Every roof replacement involves a decision that most homeowners never see but always live with — choosing the right underlayment. Installed directly onto the roof deck beneath shingles, tile, or metal panels, roof underlayment is the secondary water-resistant barrier that protects your home when the primary roofing material is compromised. In Florida’s climate of sustained UV exposure, heavy seasonal rainfall, hurricane-force winds, and high ambient humidity, that secondary barrier carries real weight.

The two most common types are felt underlayment and synthetic underlayment. Both serve the same core function, but they differ significantly in durability, weight, moisture performance, UV tolerance, and long-term cost. This guide breaks down exactly how they compare — and which one is the stronger choice for Tampa Bay homeowners.

What Is Roof Underlayment and Why Does It Matter?

Roof underlayment is a water-resistant membrane rolled out across the roof deck — typically OSB or plywood — before the final roofing material is installed. It serves as a layered defense system: the outer roofing material (shingles, tile, metal panels) handles primary weather exposure, while the underlayment acts as a backup barrier that prevents moisture infiltration at seams, fastener penetrations, and any point where the outer layer is lifted, cracked, or temporarily removed.

Beyond moisture protection, underlayment serves several other functions that directly affect roof system performance in Florida:

Secondary moisture barrier: Prevents wind-driven rain from reaching the roof deck if outer roofing is damaged during a storm — a critical function in hurricane season.
Roof deck protection during installation: Keeps the structural decking dry if roofing material installation is delayed by weather or scheduling.
Shingle and tile stability: Provides a smoother, more uniform surface for roofing material installation, reducing irregularities that can cause premature wear.
Sound dampening: Adds a buffer that reduces noise transmission from rain and hail into living spaces.
Code compliance and insurance eligibility: Florida Building Code updates following Hurricane Michael tightened underlayment requirements for both residential and commercial roofing. Meeting those requirements is also a prerequisite for wind mitigation credits that lower homeowners insurance premiums. You can find the full details in our guide to Florida roofing underlayment requirements.

What Is Felt Underlayment?

Felt underlayment — also called tar paper, asphalt-saturated felt, or roofing felt — is the oldest and most traditional underlayment type still in active use. It is made by saturating a base mat of organic cellulose fibers, recycled paper, or fiberglass with asphalt, which gives it its characteristic water-resistant properties.

Felt underlayment comes in two standard weights:

Grade	Weight per Square (100 sq ft)	Thickness	Best Application
#15 felt	7.5–12.5 lbs	Thinner	Low-slope, budget-focused projects
#30 felt	16–27 lbs	Thicker	Steep-slope roofs, heavier duty applications

Despite its name, #15 felt does not actually weigh 15 pounds per square — the number refers to a historical designation, not a precise weight. #30 felt is thicker, more durable during installation, and better suited to Florida’s steep-slope residential roofs where physical handling stress during installation is higher.

Felt underlayment has been manufactured essentially the same way since the 1960s. While it remains a functional product, the roofing industry has shifted substantially toward synthetic alternatives — for reasons that become clear when comparing performance in Florida’s demanding environment.

What Is Synthetic Underlayment?

Synthetic underlayment is a non-bitumen product made from woven or spun polymers — typically polyethylene or polypropylene. Introduced commercially in the early 2000s, it was developed to address the performance limitations of felt under modern roofing conditions: higher UV exposure during extended installation windows, greater wind loads, and stricter code requirements after major storm seasons.

Unlike felt, which is rated by weight, synthetic underlayments are rated by grams per square meter (GSM) and vary in thickness, UV exposure tolerance, and temperature rating by manufacturer. Key product characteristics to look for include:

UV exposure rating: How long the bare underlayment can remain exposed before the outer roofing material must be installed. Synthetics typically range from 6 months to 12+ months; felt lasts 30–90 days at most.
Temperature rating: Critical for metal roofing applications, where panel heat can exceed 250°F at the underlayment surface. Standard synthetics may not be rated for direct metal contact — high-temp variants are required.
Permeability: Synthetic underlayments have low permeability, meaning they resist moisture passage in both directions. This is an advantage for water protection but requires adequate attic and roof ventilation to prevent trapped condensation — a factor worth reviewing alongside our guide on roof vapor barriers and moisture management.

Synthetic vs. Felt Underlayment: Full Comparison

Durability and Tear Resistance

Synthetic underlayment wins this category clearly. Its polymer construction makes it highly resistant to tearing from wind, foot traffic, and the mechanical stress of installation. Roofers routinely walk across it, drop tools on it, and drag materials over it without compromising the membrane. Felt underlayment, by contrast, tears easily under physical stress — particularly #15 felt, which can shred during installation on a steep or complex roof in Florida’s humid summer conditions.

Felt also becomes brittle under prolonged UV exposure, further reducing its tear resistance after even short periods of exposure to the sun.

Moisture Resistance

Synthetic underlayment repels water rather than absorbing it, maintaining its protective properties even when exposed to rain between installation phases. Felt absorbs moisture. When felt gets wet, it swells, buckles, and wrinkles — creating an uneven surface that affects how flat shingles or tiles lay above it, and potentially introducing moisture pathways that compromise the finished roof.

This distinction is particularly consequential in Florida, where afternoon thunderstorms can interrupt roof installations daily during summer months. Roof underlayment waterproofing performance varies significantly between the two types, with synthetics providing a more reliable water-shedding barrier under real-world installation conditions.

UV and Heat Tolerance

Synthetic underlayment can remain exposed to direct sunlight for 6 months to over a year, depending on the product — a practical advantage during phased installations or projects where material delivery delays are common. Felt begins to degrade within 30–90 days of UV exposure. As the asphalt saturant dries out in Florida’s heat, felt can become brittle, leach petroleum compounds, and lose its moisture resistance entirely before the outer roofing material is installed.

For metal roofing applications specifically, temperature tolerance at the underlayment surface is a distinct concern. Metal panels can heat the underlayment surface to 250°F or higher — a temperature that standard felt and many general-purpose synthetics are not rated to handle. High-temperature synthetic variants are required for metal roofing systems. See our metal roof underlayment guide for product-specific guidance.

Weight and Installation Speed

Synthetic underlayment is dramatically lighter than felt — roughly four times lighter by weight per square. This translates into meaningful differences in both installation labor and roof deck loading:

Metric	Felt (#30)	Synthetic
Weight per square (100 sq ft)	16–27 lbs	~4–7 lbs
Coverage per roll	200–400 sq ft	1,000+ sq ft
Rolls needed for 2,700 sq ft roof	~14 rolls	~3 rolls
UV exposure tolerance	30–90 days	6–18 months
Typical lifespan	12–20 years	25–40 years

Fewer, lighter rolls mean fewer trips up a ladder, faster installation, and lower labor costs — a factor that can partially or fully offset the higher material cost of synthetic underlayment on larger roofing projects.

Slip Resistance and Worker Safety

Synthetic underlayment is manufactured with textured, slip-resistant surfaces and typically includes printed overlap guides and fastener placement indicators. Felt, particularly when wet, is notably slippery — creating a fall hazard for roofing crews on Tampa Bay’s common 4:12 to 6:12 pitch residential roofs. Safer installation conditions also mean fewer delays from weather-interrupted work and reduced liability exposure for roofing contractors.

Mold and Mildew Resistance

Because synthetic underlayment is polymer-based and does not absorb moisture, it is naturally resistant to mold and mildew growth — a meaningful advantage in Florida’s high-humidity environment where roof deck condensation is a persistent concern. Felt, which is an organic-based product, can harbor mold growth if it becomes saturated with moisture before or during installation.

Cost Comparison

Felt underlayment has a lower upfront material cost:

Felt underlayment: Approximately $5–$10 per square (100 sq ft) of material cost
Synthetic underlayment: Approximately $17–$25 per square for standard products; $50–$75 per square for self-adhering (peel-and-stick) membranes

However, the cost comparison is not as simple as the material price suggests. Because synthetic rolls cover significantly more area per roll, total material quantities are lower. Labor costs are also reduced with synthetic due to faster installation and safer working conditions. And over the life of the roof, synthetic’s longer lifespan — 25–40 years versus 12–20 years for felt — means fewer replacement cycles, particularly relevant on long-lived metal roofs or tile roofs that routinely outlast felt underlayment by decades.

Does Florida Building Code Require Synthetic or Felt Underlayment?

Florida Building Code accepts both felt and synthetic underlayment, but with specific installation requirements that reflect the state’s hurricane risk exposure. Following Hurricane Michael, the 2020 Florida Building Code (7th Edition) updated underlayment standards to require one of the following for wood deck residential roofs:

Two layers of #30 felt or equivalent synthetic underlayment
One layer of felt or synthetic underlayment with a secondary self-adhering peel-and-stick layer that creates a sealed roof deck

The sealed roof deck option is particularly important because it qualifies homes for a Secondary Water Resistance (SWR) discount on homeowners insurance — a meaningful long-term financial benefit. A properly installed self-adhered underlayment system can reduce water entry through a compromised roof by up to 95%, according to industry research from the Insurance Institute for Business & Home Safety.

Most roofing contractors in the Tampa Bay area now install synthetic roof underlayment as the standard choice, with self-adhered peel-and-stick membrane used at eaves, valleys, and all roof penetrations to meet sealed roof deck requirements.

Are There Any Downsides to Synthetic Underlayment?

Synthetic underlayment is the stronger performer in almost every category, but there are a few considerations worth knowing before installation:

Low permeability requires proper ventilation: Synthetic’s resistance to moisture vapor passage means that interior moisture cannot escape upward through the roof deck. Without adequate attic ventilation, this can lead to condensation buildup on the underside of the deck. This is a ventilation design issue, not a product defect, but it needs to be planned for — particularly in Florida’s humid climate. Roof insulation and attic ventilation in Florida interact directly with underlayment permeability.
No manufacturing standard: Unlike felt, which is classified under ASTM D4869 (for #15 and #30 grades), synthetic underlayment products are not standardized across manufacturers. Product composition, GSM weight, UV ratings, and temperature tolerances vary significantly. Always confirm that the specific product meets Florida Building Code and any roofing manufacturer warranty requirements.
Higher upfront cost: The material price premium is real, even if the total installed cost difference is smaller than it appears. For short-term budgets, this matters.
Removal difficulty: Peel-and-stick self-adhered synthetic underlayment in particular can be difficult to remove if the roof deck ever needs repair or replacement. This is a minor long-term consideration for most installations.

Which Underlayment Is Right for Your Florida Roof?

For the vast majority of residential roofing projects in Tampa Bay — whether you’re installing asphalt shingles, tile, or metal panels — synthetic underlayment is the stronger choice. Florida’s climate demands a product that can handle UV exposure during installation delays, repel moisture during afternoon storm interruptions, tolerate the thermal stress of summer heat, and meet Florida Building Code’s post-hurricane requirements.

Felt underlayment remains a legitimate option in specific scenarios: very tight budgets, short-timeline installations where outer roofing material will be installed within a few days, or projects where local code requirements specifically call for felt.

For any roofing system expected to last 20+ years — standing seam metal roofs, concrete tile, or architectural shingles — installing felt beneath a long-lived outer material creates a mismatch. The underlayment will need replacement well before the roofing material above it, requiring the outer material to be at least partially removed to access the deck. Synthetic underlayment, matched to the lifespan of the roofing system it supports, avoids that problem.

Get the Right Underlayment Installed the First Time

At SouthShore Roofing & Exteriors, we install code-compliant synthetic underlayment systems matched to your specific roofing material, roof pitch, and Florida exposure conditions across Tampa Bay.

Book your free roof inspection online or call (813) 400-3329 to get started.

Synthetic vs. Felt Underlayment: Which One To Choose?