underlayment—it depends on your climate, property type, and performance priorities. The right material for a coastal Florida home may not be ideal for a dry, inland commercial facility. Below, we break down how each option performs in different environments and building types to help you make the most informed decision possible.

Climate Considerations

Hot & Humid (e.g., Florida, Gulf Coast):
Synthetic underlayment is typically the better choice in these regions. It handles high humidity, heavy rains, and UV exposure without breaking down. Felt, on the other hand, can absorb moisture and degrade quickly if left exposed before shingles are installed.

Cold & Snowy (e.g., Midwest, Northeast):
Both options are viable, but synthetic still has an edge due to better resistance to ice damming and moisture infiltration. Felt may become brittle in extreme cold, especially if not installed quickly.

Dry & Arid (e.g., Southwest):
Felt can be a reasonable choice here since moisture is less of a concern, but the prolonged sun exposure may still shorten its lifespan. Synthetic holds up better under long-term UV stress.

Storm-Prone (e.g., Southeast, Tornado Alley):
Synthetic underlayment wins again. Its tear resistance and strong adhesion make it ideal for high-wind environments. Some products are even engineered to meet code requirements for storm resilience.

Property Type Considerations

Residential Homes:

• Synthetic: Great for long-term value, better protection, and minimal maintenance—especially for homeowners planning to stay in their homes for many years.
  
• Felt: Acceptable for budget-conscious projects or short-term property ownership.
  

Commercial Properties:

• Synthetic: Strongly recommended due to roof complexity, square footage, and the high cost of future repairs or tenant disruptions.
  
• Felt: Less common for commercial builds today, though still used in certain light-slope or cost-limited cases.

Best for You If…

Here’s a quick reference checklist to help you decide:

Choose…	If You…
Synthetic	Live in a humid or storm-prone region
	Manage a commercial or multi-family building
	Want maximum durability and fewer maintenance concerns
	Need code compliance or extended UV exposure during construction
	Prefer long-term value over short-term savings
Felt	Have a tight roofing budget and a simple residential roof
	Live in a dry climate with limited rainfall or storm risk
	Are replacing a roof on a short-term property or quick flip
	Are working with contractors familiar and comfortable with felt systems

Takeaway: For most modern roofing needs—especially in hot, humid, or high-wind regions—synthetic underlayment offers more protection, longevity, and flexibility. Felt may still work in drier or lower-risk areas, but it’s increasingly being replaced by synthetic as the go-to standard.

How to Choose the Right Underlayment for Your Project

With multiple factors to consider—like property type, budget, and regional climate—choosing the right roofing underlayment can feel overwhelming. 

To simplify the decision, here’s a step-by-step checklist to guide your selection process with clarity and confidence.

1. Know Your Roof Type and Slope

Different roof structures have different demands.

• Steep-slope roofs (common in residential homes) benefit from lightweight materials like synthetic underlayment, which stay put more easily and reduce seam overlap.
  
• Low-slope or flat roofs (often found on commercial buildings) may require specialized materials or additional waterproofing layers.
  

Understanding your roof’s pitch and complexity helps eliminate options that won’t perform well structurally or aesthetically.

2. Assess Local Weather Patterns

Your region’s climate plays a critical role in underlayment performance.

• In storm-prone, humid, or high-UV areas, synthetic underlayment is more resistant to moisture intrusion, wind uplift, and sun exposure.
  
• In dry or mild climates, traditional felt may still hold up adequately and offer short-term savings.
  

Tip: If your area experiences long project delays due to unpredictable weather, synthetic’s extended UV exposure rating is a key advantage.

3. Define Your Budget and Long-Term Goals

Ask yourself:

• Are you planning to live or operate in this property long term?
  
• Is the upfront cost more important, or do you prefer fewer repairs and longer protection?
  

Felt may fit the bill for quick-turnaround or low-cost jobs, but synthetic often delivers better value over the lifespan of your roof.

For homes prioritizing energy efficiency, also consider the impact of roof insulation and diminishing returns as part of your system.

4. Talk to Your Contractor

A trusted roofing contractor can provide tailored recommendations based on your roof’s condition, slope, local code requirements, and long-term needs.

• Which underlayment type do they recommend for your roof and why?
  
• Are they experienced with both materials?
  
• How does the choice impact the installation timeline or warranty coverage?

5. Consider Building Codes and Warranty Requirements

Some building codes or manufacturer warranties may require specific types of underlayment—particularly for fire ratings, wind resistance, or moisture barriers. Always verify:

• Your local code mandates (especially in hurricane or fire-risk zones)
  
• What underlayment is compatible with your roofing material’s warranty

Takeaway: By evaluating your roof type, local conditions, project goals, and contractor input, you can confidently choose the underlayment that balances performance, budget, and code compliance—whether that’s synthetic, felt, or a specialty solution tailored to your building’s needs.

Conclusion: Make a Confident, Informed Decision

Choosing between synthetic and felt roof underlayment isn’t just about cost—it’s about finding the right fit for your property, climate, and long-term goals. Synthetic underlayment offers superior durability, weather resistance, and low maintenance, making it ideal for most modern residential and commercial projects—especially in humid, storm-prone, or high-UV regions. 

Felt, while more affordable upfront, may suit short-term needs or drier climates but comes with trade-offs in longevity and resilience. Ultimately, the best underlayment for your project depends on:

• The type and slope of your roof
  
• Your regional climate
  
• Your budget and expected length of ownership
  
• Building code or warranty requirements
  
• Your contractor’s experience and recommendations
  

If you’re still unsure which direction to take, don’t hesitate to speak with a qualified roofing contractor. A professional can assess your specific roof and recommend the most effective solution based on performance needs—not just price.

Take the time to weigh your options carefully—your roof’s protection depends on what’s underneath.

FAQs: Synthetic vs. Felt Roof Underlayment