Roofing Felt vs Synthetic Underlayment: Which Is Better? (2026)

Underlayment is the unsung hero of your roofing system. This layer between your roof deck and your shingles or tiles serves as a critical moisture barrier, protecting your Pinellas County home from water intrusion when your primary roofing material is compromised by wind, impact, or age. Despite its importance, many homeowners never think about underlayment, leaving the decision entirely to their contractor.

That is a mistake. The type of underlayment on your roof directly affects how well your home withstands Florida's intense rain, humidity, and hurricane-force winds. It also affects your warranty coverage, your installation timeline, and your total project cost. In this comprehensive guide, we compare traditional roofing felt with modern synthetic underlayment and help you determine the best choice for your Pinellas County roof.

What Is Roofing Underlayment and Why Does It Matter?

Roofing underlayment is a water-resistant or waterproof barrier material installed directly over your roof deck (the plywood or OSB sheathing) and underneath your primary roofing material. Think of it as a secondary roof. If wind lifts a shingle, if a tile cracks during a storm, or if driving rain finds its way under your roofing material, the underlayment is the last line of defense preventing water from reaching your decking, attic, and interior.

In Pinellas County, this secondary protection is not optional. Florida receives an average of 50 to 55 inches of rain per year, and tropical storms and hurricanes regularly produce wind-driven rain that can penetrate even well-installed roofing systems. The Florida Building Code recognizes this reality by requiring underlayment on all sloped roofing installations.

Traditional Roofing Felt: The Original Standard

Roofing felt (also called tar paper or builder's felt) has been used as roofing underlayment for over a century. It is made from a base of natural materials (typically wood cellulose or fiberglass) saturated with asphalt to provide water resistance. Roofing felt comes in two standard weights: 15-pound and 30-pound.

15-Pound Felt

Despite its name, modern 15lb felt does not actually weigh 15 pounds per square. Manufacturing standards have changed, and today's 15lb felt typically weighs about 8 to 12 pounds per 100 square feet. It is thin (roughly 1/16 inch), lightweight, and the least expensive underlayment option available.

However, 15lb felt has significant drawbacks for Florida roofing:

• Low tear resistance: It rips easily during installation, especially in the windy conditions common across Pinellas County. Contractors working on roofs in Clearwater, St. Petersburg, and coastal areas frequently report 15lb felt tearing before the roofing material is even installed.
• Poor UV tolerance: When left exposed to sunlight (common during multi-day roofing projects or weather delays), 15lb felt degrades rapidly. Florida's intense UV radiation can compromise 15lb felt in as little as 30 days of exposure.
• Minimal water protection: While 15lb felt provides basic water resistance, it absorbs moisture and can wrinkle, creating channels for water to travel underneath.
• Single-layer weakness: A single layer of 15lb felt offers very little protection against wind-driven rain, which is a primary concern in Pinellas County.

30-Pound Felt

30lb felt is roughly twice the thickness and weight of 15lb felt, providing meaningfully better performance in every category. Modern 30lb felt typically weighs 15 to 20 pounds per square and measures approximately 1/8 inch thick.

Advantages of 30lb felt over 15lb:

• Better tear resistance: The thicker material holds up significantly better during installation and resists wind uplift forces more effectively.
• Improved water resistance: The heavier asphalt saturation provides better waterproofing and less moisture absorption.
• Longer UV exposure tolerance: While still not designed for prolonged exposure, 30lb felt can withstand 60 to 90 days of UV exposure before significant degradation.
• Code compliance: ASTM D226 Type II (30lb felt) meets Florida Building Code requirements for most residential roofing applications.

Synthetic Underlayment: The Modern Alternative

Synthetic underlayment is made from woven or spun polyethylene or polypropylene polymers. These engineered materials were developed specifically to address the weaknesses of traditional felt. Since gaining widespread adoption in the early 2010s, synthetic underlayment has become the preferred choice for most professional roofing contractors, including those serving Pinellas County.

Polyethylene-Based Synthetic

Polyethylene synthetic underlayment is the more affordable synthetic option. It offers good basic moisture protection and is lighter and more flexible than felt. Key characteristics include:

• Lightweight and easy to handle (typically 3 to 7 lbs per square)
• Good water resistance
• Moderate tear strength (significantly better than felt)
• Generally UV stable for 60 to 120 days
• More affordable than polypropylene options

Polypropylene-Based Synthetic

Polypropylene synthetic underlayment is the premium option in the synthetic category. It offers the highest performance across all metrics:

• Superior tear resistance (often 3 to 5 times stronger than 30lb felt)
• Excellent UV stability (many products rated for 180 days or more of exposure)
• Outstanding water resistance with minimal moisture absorption
• Better walkability (critical for worker safety on steep Florida roofs)
• Typically includes anti-slip surface treatment
• Longer rolls mean fewer seams and faster installation

For Pinellas County installations where hurricane protection and long-term performance matter, polypropylene-based synthetic underlayment is the top recommendation from most roofing professionals.

Head-to-Head Comparison: Felt vs Synthetic

Cost Analysis: Is Synthetic Worth the Premium?

The upfront cost difference between felt and synthetic underlayment is real but often smaller than homeowners expect when you factor in total project economics.

For a typical 2,500 square foot Pinellas County home (25 squares of roof area), here is the cost comparison:

The difference between 30lb felt and quality synthetic underlayment for a typical Pinellas County home is roughly $100 to $300. On a roofing project that typically costs $10,000 to $25,000 or more, that premium represents less than 2% of your total investment. Considering the significant performance advantages, synthetic underlayment offers excellent value.

Synthetic underlayment also saves labor costs because the larger rolls (10 squares vs 2 to 4 squares) mean fewer seams, faster installation, and less waste. Many Pinellas County contractors report completing underlayment installation 25% to 40% faster with synthetic products compared to felt.

Florida Building Code Underlayment Requirements

The Florida Building Code sets specific requirements for roofing underlayment that affect your options in Pinellas County:

General Requirements (FBC Section 1507)

• All asphalt shingle roofs require underlayment beneath the shingles
• Underlayment must comply with ASTM D226 (for felt) or ASTM D4869 (for synthetic)
• In areas with roof slope of 2:12 to 4:12, two layers of underlayment or a self-adhering modified bitumen underlayment is required
• Ice and water shield (self-adhering membrane) is required at valleys, eaves, and penetrations

Enhanced Wind-Driven Rain Requirements

Florida's code goes beyond standard IBC requirements by addressing wind-driven rain. For Pinellas County (wind speed design of 150 MPH), the code requires:

• Minimum one layer of ASTM D226 Type II (30lb felt) or equivalent synthetic product
• Proper horizontal and vertical overlap (4 inches for synthetic, 2 inches for felt minimum)
• Mechanical fastening with cap nails or staples at specified intervals
• Self-adhering underlayment at specific locations prone to water intrusion

Note that while 15lb felt is technically allowed in certain applications under the code, the enhanced wind requirements in Pinellas County essentially mandate 30lb felt as the minimum when felt is used. Most building inspectors in Clearwater, St. Petersburg, and throughout Pinellas County prefer to see synthetic underlayment and will sometimes flag 15lb felt during inspections.

Warranty Implications: A Hidden Factor

One of the most overlooked aspects of underlayment selection is its impact on your roofing material warranty. Many homeowners focus on the warranty of their shingles or tiles without realizing that the underlayment choice can void that warranty.

Here is how major manufacturers handle underlayment in their warranty requirements:

• GAF: Requires minimum ASTM D226 or ASTM D4869 compliant underlayment for standard warranty. Their premium lifetime warranties require GAF-branded synthetic underlayment as part of the GAF Roof System.
• Owens Corning: Requires minimum 30lb felt or approved synthetic for their Preferred Protection and Platinum Protection warranty levels.
• CertainTeed: Requires DiamondDeck or equivalent synthetic underlayment for their 5-Star SureStart Plus warranty.
• IKO: Requires minimum ASTM D226 Type II or ASTM D4869 synthetic underlayment for warranty coverage.

The pattern is clear: premium warranty coverage increasingly requires synthetic underlayment. If you are paying for high-quality shingles and want maximum warranty protection, skimping on underlayment to save $200 can cost you thousands in voided warranty coverage.

When Felt Paper Still Makes Sense

Despite the clear advantages of synthetic underlayment, there are limited situations where felt paper remains a reasonable choice for Pinellas County homeowners:

• Tight budgets on rental properties: If you are reroofing a rental property with basic 3-tab shingles and standard warranty is acceptable, 30lb felt provides adequate code-compliant protection at the lowest cost.
• Temporary or short-term roofing: If the property has a planned demolition or major renovation within 5 to 10 years, the long-term performance advantage of synthetic is less relevant.
• Under tile roofing with hot-mopped application: Some tile roofing systems in Pinellas County use a hot-mopped installation method that requires felt underlayment as the base layer. The hot asphalt bonds to the felt in a way that does not work with synthetic products.
• Contractor preference for specific applications: Some experienced Pinellas County tile roofing contractors prefer the moisture absorption qualities of felt under certain tile installations. The slight moisture absorption can actually help prevent condensation between tile and underlayment in Florida's humid climate.

Installation Differences in Florida's Climate

The Pinellas County climate creates unique installation challenges that affect underlayment performance:

Heat and Sun

Florida's intense heat softens asphalt felt, making it more prone to tearing and wrinkling during summer installations. Synthetic underlayment maintains its structural integrity even on 120+ degree roof surfaces. Pinellas County roofing crews working in July and August consistently report fewer installation problems with synthetic products.

Afternoon Thunderstorms

Pinellas County's famous afternoon thunderstorms can soak an exposed underlayment within minutes. Felt absorbs this water, becomes heavy, and can create wrinkles that compromise its effectiveness even after drying. Synthetic underlayment sheds water quickly and does not absorb moisture, maintaining its flat, consistent profile.

Construction Delays

Weather delays are a fact of life for Pinellas County roofing projects, especially during the June through November hurricane season. When underlayment is left exposed for days or weeks waiting for materials or clear weather, synthetic products maintain their protection far longer than felt. This extended UV tolerance provides a crucial safety margin.

Worker Safety

Florida's frequent rain makes roof surfaces dangerously slippery. Wet felt paper is one of the most hazardous surfaces a roofer can work on. Synthetic underlayment with anti-slip coating provides dramatically better footing, reducing the risk of falls. This safety advantage is why most Pinellas County roofing companies have transitioned to synthetic products as standard practice.

Self-Adhering Underlayment: The Premium Option

Beyond standard felt and synthetic, there is a third category worth mentioning: self-adhering (peel-and-stick) underlayment. This premium product provides a fully waterproof seal by bonding directly to the roof deck.

In Pinellas County, self-adhering underlayment is required by code at specific high-risk areas including roof valleys, eaves (first 24 inches minimum), around penetrations (vents, pipes, skylights), and at wall-to-roof transitions. Some homeowners and contractors choose to install self-adhering underlayment across the entire roof deck for maximum protection. While expensive ($30 to $60 per square), full-deck self-adhering underlayment creates a secondary waterproof roof that can protect your home even if the primary roofing material is completely removed by a hurricane.

Making Your Decision: Recommendations for Pinellas County

Based on Florida's climate demands, building code requirements, and the minimal cost difference relative to total project investment, here are our recommendations:

• Best overall choice: Polypropylene synthetic underlayment. It offers the best combination of tear resistance, UV tolerance, water protection, and worker safety for Pinellas County's demanding conditions.
• Best budget choice: Polyethylene synthetic underlayment. It significantly outperforms felt at a modest premium and provides the performance characteristics Florida demands.
• Acceptable minimum: 30lb felt (ASTM D226 Type II). It meets code requirements and provides adequate protection, but it is being phased out by most professional contractors in favor of synthetic products.
• Not recommended: 15lb felt. The minimal cost savings do not justify the dramatically inferior performance in Florida's climate. Avoid 15lb felt for any Pinellas County roofing project.
• Premium choice: Full-deck self-adhering underlayment combined with synthetic. This provides the ultimate protection for high-value homes or homeowners who want maximum hurricane resilience.

Frequently Asked Questions

Is synthetic underlayment worth the extra cost over felt?

For most Pinellas County roofing projects, yes. Synthetic underlayment costs $15 to $30 per square compared to $5 to $15 for felt, but it offers dramatically better tear resistance, UV exposure tolerance, and water resistance. In Florida where roofing projects may be delayed by rain and where underlayment must withstand extreme conditions during installation, synthetic pays for itself in reduced callbacks and better long-term performance. The total cost difference on a typical home is only $100 to $300.

Does Florida building code require synthetic underlayment?

Florida building code does not specifically mandate synthetic underlayment over felt. However, FBC Section 1507.1.1 requires that underlayment meet minimum standards for water resistance and wind-driven rain protection. In the High Velocity Hurricane Zone (which does not include Pinellas County), additional underlayment requirements apply. For Pinellas County, both ASTM D226 Type II (30lb felt) and ASTM D4869 Type I synthetic underlayment are code-compliant when properly installed.

Can I use 15lb felt paper on my Florida roof?

While 15lb felt is technically code-compliant in some limited applications, most Pinellas County roofing contractors and building inspectors strongly recommend against it. The thin material tears easily during installation, offers minimal water protection, and degrades quickly under Florida's UV exposure. If budget requires felt paper, use 30lb felt at minimum. For the best protection, use synthetic underlayment.

What is the difference between polyethylene and polypropylene synthetic underlayment?

Polyethylene-based synthetic underlayment is lighter, more flexible, and generally less expensive. It works well as a basic moisture barrier. Polypropylene-based synthetic underlayment is stronger, more tear-resistant, and better suited for high-wind environments like Pinellas County. Polypropylene products typically offer better UV resistance and can be left exposed longer during construction delays. For Florida installations, polypropylene-based products are the preferred choice.

Does underlayment type affect my roofing warranty?

Yes, significantly. Many roofing material manufacturers require specific underlayment types to maintain full warranty coverage. For example, most asphalt shingle manufacturers require at minimum a synthetic underlayment or 30lb felt for their premium warranty tiers. Using 15lb felt or a non-approved underlayment can void your material warranty entirely. Always check your roofing material manufacturer's warranty requirements before selecting underlayment.

Bottom Line

For Pinellas County homeowners investing in a new roof, synthetic underlayment is the clear winner in 2026. The performance advantages in Florida's demanding climate far outweigh the minimal cost premium. Your underlayment is the last line of defense between your home and water damage. Do not let your contractor cut corners on this critical component to save a few hundred dollars on a project that costs thousands.

Ask your roofing contractor specifically what underlayment they plan to use, verify it meets your shingle or tile manufacturer's warranty requirements, and ensure it is installed according to Florida Building Code standards. This one conversation can save you from costly water damage and warranty disputes down the road.

Last updated: February 2026. This guide reflects current Florida Building Code requirements and typical pricing for the Pinellas County, FL area. Always consult with a licensed local roofing contractor for project-specific advice.